Update upstream source from tag 'upstream/0.18'

Update to upstream version '0.18'
with Debian dir 4633953086f923825337301a18d98b4bc0c0e8b4

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diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,7 @@
+The Immutables project is provided under the Apache 2.0 license.  See
+LICENSE-APACHE for the full text of the license.
+
+Additionally, this software contains the following code distributed a
+different license (refer to the specific files for details):
+
+  immutables/pythoncapi_compat.h  (MIT License)
diff --git a/LICENSE-APACHE b/LICENSE-APACHE
new file mode 100644
index 0000000..bf911cf
--- /dev/null
+++ b/LICENSE-APACHE
@@ -0,0 +1,200 @@
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diff --git a/MANIFEST.in b/MANIFEST.in
new file mode 100644
index 0000000..d3f372b
--- /dev/null
+++ b/MANIFEST.in
@@ -0,0 +1,4 @@
+recursive-include tests *.py
+recursive-include immutables *.py *.c *.h *.pyi
+include LICENSE* NOTICE README.rst bench.png
+include immutables/py.typed
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..481d190
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,33 @@
+.PHONY: rtest build test clean all
+
+
+PYTHON ?= python
+ROOT = $(dir $(realpath $(firstword $(MAKEFILE_LIST))))
+
+
+all: build
+
+build:
+	$(PYTHON) setup.py build_ext --inplace
+
+debug:
+	DEBUG_IMMUTABLES=1 $(PYTHON) setup.py build_ext --inplace
+
+test:
+	$(PYTHON) -m pytest -v
+
+rtest:
+	~/dev/venvs/36-debug/bin/python setup.py build_ext --inplace
+
+	env PYTHONPATH=. \
+		~/dev/venvs/36-debug/bin/python -m test.regrtest -R3:3 --testdir tests/
+
+clean:
+	find . -name '*.pyc' | xargs rm -f
+	find . -name '*.so' | xargs rm -f
+	rm -rf ./build
+	rm -rf ./dist
+	rm -rf ./*.egg-info
+
+testinstalled:
+	cd /tmp && $(PYTHON) $(ROOT)/tests/__init__.py
diff --git a/NOTICE b/NOTICE
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index 0000000..8a796ab
--- /dev/null
+++ b/NOTICE
@@ -0,0 +1 @@
+Copyright 2018-present Contributors to the immutables project.
diff --git a/README.rst b/README.rst
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--- /dev/null
+++ b/README.rst
@@ -0,0 +1,123 @@
+immutables
+==========
+
+.. image:: https://github.com/MagicStack/immutables/workflows/Tests/badge.svg?branch=master
+    :target: https://github.com/MagicStack/immutables/actions?query=workflow%3ATests+branch%3Amaster+event%3Apush
+
+.. image:: https://img.shields.io/pypi/v/immutables.svg
+    :target: https://pypi.python.org/pypi/immutables
+
+An immutable mapping type for Python.
+
+The underlying datastructure is a Hash Array Mapped Trie (HAMT)
+used in Clojure, Scala, Haskell, and other functional languages.
+This implementation is used in CPython 3.7 in the ``contextvars``
+module (see `PEP 550 <https://www.python.org/dev/peps/pep-0550/>`_ and
+`PEP 567 <https://www.python.org/dev/peps/pep-0567/>`_ for more details).
+
+Immutable mappings based on HAMT have O(log N) performance for both
+``set()`` and ``get()`` operations, which is essentially O(1) for
+relatively small mappings.
+
+Below is a visualization of a simple get/set benchmark comparing
+HAMT to an immutable mapping implemented with a Python dict
+copy-on-write approach (the benchmark code is available
+`here <https://gist.github.com/1st1/292e3f0bbe43bd65ff3256f80aa2637d>`_):
+
+.. image:: bench.png
+
+
+Installation
+------------
+
+``immutables`` requires Python 3.6+ and is available on PyPI::
+
+    $ pip install immutables
+
+
+API
+---
+
+``immutables.Map`` is an unordered immutable mapping.  ``Map`` objects
+are hashable, comparable, and pickleable.
+
+The ``Map`` object implements the ``collections.abc.Mapping`` ABC
+so working with it is very similar to working with Python dicts:
+
+.. code-block:: python
+
+    import immutables
+
+    map = immutables.Map(a=1, b=2)
+
+    print(map['a'])
+    # will print '1'
+
+    print(map.get('z', 100))
+    # will print '100'
+
+    print('z' in map)
+    # will print 'False'
+
+Since Maps are immutable, there is a special API for mutations that
+allow apply changes to the Map object and create new (derived) Maps:
+
+.. code-block:: python
+
+    map2 = map.set('a', 10)
+    print(map, map2)
+    # will print:
+    #   <immutables.Map({'a': 1, 'b': 2})>
+    #   <immutables.Map({'a': 10, 'b': 2})>
+
+    map3 = map2.delete('b')
+    print(map, map2, map3)
+    # will print:
+    #   <immutables.Map({'a': 1, 'b': 2})>
+    #   <immutables.Map({'a': 10, 'b': 2})>
+    #   <immutables.Map({'a': 10})>
+
+Maps also implement APIs for bulk updates: ``MapMutation`` objects:
+
+.. code-block:: python
+
+    map_mutation = map.mutate()
+    map_mutation['a'] = 100
+    del map_mutation['b']
+    map_mutation.set('y', 'y')
+
+    map2 = map_mutation.finish()
+
+    print(map, map2)
+    # will print:
+    #   <immutables.Map({'a': 1, 'b': 2})>
+    #   <immutables.Map({'a': 100, 'y': 'y'})>
+
+``MapMutation`` objects are context managers.  Here's the above example
+rewritten in a more idiomatic way:
+
+.. code-block:: python
+
+    with map.mutate() as mm:
+        mm['a'] = 100
+        del mm['b']
+        mm.set('y', 'y')
+        map2 = mm.finish()
+
+    print(map, map2)
+    # will print:
+    #   <immutables.Map({'a': 1, 'b': 2})>
+    #   <immutables.Map({'a': 100, 'y': 'y'})>
+
+
+Further development
+-------------------
+
+* An immutable version of Python ``set`` type with efficient
+  ``add()`` and ``discard()`` operations.
+
+
+License
+-------
+
+Apache 2.0
diff --git a/bench.png b/bench.png
new file mode 100644
index 0000000..e77ff5d
--- /dev/null
+++ b/bench.png
diff --git a/immutables/__init__.py b/immutables/__init__.py
new file mode 100644
index 0000000..b8565b0
--- /dev/null
+++ b/immutables/__init__.py
@@ -0,0 +1,25 @@
+# flake8: noqa
+
+import sys
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from ._map import Map
+else:
+    try:
+        from ._map import Map
+    except ImportError:
+        from .map import Map
+    else:
+        import collections.abc as _abc
+        _abc.Mapping.register(Map)
+
+from ._protocols import MapKeys as MapKeys
+from ._protocols import MapValues as MapValues
+from ._protocols import MapItems as MapItems
+from ._protocols import MapMutation as MapMutation
+
+from ._version import __version__
+
+__all__ = 'Map',
diff --git a/immutables/_map.c b/immutables/_map.c
new file mode 100644
index 0000000..7e510fd
--- /dev/null
+++ b/immutables/_map.c
@@ -0,0 +1,4175 @@
+#include <stddef.h> /* For offsetof */
+#include "pythoncapi_compat.h"
+#include "_map.h"
+
+
+/*
+This file provides an implemention of an immutable mapping using the
+Hash Array Mapped Trie (or HAMT) datastructure.
+
+This design allows to have:
+
+1. Efficient copy: immutable mappings can be copied by reference,
+   making it an O(1) operation.
+
+2. Efficient mutations: due to structural sharing, only a portion of
+   the trie needs to be copied when the collection is mutated.  The
+   cost of set/delete operations is O(log N).
+
+3. Efficient lookups: O(log N).
+
+(where N is number of key/value items in the immutable mapping.)
+
+
+HAMT
+====
+
+The core idea of HAMT is that the shape of the trie is encoded into the
+hashes of keys.
+
+Say we want to store a K/V pair in our mapping.  First, we calculate the
+hash of K, let's say it's 19830128, or in binary:
+
+    0b1001011101001010101110000 = 19830128
+
+Now let's partition this bit representation of the hash into blocks of
+5 bits each:
+
+    0b00_00000_10010_11101_00101_01011_10000 = 19830128
+          (6)   (5)   (4)   (3)   (2)   (1)
+
+Each block of 5 bits represents a number between 0 and 31.  So if we have
+a tree that consists of nodes, each of which is an array of 32 pointers,
+those 5-bit blocks will encode a position on a single tree level.
+
+For example, storing the key K with hash 19830128, results in the following
+tree structure:
+
+                     (array of 32 pointers)
+                     +---+ -- +----+----+----+ -- +----+
+  root node          | 0 | .. | 15 | 16 | 17 | .. | 31 |   0b10000 = 16 (1)
+  (level 1)          +---+ -- +----+----+----+ -- +----+
+                                      |
+                     +---+ -- +----+----+----+ -- +----+
+  a 2nd level node   | 0 | .. | 10 | 11 | 12 | .. | 31 |   0b01011 = 11 (2)
+                     +---+ -- +----+----+----+ -- +----+
+                                      |
+                     +---+ -- +----+----+----+ -- +----+
+  a 3rd level node   | 0 | .. | 04 | 05 | 06 | .. | 31 |   0b00101 = 5  (3)
+                     +---+ -- +----+----+----+ -- +----+
+                                      |
+                     +---+ -- +----+----+----+----+
+  a 4th level node   | 0 | .. | 04 | 29 | 30 | 31 |        0b11101 = 29 (4)
+                     +---+ -- +----+----+----+----+
+                                      |
+                     +---+ -- +----+----+----+ -- +----+
+  a 5th level node   | 0 | .. | 17 | 18 | 19 | .. | 31 |   0b10010 = 18 (5)
+                     +---+ -- +----+----+----+ -- +----+
+                                      |
+                       +--------------+
+                       |
+                     +---+ -- +----+----+----+ -- +----+
+  a 6th level node   | 0 | .. | 15 | 16 | 17 | .. | 31 |   0b00000 = 0  (6)
+                     +---+ -- +----+----+----+ -- +----+
+                       |
+                       V -- our value (or collision)
+
+To rehash: for a K/V pair, the hash of K encodes where in the tree V will
+be stored.
+
+To optimize memory footprint and handle hash collisions, our implementation
+uses three different types of nodes:
+
+ * A Bitmap node;
+ * An Array node;
+ * A Collision node.
+
+Because we implement an immutable dictionary, our nodes are also
+immutable.  Therefore, when we need to modify a node, we copy it, and
+do that modification to the copy.
+
+
+Array Nodes
+-----------
+
+These nodes are very simple.  Essentially they are arrays of 32 pointers
+we used to illustrate the high-level idea in the previous section.
+
+We use Array nodes only when we need to store more than 16 pointers
+in a single node.
+
+Array nodes do not store key objects or value objects.  They are used
+only as an indirection level - their pointers point to other nodes in
+the tree.
+
+
+Bitmap Node
+-----------
+
+Allocating a new 32-pointers array for every node of our tree would be
+very expensive.  Unless we store millions of keys, most of tree nodes would
+be very sparse.
+
+When we have less than 16 elements in a node, we don't want to use the
+Array node, that would mean that we waste a lot of memory.  Instead,
+we can use bitmap compression and can have just as many pointers
+as we need!
+
+Bitmap nodes consist of two fields:
+
+1. An array of pointers.  If a Bitmap node holds N elements, the
+   array will be of N pointers.
+
+2. A 32bit integer -- a bitmap field.  If an N-th bit is set in the
+   bitmap, it means that the node has an N-th element.
+
+For example, say we need to store a 3 elements sparse array:
+
+   +---+  --  +---+  --  +----+  --  +----+
+   | 0 |  ..  | 4 |  ..  | 11 |  ..  | 17 |
+   +---+  --  +---+  --  +----+  --  +----+
+                |          |           |
+                o1         o2          o3
+
+We allocate a three-pointer Bitmap node.  Its bitmap field will be
+then set to:
+
+   0b_00100_00010_00000_10000 == (1 << 17) | (1 << 11) | (1 << 4)
+
+To check if our Bitmap node has an I-th element we can do:
+
+   bitmap & (1 << I)
+
+
+And here's a formula to calculate a position in our pointer array
+which would correspond to an I-th element:
+
+   popcount(bitmap & ((1 << I) - 1))
+
+
+Let's break it down:
+
+ * `popcount` is a function that returns a number of bits set to 1;
+
+ * `((1 << I) - 1)` is a mask to filter the bitmask to contain bits
+   set to the *right* of our bit.
+
+
+So for our 17, 11, and 4 indexes:
+
+ * bitmap & ((1 << 17) - 1) == 0b100000010000 => 2 bits are set => index is 2.
+
+ * bitmap & ((1 << 11) - 1) == 0b10000 => 1 bit is set => index is 1.
+
+ * bitmap & ((1 << 4) - 1) == 0b0 => 0 bits are set => index is 0.
+
+
+To conclude: Bitmap nodes are just like Array nodes -- they can store
+a number of pointers, but use bitmap compression to eliminate unused
+pointers.
+
+
+Bitmap nodes have two pointers for each item:
+
+  +----+----+----+----+  --  +----+----+
+  | k1 | v1 | k2 | v2 |  ..  | kN | vN |
+  +----+----+----+----+  --  +----+----+
+
+When kI == NULL, vI points to another tree level.
+
+When kI != NULL, the actual key object is stored in kI, and its
+value is stored in vI.
+
+
+Collision Nodes
+---------------
+
+Collision nodes are simple arrays of pointers -- two pointers per
+key/value.  When there's a hash collision, say for k1/v1 and k2/v2
+we have `hash(k1)==hash(k2)`.  Then our collision node will be:
+
+  +----+----+----+----+
+  | k1 | v1 | k2 | v2 |
+  +----+----+----+----+
+
+
+Tree Structure
+--------------
+
+All nodes are PyObjects.
+
+The `MapObject` object has a pointer to the root node (h_root),
+and has a length field (h_count).
+
+High-level functions accept a MapObject object and dispatch to
+lower-level functions depending on what kind of node h_root points to.
+
+
+Operations
+==========
+
+There are three fundamental operations on an immutable dictionary:
+
+1. "o.assoc(k, v)" will return a new immutable dictionary, that will be
+   a copy of "o", but with the "k/v" item set.
+
+   Functions in this file:
+
+        map_node_assoc, map_node_bitmap_assoc,
+        map_node_array_assoc, map_node_collision_assoc
+
+   `map_node_assoc` function accepts a node object, and calls
+   other functions depending on its actual type.
+
+2. "o.find(k)" will lookup key "k" in "o".
+
+   Functions:
+
+        map_node_find, map_node_bitmap_find,
+        map_node_array_find, map_node_collision_find
+
+3. "o.without(k)" will return a new immutable dictionary, that will be
+   a copy of "o", buth without the "k" key.
+
+   Functions:
+
+        map_node_without, map_node_bitmap_without,
+        map_node_array_without, map_node_collision_without
+
+
+Further Reading
+===============
+
+1. http://blog.higher-order.net/2009/09/08/understanding-clojures-persistenthashmap-deftwice.html
+
+2. http://blog.higher-order.net/2010/08/16/assoc-and-clojures-persistenthashmap-part-ii.html
+
+3. Clojure's PersistentHashMap implementation:
+   https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/PersistentHashMap.java
+*/
+
+
+#define IS_ARRAY_NODE(node)     (Py_TYPE(node) == &_Map_ArrayNode_Type)
+#define IS_BITMAP_NODE(node)    (Py_TYPE(node) == &_Map_BitmapNode_Type)
+#define IS_COLLISION_NODE(node) (Py_TYPE(node) == &_Map_CollisionNode_Type)
+
+
+/* Return type for 'find' (lookup a key) functions.
+
+   * F_ERROR - an error occurred;
+   * F_NOT_FOUND - the key was not found;
+   * F_FOUND - the key was found.
+*/
+typedef enum {F_ERROR, F_NOT_FOUND, F_FOUND} map_find_t;
+
+
+/* Return type for 'without' (delete a key) functions.
+
+   * W_ERROR - an error occurred;
+   * W_NOT_FOUND - the key was not found: there's nothing to delete;
+   * W_EMPTY - the key was found: the node/tree would be empty
+     if the key is deleted;
+   * W_NEWNODE - the key was found: a new node/tree is returned
+     without that key.
+*/
+typedef enum {W_ERROR, W_NOT_FOUND, W_EMPTY, W_NEWNODE} map_without_t;
+
+
+/* Low-level iterator protocol type.
+
+   * I_ITEM - a new item has been yielded;
+   * I_END - the whole tree was visited (similar to StopIteration).
+*/
+typedef enum {I_ITEM, I_END} map_iter_t;
+
+
+#define HAMT_ARRAY_NODE_SIZE 32
+
+
+typedef struct {
+    PyObject_HEAD
+    MapNode *a_array[HAMT_ARRAY_NODE_SIZE];
+    Py_ssize_t a_count;
+    uint64_t a_mutid;
+} MapNode_Array;
+
+
+typedef struct {
+    PyObject_VAR_HEAD
+    uint64_t b_mutid;
+    uint32_t b_bitmap;
+    PyObject *b_array[1];
+} MapNode_Bitmap;
+
+
+typedef struct {
+    PyObject_VAR_HEAD
+    uint64_t c_mutid;
+    int32_t c_hash;
+    PyObject *c_array[1];
+} MapNode_Collision;
+
+
+static volatile uint64_t mutid_counter = 1;
+
+static MapNode_Bitmap *_empty_bitmap_node;
+
+
+/* Create a new HAMT immutable mapping. */
+static MapObject *
+map_new(void);
+
+/* Return a new collection based on "o", but with an additional
+   key/val pair. */
+static MapObject *
+map_assoc(MapObject *o, PyObject *key, PyObject *val);
+
+/* Return a new collection based on "o", but without "key". */
+static MapObject *
+map_without(MapObject *o, PyObject *key);
+
+/* Check if "v" is equal to "w".
+
+   Return:
+   - 0: v != w
+   - 1: v == w
+   - -1: An error occurred.
+*/
+static int
+map_eq(BaseMapObject *v, BaseMapObject *w);
+
+static map_find_t
+map_find(BaseMapObject *o, PyObject *key, PyObject **val);
+
+/* Return the size of "o"; equivalent of "len(o)". */
+static Py_ssize_t
+map_len(BaseMapObject *o);
+
+
+static MapObject *
+map_alloc(void);
+
+static MapNode *
+map_node_assoc(MapNode *node,
+               uint32_t shift, int32_t hash,
+               PyObject *key, PyObject *val, int* added_leaf,
+               uint64_t mutid);
+
+static map_without_t
+map_node_without(MapNode *node,
+                 uint32_t shift, int32_t hash,
+                 PyObject *key,
+                 MapNode **new_node,
+                 uint64_t mutid);
+
+static map_find_t
+map_node_find(MapNode *node,
+              uint32_t shift, int32_t hash,
+              PyObject *key, PyObject **val);
+
+static int
+map_node_dump(MapNode *node,
+              _PyUnicodeWriter *writer, int level);
+
+static MapNode *
+map_node_array_new(Py_ssize_t, uint64_t mutid);
+
+static MapNode *
+map_node_collision_new(int32_t hash, Py_ssize_t size, uint64_t mutid);
+
+static inline Py_ssize_t
+map_node_collision_count(MapNode_Collision *node);
+
+static int
+map_node_update(uint64_t mutid,
+                PyObject *seq,
+                MapNode *root, Py_ssize_t count,
+                MapNode **new_root, Py_ssize_t *new_count);
+
+
+static int
+map_update_inplace(uint64_t mutid, BaseMapObject *o, PyObject *src);
+
+static MapObject *
+map_update(uint64_t mutid, MapObject *o, PyObject *src);
+
+
+#if !defined(NDEBUG)
+static void
+_map_node_array_validate(void *o)
+{
+    assert(IS_ARRAY_NODE(o));
+    MapNode_Array *node = (MapNode_Array*)(o);
+    assert(node->a_count <= HAMT_ARRAY_NODE_SIZE);
+    Py_ssize_t i = 0, count = 0;
+    for (; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        if (node->a_array[i] != NULL) {
+            count++;
+        }
+    }
+    assert(count == node->a_count);
+}
+
+#define VALIDATE_ARRAY_NODE(NODE) \
+    do { _map_node_array_validate(NODE); } while (0);
+#else
+#define VALIDATE_ARRAY_NODE(NODE)
+#endif
+
+
+/* Returns -1 on error */
+static inline int32_t
+map_hash(PyObject *o)
+{
+    Py_hash_t hash = PyObject_Hash(o);
+
+#if SIZEOF_PY_HASH_T <= 4
+    return hash;
+#else
+    if (hash == -1) {
+        /* exception */
+        return -1;
+    }
+
+    /* While it's suboptimal to reduce Python's 64 bit hash to
+       32 bits via XOR, it seems that the resulting hash function
+       is good enough (this is also how Long type is hashed in Java.)
+       Storing 10, 100, 1000 Python strings results in a relatively
+       shallow and uniform tree structure.
+
+       Please don't change this hashing algorithm, as there are many
+       tests that test some exact tree shape to cover all code paths.
+    */
+    int32_t xored = (int32_t)(hash & 0xffffffffl) ^ (int32_t)(hash >> 32);
+    return xored == -1 ? -2 : xored;
+#endif
+}
+
+static inline uint32_t
+map_mask(int32_t hash, uint32_t shift)
+{
+    return (((uint32_t)hash >> shift) & 0x01f);
+}
+
+static inline uint32_t
+map_bitpos(int32_t hash, uint32_t shift)
+{
+    return (uint32_t)1 << map_mask(hash, shift);
+}
+
+static inline uint32_t
+map_bitcount(uint32_t i)
+{
+    /* We could use native popcount instruction but that would
+       require to either add configure flags to enable SSE4.2
+       support or to detect it dynamically.  Otherwise, we have
+       a risk of CPython not working properly on older hardware.
+
+       In practice, there's no observable difference in
+       performance between using a popcount instruction or the
+       following fallback code.
+
+       The algorithm is copied from:
+       https://graphics.stanford.edu/~seander/bithacks.html
+    */
+    i = i - ((i >> 1) & 0x55555555);
+    i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
+    return (((i + (i >> 4)) & 0xF0F0F0F) * 0x1010101) >> 24;
+}
+
+static inline uint32_t
+map_bitindex(uint32_t bitmap, uint32_t bit)
+{
+    return map_bitcount(bitmap & (bit - 1));
+}
+
+
+/////////////////////////////////// Dump Helpers
+
+static int
+_map_dump_ident(_PyUnicodeWriter *writer, int level)
+{
+    /* Write `'    ' * level` to the `writer` */
+    PyObject *str = NULL;
+    PyObject *num = NULL;
+    PyObject *res = NULL;
+    int ret = -1;
+
+    str = PyUnicode_FromString("    ");
+    if (str == NULL) {
+        goto error;
+    }
+
+    num = PyLong_FromLong((long)level);
+    if (num == NULL) {
+        goto error;
+    }
+
+    res = PyNumber_Multiply(str, num);
+    if (res == NULL) {
+        goto error;
+    }
+
+    ret = _PyUnicodeWriter_WriteStr(writer, res);
+
+error:
+    Py_XDECREF(res);
+    Py_XDECREF(str);
+    Py_XDECREF(num);
+    return ret;
+}
+
+static int
+_map_dump_format(_PyUnicodeWriter *writer, const char *format, ...)
+{
+    /* A convenient helper combining _PyUnicodeWriter_WriteStr and
+       PyUnicode_FromFormatV.
+    */
+    PyObject* msg;
+    int ret;
+
+    va_list vargs;
+#ifdef HAVE_STDARG_PROTOTYPES
+    va_start(vargs, format);
+#else
+    va_start(vargs);
+#endif
+    msg = PyUnicode_FromFormatV(format, vargs);
+    va_end(vargs);
+
+    if (msg == NULL) {
+        return -1;
+    }
+
+    ret = _PyUnicodeWriter_WriteStr(writer, msg);
+    Py_DECREF(msg);
+    return ret;
+}
+
+/////////////////////////////////// Bitmap Node
+
+
+static MapNode *
+map_node_bitmap_new(Py_ssize_t size, uint64_t mutid)
+{
+    /* Create a new bitmap node of size 'size' */
+
+    MapNode_Bitmap *node;
+    Py_ssize_t i;
+
+    assert(size >= 0);
+    assert(size % 2 == 0);
+
+    if (size == 0 && _empty_bitmap_node != NULL && mutid == 0) {
+        Py_INCREF(_empty_bitmap_node);
+        return (MapNode *)_empty_bitmap_node;
+    }
+
+    /* No freelist; allocate a new bitmap node */
+    node = PyObject_GC_NewVar(
+        MapNode_Bitmap, &_Map_BitmapNode_Type, size);
+    if (node == NULL) {
+        return NULL;
+    }
+
+    Py_SET_SIZE(node, size);
+
+    for (i = 0; i < size; i++) {
+        node->b_array[i] = NULL;
+    }
+
+    node->b_bitmap = 0;
+    node->b_mutid = mutid;
+
+    PyObject_GC_Track(node);
+
+    if (size == 0 && _empty_bitmap_node == NULL && mutid == 0) {
+        /* Since bitmap nodes are immutable, we can cache the instance
+           for size=0 and reuse it whenever we need an empty bitmap node.
+        */
+        _empty_bitmap_node = node;
+        Py_INCREF(_empty_bitmap_node);
+    }
+
+    return (MapNode *)node;
+}
+
+static inline Py_ssize_t
+map_node_bitmap_count(MapNode_Bitmap *node)
+{
+    return Py_SIZE(node) / 2;
+}
+
+static MapNode_Bitmap *
+map_node_bitmap_clone(MapNode_Bitmap *node, uint64_t mutid)
+{
+    /* Clone a bitmap node; return a new one with the same child notes. */
+
+    MapNode_Bitmap *clone;
+    Py_ssize_t i;
+
+    clone = (MapNode_Bitmap *)map_node_bitmap_new(
+        Py_SIZE(node), mutid);
+    if (clone == NULL) {
+        return NULL;
+    }
+
+    for (i = 0; i < Py_SIZE(node); i++) {
+        Py_XINCREF(node->b_array[i]);
+        clone->b_array[i] = node->b_array[i];
+    }
+
+    clone->b_bitmap = node->b_bitmap;
+    return clone;
+}
+
+static MapNode_Bitmap *
+map_node_bitmap_clone_without(MapNode_Bitmap *o, uint32_t bit, uint64_t mutid)
+{
+    assert(bit & o->b_bitmap);
+    assert(map_node_bitmap_count(o) > 1);
+
+    MapNode_Bitmap *new = (MapNode_Bitmap *)map_node_bitmap_new(
+        Py_SIZE(o) - 2, mutid);
+    if (new == NULL) {
+        return NULL;
+    }
+
+    uint32_t idx = map_bitindex(o->b_bitmap, bit);
+    uint32_t key_idx = 2 * idx;
+    uint32_t val_idx = key_idx + 1;
+    uint32_t i;
+
+    for (i = 0; i < key_idx; i++) {
+        Py_XINCREF(o->b_array[i]);
+        new->b_array[i] = o->b_array[i];
+    }
+
+    assert(Py_SIZE(o) >= 0 && Py_SIZE(o) <= 32);
+    for (i = val_idx + 1; i < (uint32_t)Py_SIZE(o); i++) {
+        Py_XINCREF(o->b_array[i]);
+        new->b_array[i - 2] = o->b_array[i];
+    }
+
+    new->b_bitmap = o->b_bitmap & ~bit;
+    return new;
+}
+
+static MapNode *
+map_node_new_bitmap_or_collision(uint32_t shift,
+                                 PyObject *key1, PyObject *val1,
+                                 int32_t key2_hash,
+                                 PyObject *key2, PyObject *val2,
+                                 uint64_t mutid)
+{
+    /* Helper method.  Creates a new node for key1/val and key2/val2
+       pairs.
+
+       If key1 hash is equal to the hash of key2, a Collision node
+       will be created.  If they are not equal, a Bitmap node is
+       created.
+    */
+
+    int32_t key1_hash = map_hash(key1);
+    if (key1_hash == -1) {
+        return NULL;
+    }
+
+    if (key1_hash == key2_hash) {
+        MapNode_Collision *n;
+        n = (MapNode_Collision *)map_node_collision_new(key1_hash, 4, mutid);
+        if (n == NULL) {
+            return NULL;
+        }
+
+        Py_INCREF(key1);
+        n->c_array[0] = key1;
+        Py_INCREF(val1);
+        n->c_array[1] = val1;
+
+        Py_INCREF(key2);
+        n->c_array[2] = key2;
+        Py_INCREF(val2);
+        n->c_array[3] = val2;
+
+        return (MapNode *)n;
+    }
+    else {
+        int added_leaf = 0;
+        MapNode *n = map_node_bitmap_new(0, mutid);
+        if (n == NULL) {
+            return NULL;
+        }
+
+        MapNode *n2 = map_node_assoc(
+            n, shift, key1_hash, key1, val1, &added_leaf, mutid);
+        Py_DECREF(n);
+        if (n2 == NULL) {
+            return NULL;
+        }
+
+        n = map_node_assoc(
+            n2, shift, key2_hash, key2, val2, &added_leaf, mutid);
+        Py_DECREF(n2);
+        if (n == NULL) {
+            return NULL;
+        }
+
+        return n;
+    }
+}
+
+static MapNode *
+map_node_bitmap_assoc(MapNode_Bitmap *self,
+                      uint32_t shift, int32_t hash,
+                      PyObject *key, PyObject *val, int* added_leaf,
+                      uint64_t mutid)
+{
+    /* assoc operation for bitmap nodes.
+
+       Return: a new node, or self if key/val already is in the
+       collection.
+
+       'added_leaf' is later used in 'map_assoc' to determine if
+       `map.set(key, val)` increased the size of the collection.
+    */
+
+    uint32_t bit = map_bitpos(hash, shift);
+    uint32_t idx = map_bitindex(self->b_bitmap, bit);
+
+    /* Bitmap node layout:
+
+    +------+------+------+------+  ---  +------+------+
+    | key1 | val1 | key2 | val2 |  ...  | keyN | valN |
+    +------+------+------+------+  ---  +------+------+
+    where `N < Py_SIZE(node)`.
+
+    The `node->b_bitmap` field is a bitmap.  For a given
+    `(shift, hash)` pair we can determine:
+
+     - If this node has the corresponding key/val slots.
+     - The index of key/val slots.
+    */
+
+    if (self->b_bitmap & bit) {
+        /* The key is set in this node */
+
+        uint32_t key_idx = 2 * idx;
+        uint32_t val_idx = key_idx + 1;
+
+        assert(val_idx < (size_t)Py_SIZE(self));
+
+        PyObject *key_or_null = self->b_array[key_idx];
+        PyObject *val_or_node = self->b_array[val_idx];
+
+        if (key_or_null == NULL) {
+            /* key is NULL.  This means that we have a few keys
+               that have the same (hash, shift) pair. */
+
+            assert(val_or_node != NULL);
+
+            MapNode *sub_node = map_node_assoc(
+                (MapNode *)val_or_node,
+                shift + 5, hash, key, val, added_leaf,
+                mutid);
+            if (sub_node == NULL) {
+                return NULL;
+            }
+
+            if (val_or_node == (PyObject *)sub_node) {
+                Py_DECREF(sub_node);
+                Py_INCREF(self);
+                return (MapNode *)self;
+            }
+
+            if (mutid != 0 && self->b_mutid == mutid) {
+                Py_SETREF(self->b_array[val_idx], (PyObject*)sub_node);
+                Py_INCREF(self);
+                return (MapNode *)self;
+            }
+            else {
+                MapNode_Bitmap *ret = map_node_bitmap_clone(self, mutid);
+                if (ret == NULL) {
+                    return NULL;
+                }
+                Py_SETREF(ret->b_array[val_idx], (PyObject*)sub_node);
+                return (MapNode *)ret;
+            }
+        }
+
+        assert(key != NULL);
+        /* key is not NULL.  This means that we have only one other
+           key in this collection that matches our hash for this shift. */
+
+        int comp_err = PyObject_RichCompareBool(key, key_or_null, Py_EQ);
+        if (comp_err < 0) {  /* exception in __eq__ */
+            return NULL;
+        }
+        if (comp_err == 1) {  /* key == key_or_null */
+            if (val == val_or_node) {
+                /* we already have the same key/val pair; return self. */
+                Py_INCREF(self);
+                return (MapNode *)self;
+            }
+
+            /* We're setting a new value for the key we had before. */
+            if (mutid != 0 && self->b_mutid == mutid) {
+                /* We've been mutating this node before: update inplace. */
+                Py_INCREF(val);
+                Py_SETREF(self->b_array[val_idx], val);
+                Py_INCREF(self);
+                return (MapNode *)self;
+            }
+            else {
+                /* Make a new bitmap node with a replaced value,
+                   and return it. */
+                MapNode_Bitmap *ret = map_node_bitmap_clone(self, mutid);
+                if (ret == NULL) {
+                    return NULL;
+                }
+                Py_INCREF(val);
+                Py_SETREF(ret->b_array[val_idx], val);
+                return (MapNode *)ret;
+            }
+        }
+
+        /* It's a new key, and it has the same index as *one* another key.
+           We have a collision.  We need to create a new node which will
+           combine the existing key and the key we're adding.
+
+           `map_node_new_bitmap_or_collision` will either create a new
+           Collision node if the keys have identical hashes, or
+           a new Bitmap node.
+        */
+        MapNode *sub_node = map_node_new_bitmap_or_collision(
+            shift + 5,
+            key_or_null, val_or_node,  /* existing key/val */
+            hash,
+            key, val,  /* new key/val */
+            self->b_mutid
+        );
+        if (sub_node == NULL) {
+            return NULL;
+        }
+
+        if (mutid != 0 && self->b_mutid == mutid) {
+            Py_SETREF(self->b_array[key_idx], NULL);
+            Py_SETREF(self->b_array[val_idx], (PyObject *)sub_node);
+            Py_INCREF(self);
+
+            *added_leaf = 1;
+            return (MapNode *)self;
+        }
+        else {
+            MapNode_Bitmap *ret = map_node_bitmap_clone(self, mutid);
+            if (ret == NULL) {
+                Py_DECREF(sub_node);
+                return NULL;
+            }
+            Py_SETREF(ret->b_array[key_idx], NULL);
+            Py_SETREF(ret->b_array[val_idx], (PyObject *)sub_node);
+
+            *added_leaf = 1;
+            return (MapNode *)ret;
+        }
+    }
+    else {
+        /* There was no key before with the same (shift,hash). */
+
+        uint32_t n = map_bitcount(self->b_bitmap);
+
+        if (n >= 16) {
+            /* When we have a situation where we want to store more
+               than 16 nodes at one level of the tree, we no longer
+               want to use the Bitmap node with bitmap encoding.
+
+               Instead we start using an Array node, which has
+               simpler (faster) implementation at the expense of
+               having prealocated 32 pointers for its keys/values
+               pairs.
+
+               Small map objects (<30 keys) usually don't have any
+               Array nodes at all.  Between ~30 and ~400 keys map
+               objects usually have one Array node, and usually it's
+               a root node.
+            */
+
+            uint32_t jdx = map_mask(hash, shift);
+            /* 'jdx' is the index of where the new key should be added
+               in the new Array node we're about to create. */
+
+            MapNode *empty = NULL;
+            MapNode_Array *new_node = NULL;
+            MapNode *res = NULL;
+
+            /* Create a new Array node. */
+            new_node = (MapNode_Array *)map_node_array_new(n + 1, mutid);
+            if (new_node == NULL) {
+                goto fin;
+            }
+
+            /* Create an empty bitmap node for the next
+               map_node_assoc call. */
+            empty = map_node_bitmap_new(0, mutid);
+            if (empty == NULL) {
+                goto fin;
+            }
+
+            /* Make a new bitmap node for the key/val we're adding.
+               Set that bitmap node to new-array-node[jdx]. */
+            new_node->a_array[jdx] = map_node_assoc(
+                empty, shift + 5, hash, key, val, added_leaf, mutid);
+            if (new_node->a_array[jdx] == NULL) {
+                goto fin;
+            }
+
+            /* Copy existing key/value pairs from the current Bitmap
+               node to the new Array node we've just created. */
+            Py_ssize_t i, j;
+            for (i = 0, j = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+                if (((self->b_bitmap >> i) & 1) != 0) {
+                    /* Ensure we don't accidentally override `jdx` element
+                       we set few lines above.
+                    */
+                    assert(new_node->a_array[i] == NULL);
+
+                    if (self->b_array[j] == NULL) {
+                        new_node->a_array[i] =
+                            (MapNode *)self->b_array[j + 1];
+                        Py_INCREF(new_node->a_array[i]);
+                    }
+                    else {
+                        int32_t rehash = map_hash(self->b_array[j]);
+                        if (rehash == -1) {
+                            goto fin;
+                        }
+
+                        new_node->a_array[i] = map_node_assoc(
+                            empty, shift + 5,
+                            rehash,
+                            self->b_array[j],
+                            self->b_array[j + 1],
+                            added_leaf,
+                            mutid);
+
+                        if (new_node->a_array[i] == NULL) {
+                            goto fin;
+                        }
+                    }
+                    j += 2;
+                }
+            }
+
+            VALIDATE_ARRAY_NODE(new_node)
+
+            /* That's it! */
+            res = (MapNode *)new_node;
+
+        fin:
+            Py_XDECREF(empty);
+            if (res == NULL) {
+                Py_XDECREF(new_node);
+            }
+            return res;
+        }
+        else {
+            /* We have less than 16 keys at this level; let's just
+               create a new bitmap node out of this node with the
+               new key/val pair added. */
+
+            uint32_t key_idx = 2 * idx;
+            uint32_t val_idx = key_idx + 1;
+            uint32_t i;
+
+            *added_leaf = 1;
+
+            /* Allocate new Bitmap node which can have one more key/val
+               pair in addition to what we have already. */
+            MapNode_Bitmap *new_node =
+                (MapNode_Bitmap *)map_node_bitmap_new(2 * (n + 1), mutid);
+            if (new_node == NULL) {
+                return NULL;
+            }
+
+            /* Copy all keys/values that will be before the new key/value
+               we are adding. */
+            for (i = 0; i < key_idx; i++) {
+                Py_XINCREF(self->b_array[i]);
+                new_node->b_array[i] = self->b_array[i];
+            }
+
+            /* Set the new key/value to the new Bitmap node. */
+            Py_INCREF(key);
+            new_node->b_array[key_idx] = key;
+            Py_INCREF(val);
+            new_node->b_array[val_idx] = val;
+
+            /* Copy all keys/values that will be after the new key/value
+               we are adding. */
+            assert(Py_SIZE(self) >= 0 && Py_SIZE(self) <= 32);
+            for (i = key_idx; i < (uint32_t)Py_SIZE(self); i++) {
+                Py_XINCREF(self->b_array[i]);
+                new_node->b_array[i + 2] = self->b_array[i];
+            }
+
+            new_node->b_bitmap = self->b_bitmap | bit;
+            return (MapNode *)new_node;
+        }
+    }
+}
+
+static map_without_t
+map_node_bitmap_without(MapNode_Bitmap *self,
+                        uint32_t shift, int32_t hash,
+                        PyObject *key,
+                        MapNode **new_node,
+                        uint64_t mutid)
+{
+    uint32_t bit = map_bitpos(hash, shift);
+    if ((self->b_bitmap & bit) == 0) {
+        return W_NOT_FOUND;
+    }
+
+    uint32_t idx = map_bitindex(self->b_bitmap, bit);
+
+    uint32_t key_idx = 2 * idx;
+    uint32_t val_idx = key_idx + 1;
+
+    PyObject *key_or_null = self->b_array[key_idx];
+    PyObject *val_or_node = self->b_array[val_idx];
+
+    if (key_or_null == NULL) {
+        /* key == NULL means that 'value' is another tree node. */
+
+        MapNode *sub_node = NULL;
+        MapNode_Bitmap *target = NULL;
+
+        map_without_t res = map_node_without(
+            (MapNode *)val_or_node,
+            shift + 5, hash, key, &sub_node,
+            mutid);
+
+        switch (res) {
+            case W_EMPTY:
+                /* It's impossible for us to receive a W_EMPTY here:
+
+                    - Array nodes are converted to Bitmap nodes when
+                      we delete 16th item from them;
+
+                    - Collision nodes are converted to Bitmap when
+                      there is one item in them;
+
+                    - Bitmap node's without() inlines single-item
+                      sub-nodes.
+
+                   So in no situation we can have a single-item
+                   Bitmap child of another Bitmap node.
+                */
+                abort();
+
+            case W_NEWNODE: {
+                assert(sub_node != NULL);
+
+                if (IS_BITMAP_NODE(sub_node)) {
+                    MapNode_Bitmap *sub_tree = (MapNode_Bitmap *)sub_node;
+                    if (map_node_bitmap_count(sub_tree) == 1 &&
+                            sub_tree->b_array[0] != NULL)
+                    {
+                        /* A bitmap node with one key/value pair.  Just
+                           merge it into this node.
+
+                           Note that we don't inline Bitmap nodes that
+                           have a NULL key -- those nodes point to another
+                           tree level, and we cannot simply move tree levels
+                           up or down.
+                        */
+
+                        if (mutid != 0 && self->b_mutid == mutid) {
+                            target = self;
+                            Py_INCREF(target);
+                        }
+                        else {
+                            target = map_node_bitmap_clone(self, mutid);
+                            if (target == NULL) {
+                                Py_DECREF(sub_node);
+                                return W_ERROR;
+                            }
+                        }
+
+                        PyObject *key = sub_tree->b_array[0];
+                        PyObject *val = sub_tree->b_array[1];
+
+                        Py_INCREF(key);
+                        Py_XSETREF(target->b_array[key_idx], key);
+                        Py_INCREF(val);
+                        Py_SETREF(target->b_array[val_idx], val);
+
+                        Py_DECREF(sub_tree);
+
+                        *new_node = (MapNode *)target;
+                        return W_NEWNODE;
+                    }
+                }
+
+#if !defined(NDEBUG)
+                /* Ensure that Collision.without implementation
+                   converts to Bitmap nodes itself.
+                */
+                if (IS_COLLISION_NODE(sub_node)) {
+                    assert(map_node_collision_count(
+                            (MapNode_Collision*)sub_node) > 1);
+                }
+#endif
+
+                if (mutid != 0 && self->b_mutid == mutid) {
+                    target = self;
+                    Py_INCREF(target);
+                }
+                else {
+                    target = map_node_bitmap_clone(self, mutid);
+                    if (target == NULL) {
+                        return W_ERROR;
+                    }
+                }
+
+                Py_SETREF(target->b_array[val_idx],
+                          (PyObject *)sub_node);  /* borrow */
+
+                *new_node = (MapNode *)target;
+                return W_NEWNODE;
+            }
+
+            case W_ERROR:
+            case W_NOT_FOUND:
+                assert(sub_node == NULL);
+                return res;
+
+            default:
+                abort();
+        }
+    }
+    else {
+        /* We have a regular key/value pair */
+
+        int cmp = PyObject_RichCompareBool(key_or_null, key, Py_EQ);
+        if (cmp < 0) {
+            return W_ERROR;
+        }
+        if (cmp == 0) {
+            return W_NOT_FOUND;
+        }
+
+        if (map_node_bitmap_count(self) == 1) {
+            return W_EMPTY;
+        }
+
+        *new_node = (MapNode *)
+            map_node_bitmap_clone_without(self, bit, mutid);
+        if (*new_node == NULL) {
+            return W_ERROR;
+        }
+
+        return W_NEWNODE;
+    }
+}
+
+static map_find_t
+map_node_bitmap_find(MapNode_Bitmap *self,
+                     uint32_t shift, int32_t hash,
+                     PyObject *key, PyObject **val)
+{
+    /* Lookup a key in a Bitmap node. */
+
+    uint32_t bit = map_bitpos(hash, shift);
+    uint32_t idx;
+    uint32_t key_idx;
+    uint32_t val_idx;
+    PyObject *key_or_null;
+    PyObject *val_or_node;
+    int comp_err;
+
+    if ((self->b_bitmap & bit) == 0) {
+        return F_NOT_FOUND;
+    }
+
+    idx = map_bitindex(self->b_bitmap, bit);
+    key_idx = idx * 2;
+    val_idx = key_idx + 1;
+
+    assert(val_idx < (size_t)Py_SIZE(self));
+
+    key_or_null = self->b_array[key_idx];
+    val_or_node = self->b_array[val_idx];
+
+    if (key_or_null == NULL) {
+        /* There are a few keys that have the same hash at the current shift
+           that match our key.  Dispatch the lookup further down the tree. */
+        assert(val_or_node != NULL);
+        return map_node_find((MapNode *)val_or_node,
+                             shift + 5, hash, key, val);
+    }
+
+    /* We have only one key -- a potential match.  Let's compare if the
+       key we are looking at is equal to the key we are looking for. */
+    assert(key != NULL);
+    comp_err = PyObject_RichCompareBool(key, key_or_null, Py_EQ);
+    if (comp_err < 0) {  /* exception in __eq__ */
+        return F_ERROR;
+    }
+    if (comp_err == 1) {  /* key == key_or_null */
+        *val = val_or_node;
+        return F_FOUND;
+    }
+
+    return F_NOT_FOUND;
+}
+
+static int
+map_node_bitmap_traverse(MapNode_Bitmap *self, visitproc visit, void *arg)
+{
+    /* Bitmap's tp_traverse */
+
+    Py_ssize_t i;
+
+    for (i = Py_SIZE(self); --i >= 0; ) {
+        Py_VISIT(self->b_array[i]);
+    }
+
+    return 0;
+}
+
+static void
+map_node_bitmap_dealloc(MapNode_Bitmap *self)
+{
+    /* Bitmap's tp_dealloc */
+
+    Py_ssize_t len = Py_SIZE(self);
+    Py_ssize_t i;
+
+    PyObject_GC_UnTrack(self);
+    Py_TRASHCAN_SAFE_BEGIN(self)
+
+    if (len > 0) {
+        i = len;
+        while (--i >= 0) {
+            Py_XDECREF(self->b_array[i]);
+        }
+    }
+
+    Py_TYPE(self)->tp_free((PyObject *)self);
+    Py_TRASHCAN_SAFE_END(self)
+}
+
+static int
+map_node_bitmap_dump(MapNode_Bitmap *node,
+                     _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for Bitmap nodes. */
+
+    Py_ssize_t i;
+    PyObject *tmp1;
+    PyObject *tmp2;
+
+    if (_map_dump_ident(writer, level + 1)) {
+        goto error;
+    }
+
+    if (_map_dump_format(writer, "BitmapNode(size=%zd count=%zd ",
+                         Py_SIZE(node), Py_SIZE(node) / 2))
+    {
+        goto error;
+    }
+
+    tmp1 = PyLong_FromUnsignedLong(node->b_bitmap);
+    if (tmp1 == NULL) {
+        goto error;
+    }
+    tmp2 = _PyLong_Format(tmp1, 2);
+    Py_DECREF(tmp1);
+    if (tmp2 == NULL) {
+        goto error;
+    }
+    if (_map_dump_format(writer, "bitmap=%S id=%p):\n", tmp2, node)) {
+        Py_DECREF(tmp2);
+        goto error;
+    }
+    Py_DECREF(tmp2);
+
+    for (i = 0; i < Py_SIZE(node); i += 2) {
+        PyObject *key_or_null = node->b_array[i];
+        PyObject *val_or_node = node->b_array[i + 1];
+
+        if (_map_dump_ident(writer, level + 2)) {
+            goto error;
+        }
+
+        if (key_or_null == NULL) {
+            if (_map_dump_format(writer, "NULL:\n")) {
+                goto error;
+            }
+
+            if (map_node_dump((MapNode *)val_or_node,
+                              writer, level + 2))
+            {
+                goto error;
+            }
+        }
+        else {
+            if (_map_dump_format(writer, "%R: %R", key_or_null,
+                                 val_or_node))
+            {
+                goto error;
+            }
+        }
+
+        if (_map_dump_format(writer, "\n")) {
+            goto error;
+        }
+    }
+
+    return 0;
+error:
+    return -1;
+}
+
+
+/////////////////////////////////// Collision Node
+
+
+static MapNode *
+map_node_collision_new(int32_t hash, Py_ssize_t size, uint64_t mutid)
+{
+    /* Create a new Collision node. */
+
+    MapNode_Collision *node;
+    Py_ssize_t i;
+
+    assert(size >= 4);
+    assert(size % 2 == 0);
+
+    node = PyObject_GC_NewVar(
+        MapNode_Collision, &_Map_CollisionNode_Type, size);
+    if (node == NULL) {
+        return NULL;
+    }
+
+    for (i = 0; i < size; i++) {
+        node->c_array[i] = NULL;
+    }
+
+    Py_SET_SIZE(node, size);
+    node->c_hash = hash;
+
+    node->c_mutid = mutid;
+
+    PyObject_GC_Track(node);
+    return (MapNode *)node;
+}
+
+static map_find_t
+map_node_collision_find_index(MapNode_Collision *self, PyObject *key,
+                              Py_ssize_t *idx)
+{
+    /* Lookup `key` in the Collision node `self`.  Set the index of the
+       found key to 'idx'. */
+
+    Py_ssize_t i;
+    PyObject *el;
+
+    for (i = 0; i < Py_SIZE(self); i += 2) {
+        el = self->c_array[i];
+
+        assert(el != NULL);
+        int cmp = PyObject_RichCompareBool(key, el, Py_EQ);
+        if (cmp < 0) {
+            return F_ERROR;
+        }
+        if (cmp == 1) {
+            *idx = i;
+            return F_FOUND;
+        }
+    }
+
+    return F_NOT_FOUND;
+}
+
+static MapNode *
+map_node_collision_assoc(MapNode_Collision *self,
+                         uint32_t shift, int32_t hash,
+                         PyObject *key, PyObject *val, int* added_leaf,
+                         uint64_t mutid)
+{
+    /* Set a new key to this level (currently a Collision node)
+       of the tree. */
+
+    if (hash == self->c_hash) {
+        /* The hash of the 'key' we are adding matches the hash of
+           other keys in this Collision node. */
+
+        Py_ssize_t key_idx = -1;
+        map_find_t found;
+        MapNode_Collision *new_node;
+        Py_ssize_t i;
+
+        /* Let's try to lookup the new 'key', maybe we already have it. */
+        found = map_node_collision_find_index(self, key, &key_idx);
+        switch (found) {
+            case F_ERROR:
+                /* Exception. */
+                return NULL;
+
+            case F_NOT_FOUND:
+                /* This is a totally new key.  Clone the current node,
+                   add a new key/value to the cloned node. */
+
+                new_node = (MapNode_Collision *)map_node_collision_new(
+                    self->c_hash, Py_SIZE(self) + 2, mutid);
+                if (new_node == NULL) {
+                    return NULL;
+                }
+
+                for (i = 0; i < Py_SIZE(self); i++) {
+                    Py_INCREF(self->c_array[i]);
+                    new_node->c_array[i] = self->c_array[i];
+                }
+
+                Py_INCREF(key);
+                new_node->c_array[i] = key;
+                Py_INCREF(val);
+                new_node->c_array[i + 1] = val;
+
+                *added_leaf = 1;
+                return (MapNode *)new_node;
+
+            case F_FOUND:
+                /* There's a key which is equal to the key we are adding. */
+
+                assert(key_idx >= 0);
+                assert(key_idx < Py_SIZE(self));
+                Py_ssize_t val_idx = key_idx + 1;
+
+                if (self->c_array[val_idx] == val) {
+                    /* We're setting a key/value pair that's already set. */
+                    Py_INCREF(self);
+                    return (MapNode *)self;
+                }
+
+                /* We need to replace old value for the key with
+                   a new value. */
+
+                if (mutid != 0 && self->c_mutid == mutid) {
+                    new_node = self;
+                    Py_INCREF(self);
+                }
+                else {
+                    /* Create a new Collision node.*/
+                    new_node = (MapNode_Collision *)map_node_collision_new(
+                        self->c_hash, Py_SIZE(self), mutid);
+                    if (new_node == NULL) {
+                        return NULL;
+                    }
+
+                    /* Copy all elements of the old node to the new one. */
+                    for (i = 0; i < Py_SIZE(self); i++) {
+                        Py_INCREF(self->c_array[i]);
+                        new_node->c_array[i] = self->c_array[i];
+                    }
+                }
+
+                /* Replace the old value with the new value for the our key. */
+                Py_DECREF(new_node->c_array[val_idx]);
+                Py_INCREF(val);
+                new_node->c_array[val_idx] = val;
+
+                return (MapNode *)new_node;
+
+            default:
+                abort();
+        }
+    }
+    else {
+        /* The hash of the new key is different from the hash that
+           all keys of this Collision node have.
+
+           Create a Bitmap node inplace with two children:
+           key/value pair that we're adding, and the Collision node
+           we're replacing on this tree level.
+        */
+
+        MapNode_Bitmap *new_node;
+        MapNode *assoc_res;
+
+        new_node = (MapNode_Bitmap *)map_node_bitmap_new(2, mutid);
+        if (new_node == NULL) {
+            return NULL;
+        }
+        new_node->b_bitmap = map_bitpos(self->c_hash, shift);
+        Py_INCREF(self);
+        new_node->b_array[1] = (PyObject*) self;
+
+        assoc_res = map_node_bitmap_assoc(
+            new_node, shift, hash, key, val, added_leaf, mutid);
+        Py_DECREF(new_node);
+        return assoc_res;
+    }
+}
+
+static inline Py_ssize_t
+map_node_collision_count(MapNode_Collision *node)
+{
+    return Py_SIZE(node) / 2;
+}
+
+static map_without_t
+map_node_collision_without(MapNode_Collision *self,
+                           uint32_t shift, int32_t hash,
+                           PyObject *key,
+                           MapNode **new_node,
+                           uint64_t mutid)
+{
+    if (hash != self->c_hash) {
+        return W_NOT_FOUND;
+    }
+
+    Py_ssize_t key_idx = -1;
+    map_find_t found = map_node_collision_find_index(self, key, &key_idx);
+
+    switch (found) {
+        case F_ERROR:
+            return W_ERROR;
+
+        case F_NOT_FOUND:
+            return W_NOT_FOUND;
+
+        case F_FOUND:
+            assert(key_idx >= 0);
+            assert(key_idx < Py_SIZE(self));
+
+            Py_ssize_t new_count = map_node_collision_count(self) - 1;
+
+            if (new_count == 0) {
+                /* The node has only one key/value pair and it's for the
+                   key we're trying to delete.  So a new node will be empty
+                   after the removal.
+                */
+                return W_EMPTY;
+            }
+
+            if (new_count == 1) {
+                /* The node has two keys, and after deletion the
+                   new Collision node would have one.  Collision nodes
+                   with one key shouldn't exist, so convert it to a
+                   Bitmap node.
+                */
+                MapNode_Bitmap *node = (MapNode_Bitmap *)
+                    map_node_bitmap_new(2, mutid);
+                if (node == NULL) {
+                    return W_ERROR;
+                }
+
+                if (key_idx == 0) {
+                    Py_INCREF(self->c_array[2]);
+                    node->b_array[0] = self->c_array[2];
+                    Py_INCREF(self->c_array[3]);
+                    node->b_array[1] = self->c_array[3];
+                }
+                else {
+                    assert(key_idx == 2);
+                    Py_INCREF(self->c_array[0]);
+                    node->b_array[0] = self->c_array[0];
+                    Py_INCREF(self->c_array[1]);
+                    node->b_array[1] = self->c_array[1];
+                }
+
+                node->b_bitmap = map_bitpos(hash, shift);
+
+                *new_node = (MapNode *)node;
+                return W_NEWNODE;
+            }
+
+            /* Allocate a new Collision node with capacity for one
+               less key/value pair */
+            MapNode_Collision *new = (MapNode_Collision *)
+                map_node_collision_new(
+                    self->c_hash, Py_SIZE(self) - 2, mutid);
+            if (new == NULL) {
+                return W_ERROR;
+            }
+
+            /* Copy all other keys from `self` to `new` */
+            Py_ssize_t i;
+            for (i = 0; i < key_idx; i++) {
+                Py_INCREF(self->c_array[i]);
+                new->c_array[i] = self->c_array[i];
+            }
+            for (i = key_idx + 2; i < Py_SIZE(self); i++) {
+                Py_INCREF(self->c_array[i]);
+                new->c_array[i - 2] = self->c_array[i];
+            }
+
+            *new_node = (MapNode*)new;
+            return W_NEWNODE;
+
+        default:
+            abort();
+    }
+}
+
+static map_find_t
+map_node_collision_find(MapNode_Collision *self,
+                        uint32_t shift, int32_t hash,
+                        PyObject *key, PyObject **val)
+{
+    /* Lookup `key` in the Collision node `self`.  Set the value
+       for the found key to 'val'. */
+
+    Py_ssize_t idx = -1;
+    map_find_t res;
+
+    res = map_node_collision_find_index(self, key, &idx);
+    if (res == F_ERROR || res == F_NOT_FOUND) {
+        return res;
+    }
+
+    assert(idx >= 0);
+    assert(idx + 1 < Py_SIZE(self));
+
+    *val = self->c_array[idx + 1];
+    assert(*val != NULL);
+
+    return F_FOUND;
+}
+
+
+static int
+map_node_collision_traverse(MapNode_Collision *self,
+                            visitproc visit, void *arg)
+{
+    /* Collision's tp_traverse */
+
+    Py_ssize_t i;
+
+    for (i = Py_SIZE(self); --i >= 0; ) {
+        Py_VISIT(self->c_array[i]);
+    }
+
+    return 0;
+}
+
+static void
+map_node_collision_dealloc(MapNode_Collision *self)
+{
+    /* Collision's tp_dealloc */
+
+    Py_ssize_t len = Py_SIZE(self);
+
+    PyObject_GC_UnTrack(self);
+    Py_TRASHCAN_SAFE_BEGIN(self)
+
+    if (len > 0) {
+
+        while (--len >= 0) {
+            Py_XDECREF(self->c_array[len]);
+        }
+    }
+
+    Py_TYPE(self)->tp_free((PyObject *)self);
+    Py_TRASHCAN_SAFE_END(self)
+}
+
+static int
+map_node_collision_dump(MapNode_Collision *node,
+                        _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for Collision nodes. */
+
+    Py_ssize_t i;
+
+    if (_map_dump_ident(writer, level + 1)) {
+        goto error;
+    }
+
+    if (_map_dump_format(writer, "CollisionNode(size=%zd id=%p):\n",
+                         Py_SIZE(node), node))
+    {
+        goto error;
+    }
+
+    for (i = 0; i < Py_SIZE(node); i += 2) {
+        PyObject *key = node->c_array[i];
+        PyObject *val = node->c_array[i + 1];
+
+        if (_map_dump_ident(writer, level + 2)) {
+            goto error;
+        }
+
+        if (_map_dump_format(writer, "%R: %R\n", key, val)) {
+            goto error;
+        }
+    }
+
+    return 0;
+error:
+    return -1;
+}
+
+
+/////////////////////////////////// Array Node
+
+
+static MapNode *
+map_node_array_new(Py_ssize_t count, uint64_t mutid)
+{
+    Py_ssize_t i;
+
+    MapNode_Array *node = PyObject_GC_New(
+        MapNode_Array, &_Map_ArrayNode_Type);
+    if (node == NULL) {
+        return NULL;
+    }
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        node->a_array[i] = NULL;
+    }
+
+    node->a_count = count;
+    node->a_mutid = mutid;
+
+    PyObject_GC_Track(node);
+    return (MapNode *)node;
+}
+
+static MapNode_Array *
+map_node_array_clone(MapNode_Array *node, uint64_t mutid)
+{
+    MapNode_Array *clone;
+    Py_ssize_t i;
+
+    VALIDATE_ARRAY_NODE(node)
+    assert(node->a_count <= HAMT_ARRAY_NODE_SIZE);
+
+    /* Create a new Array node. */
+    clone = (MapNode_Array *)map_node_array_new(node->a_count, mutid);
+    if (clone == NULL) {
+        return NULL;
+    }
+
+    /* Copy all elements from the current Array node to the new one. */
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        Py_XINCREF(node->a_array[i]);
+        clone->a_array[i] = node->a_array[i];
+    }
+
+    clone->a_mutid = mutid;
+
+    VALIDATE_ARRAY_NODE(clone)
+    return clone;
+}
+
+static MapNode *
+map_node_array_assoc(MapNode_Array *self,
+                     uint32_t shift, int32_t hash,
+                     PyObject *key, PyObject *val, int* added_leaf,
+                     uint64_t mutid)
+{
+    /* Set a new key to this level (currently a Collision node)
+       of the tree.
+
+       Array nodes don't store values, they can only point to
+       other nodes.  They are simple arrays of 32 BaseNode pointers/
+     */
+
+    uint32_t idx = map_mask(hash, shift);
+    MapNode *node = self->a_array[idx];
+    MapNode *child_node;
+    MapNode_Array *new_node;
+    Py_ssize_t i;
+
+    if (node == NULL) {
+        /* There's no child node for the given hash.  Create a new
+           Bitmap node for this key. */
+
+        MapNode_Bitmap *empty = NULL;
+
+        /* Get an empty Bitmap node to work with. */
+        empty = (MapNode_Bitmap *)map_node_bitmap_new(0, mutid);
+        if (empty == NULL) {
+            return NULL;
+        }
+
+        /* Set key/val to the newly created empty Bitmap, thus
+           creating a new Bitmap node with our key/value pair. */
+        child_node = map_node_bitmap_assoc(
+            empty,
+            shift + 5, hash, key, val, added_leaf, mutid);
+        Py_DECREF(empty);
+        if (child_node == NULL) {
+            return NULL;
+        }
+
+        if (mutid != 0 && self->a_mutid == mutid) {
+            new_node = self;
+            self->a_count++;
+            Py_INCREF(self);
+        }
+        else {
+            /* Create a new Array node. */
+            new_node = (MapNode_Array *)map_node_array_new(
+                self->a_count + 1, mutid);
+            if (new_node == NULL) {
+                Py_DECREF(child_node);
+                return NULL;
+            }
+
+            /* Copy all elements from the current Array node to the
+               new one. */
+            for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+                Py_XINCREF(self->a_array[i]);
+                new_node->a_array[i] = self->a_array[i];
+            }
+        }
+
+        assert(new_node->a_array[idx] == NULL);
+        new_node->a_array[idx] = child_node;  /* borrow */
+        VALIDATE_ARRAY_NODE(new_node)
+    }
+    else {
+        /* There's a child node for the given hash.
+           Set the key to it./ */
+
+        child_node = map_node_assoc(
+            node, shift + 5, hash, key, val, added_leaf, mutid);
+        if (child_node == NULL) {
+            return NULL;
+        }
+        else if (child_node == (MapNode *)self) {
+            Py_DECREF(child_node);
+            return (MapNode *)self;
+        }
+
+        if (mutid != 0 && self->a_mutid == mutid) {
+            new_node = self;
+            Py_INCREF(self);
+        }
+        else {
+            new_node = map_node_array_clone(self, mutid);
+        }
+
+        if (new_node == NULL) {
+            Py_DECREF(child_node);
+            return NULL;
+        }
+
+        Py_SETREF(new_node->a_array[idx], child_node);  /* borrow */
+        VALIDATE_ARRAY_NODE(new_node)
+    }
+
+    return (MapNode *)new_node;
+}
+
+static map_without_t
+map_node_array_without(MapNode_Array *self,
+                       uint32_t shift, int32_t hash,
+                       PyObject *key,
+                       MapNode **new_node,
+                       uint64_t mutid)
+{
+    uint32_t idx = map_mask(hash, shift);
+    MapNode *node = self->a_array[idx];
+
+    if (node == NULL) {
+        return W_NOT_FOUND;
+    }
+
+    MapNode *sub_node = NULL;
+    MapNode_Array *target = NULL;
+    map_without_t res = map_node_without(
+        (MapNode *)node,
+        shift + 5, hash, key, &sub_node, mutid);
+
+    switch (res) {
+        case W_NOT_FOUND:
+        case W_ERROR:
+            assert(sub_node == NULL);
+            return res;
+
+        case W_NEWNODE: {
+            /* We need to replace a node at the `idx` index.
+               Clone this node and replace.
+            */
+            assert(sub_node != NULL);
+
+            if (mutid != 0 && self->a_mutid == mutid) {
+                target = self;
+                Py_INCREF(self);
+            }
+            else {
+                target = map_node_array_clone(self, mutid);
+                if (target == NULL) {
+                    Py_DECREF(sub_node);
+                    return W_ERROR;
+                }
+            }
+
+            Py_SETREF(target->a_array[idx], sub_node);  /* borrow */
+            *new_node = (MapNode*)target;  /* borrow */
+            return W_NEWNODE;
+        }
+
+        case W_EMPTY: {
+            assert(sub_node == NULL);
+            /* We need to remove a node at the `idx` index.
+               Calculate the size of the replacement Array node.
+            */
+            Py_ssize_t new_count = self->a_count - 1;
+
+            if (new_count == 0) {
+                return W_EMPTY;
+            }
+
+            if (new_count >= 16) {
+                /* We convert Bitmap nodes to Array nodes, when a
+                   Bitmap node needs to store more than 15 key/value
+                   pairs.  So we will create a new Array node if we
+                   the number of key/values after deletion is still
+                   greater than 15.
+                */
+
+                if (mutid != 0 && self->a_mutid == mutid) {
+                    target = self;
+                    Py_INCREF(self);
+                }
+                else {
+                    target = map_node_array_clone(self, mutid);
+                    if (target == NULL) {
+                        return W_ERROR;
+                    }
+                }
+
+                target->a_count = new_count;
+                Py_CLEAR(target->a_array[idx]);
+
+                *new_node = (MapNode*)target;  /* borrow */
+                return W_NEWNODE;
+            }
+
+            /* New Array node would have less than 16 key/value
+               pairs.  We need to create a replacement Bitmap node. */
+
+            Py_ssize_t bitmap_size = new_count * 2;
+            uint32_t bitmap = 0;
+
+            MapNode_Bitmap *new = (MapNode_Bitmap *)
+                map_node_bitmap_new(bitmap_size, mutid);
+            if (new == NULL) {
+                return W_ERROR;
+            }
+
+            Py_ssize_t new_i = 0;
+            for (uint32_t i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+                if (i == idx) {
+                    /* Skip the node we are deleting. */
+                    continue;
+                }
+
+                MapNode *node = self->a_array[i];
+                if (node == NULL) {
+                    /* Skip any missing nodes. */
+                    continue;
+                }
+
+                bitmap |= 1u << i;
+
+                if (IS_BITMAP_NODE(node)) {
+                    MapNode_Bitmap *child = (MapNode_Bitmap *)node;
+
+                    if (map_node_bitmap_count(child) == 1 &&
+                            child->b_array[0] != NULL)
+                    {
+                        /* node is a Bitmap with one key/value pair, just
+                           merge it into the new Bitmap node we're building.
+
+                           Note that we don't inline Bitmap nodes that
+                           have a NULL key -- those nodes point to another
+                           tree level, and we cannot simply move tree levels
+                           up or down.
+                        */
+                        PyObject *key = child->b_array[0];
+                        PyObject *val = child->b_array[1];
+
+                        Py_INCREF(key);
+                        new->b_array[new_i] = key;
+                        Py_INCREF(val);
+                        new->b_array[new_i + 1] = val;
+                    }
+                    else {
+                        new->b_array[new_i] = NULL;
+                        Py_INCREF(node);
+                        new->b_array[new_i + 1] = (PyObject*)node;
+                    }
+                }
+                else {
+
+#if !defined(NDEBUG)
+                    if (IS_COLLISION_NODE(node)) {
+                        assert(
+                            (map_node_collision_count(
+                                (MapNode_Collision*)node)) > 1);
+                    }
+                    else if (IS_ARRAY_NODE(node)) {
+                        assert(((MapNode_Array*)node)->a_count >= 16);
+                    }
+#endif
+
+                    /* Just copy the node into our new Bitmap */
+                    new->b_array[new_i] = NULL;
+                    Py_INCREF(node);
+                    new->b_array[new_i + 1] = (PyObject*)node;
+                }
+
+                new_i += 2;
+            }
+
+            new->b_bitmap = bitmap;
+            *new_node = (MapNode*)new;  /* borrow */
+            return W_NEWNODE;
+        }
+
+        default:
+            abort();
+    }
+}
+
+static map_find_t
+map_node_array_find(MapNode_Array *self,
+                    uint32_t shift, int32_t hash,
+                    PyObject *key, PyObject **val)
+{
+    /* Lookup `key` in the Array node `self`.  Set the value
+       for the found key to 'val'. */
+
+    uint32_t idx = map_mask(hash, shift);
+    MapNode *node;
+
+    node = self->a_array[idx];
+    if (node == NULL) {
+        return F_NOT_FOUND;
+    }
+
+    /* Dispatch to the generic map_node_find */
+    return map_node_find(node, shift + 5, hash, key, val);
+}
+
+static int
+map_node_array_traverse(MapNode_Array *self,
+                        visitproc visit, void *arg)
+{
+    /* Array's tp_traverse */
+
+    Py_ssize_t i;
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        Py_VISIT(self->a_array[i]);
+    }
+
+    return 0;
+}
+
+static void
+map_node_array_dealloc(MapNode_Array *self)
+{
+    /* Array's tp_dealloc */
+
+    Py_ssize_t i;
+
+    PyObject_GC_UnTrack(self);
+    Py_TRASHCAN_SAFE_BEGIN(self)
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        Py_XDECREF(self->a_array[i]);
+    }
+
+    Py_TYPE(self)->tp_free((PyObject *)self);
+    Py_TRASHCAN_SAFE_END(self)
+}
+
+static int
+map_node_array_dump(MapNode_Array *node,
+                    _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for Array nodes. */
+
+    Py_ssize_t i;
+
+    if (_map_dump_ident(writer, level + 1)) {
+        goto error;
+    }
+
+    if (_map_dump_format(writer, "ArrayNode(id=%p count=%zd):\n",
+                         node, node->a_count)
+    ) {
+        goto error;
+    }
+
+    for (i = 0; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        if (node->a_array[i] == NULL) {
+            continue;
+        }
+
+        if (_map_dump_ident(writer, level + 2)) {
+            goto error;
+        }
+
+        if (_map_dump_format(writer, "%d::\n", i)) {
+            goto error;
+        }
+
+        if (map_node_dump(node->a_array[i], writer, level + 1)) {
+            goto error;
+        }
+
+        if (_map_dump_format(writer, "\n")) {
+            goto error;
+        }
+    }
+
+    return 0;
+error:
+    return -1;
+}
+
+
+/////////////////////////////////// Node Dispatch
+
+
+static MapNode *
+map_node_assoc(MapNode *node,
+               uint32_t shift, int32_t hash,
+               PyObject *key, PyObject *val, int* added_leaf,
+               uint64_t mutid)
+{
+    /* Set key/value to the 'node' starting with the given shift/hash.
+       Return a new node, or the same node if key/value already
+       set.
+
+       added_leaf will be set to 1 if key/value wasn't in the
+       tree before.
+
+       This method automatically dispatches to the suitable
+       map_node_{nodetype}_assoc method.
+    */
+
+    *added_leaf = 0;
+
+    if (IS_BITMAP_NODE(node)) {
+        return map_node_bitmap_assoc(
+            (MapNode_Bitmap *)node,
+            shift, hash, key, val, added_leaf, mutid);
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return map_node_array_assoc(
+            (MapNode_Array *)node,
+            shift, hash, key, val, added_leaf, mutid);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return map_node_collision_assoc(
+            (MapNode_Collision *)node,
+            shift, hash, key, val, added_leaf, mutid);
+    }
+}
+
+static map_without_t
+map_node_without(MapNode *node,
+                 uint32_t shift, int32_t hash,
+                 PyObject *key,
+                 MapNode **new_node,
+                 uint64_t mutid)
+{
+    if (IS_BITMAP_NODE(node)) {
+        return map_node_bitmap_without(
+            (MapNode_Bitmap *)node,
+            shift, hash, key,
+            new_node,
+            mutid);
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return map_node_array_without(
+            (MapNode_Array *)node,
+            shift, hash, key,
+            new_node,
+            mutid);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return map_node_collision_without(
+            (MapNode_Collision *)node,
+            shift, hash, key,
+            new_node,
+            mutid);
+    }
+}
+
+static map_find_t
+map_node_find(MapNode *node,
+              uint32_t shift, int32_t hash,
+              PyObject *key, PyObject **val)
+{
+    /* Find the key in the node starting with the given shift/hash.
+
+       If a value is found, the result will be set to F_FOUND, and
+       *val will point to the found value object.
+
+       If a value wasn't found, the result will be set to F_NOT_FOUND.
+
+       If an exception occurs during the call, the result will be F_ERROR.
+
+       This method automatically dispatches to the suitable
+       map_node_{nodetype}_find method.
+    */
+
+    if (IS_BITMAP_NODE(node)) {
+        return map_node_bitmap_find(
+            (MapNode_Bitmap *)node,
+            shift, hash, key, val);
+
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return map_node_array_find(
+            (MapNode_Array *)node,
+            shift, hash, key, val);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return map_node_collision_find(
+            (MapNode_Collision *)node,
+            shift, hash, key, val);
+    }
+}
+
+static int
+map_node_dump(MapNode *node,
+              _PyUnicodeWriter *writer, int level)
+{
+    /* Debug build: __dump__() method implementation for a node.
+
+       This method automatically dispatches to the suitable
+       map_node_{nodetype})_dump method.
+    */
+
+    if (IS_BITMAP_NODE(node)) {
+        return map_node_bitmap_dump(
+            (MapNode_Bitmap *)node, writer, level);
+    }
+    else if (IS_ARRAY_NODE(node)) {
+        return map_node_array_dump(
+            (MapNode_Array *)node, writer, level);
+    }
+    else {
+        assert(IS_COLLISION_NODE(node));
+        return map_node_collision_dump(
+            (MapNode_Collision *)node, writer, level);
+    }
+}
+
+
+/////////////////////////////////// Iterators: Machinery
+
+
+static map_iter_t
+map_iterator_next(MapIteratorState *iter, PyObject **key, PyObject **val);
+
+
+static void
+map_iterator_init(MapIteratorState *iter, MapNode *root)
+{
+    for (uint32_t i = 0; i < _Py_HAMT_MAX_TREE_DEPTH; i++) {
+        iter->i_nodes[i] = NULL;
+        iter->i_pos[i] = 0;
+    }
+
+    iter->i_level = 0;
+
+    /* Note: we don't incref/decref nodes in i_nodes. */
+    iter->i_nodes[0] = root;
+}
+
+static map_iter_t
+map_iterator_bitmap_next(MapIteratorState *iter,
+                         PyObject **key, PyObject **val)
+{
+    int8_t level = iter->i_level;
+
+    MapNode_Bitmap *node = (MapNode_Bitmap *)(iter->i_nodes[level]);
+    Py_ssize_t pos = iter->i_pos[level];
+
+    if (pos + 1 >= Py_SIZE(node)) {
+#if !defined(NDEBUG)
+        assert(iter->i_level >= 0);
+        iter->i_nodes[iter->i_level] = NULL;
+#endif
+        iter->i_level--;
+        return map_iterator_next(iter, key, val);
+    }
+
+    if (node->b_array[pos] == NULL) {
+        iter->i_pos[level] = pos + 2;
+
+        assert(level + 1 < _Py_HAMT_MAX_TREE_DEPTH);
+        int8_t next_level = (int8_t)(level + 1);
+        iter->i_level = next_level;
+        iter->i_pos[next_level] = 0;
+        iter->i_nodes[next_level] = (MapNode *)
+            node->b_array[pos + 1];
+
+        return map_iterator_next(iter, key, val);
+    }
+
+    *key = node->b_array[pos];
+    *val = node->b_array[pos + 1];
+    iter->i_pos[level] = pos + 2;
+    return I_ITEM;
+}
+
+static map_iter_t
+map_iterator_collision_next(MapIteratorState *iter,
+                            PyObject **key, PyObject **val)
+{
+    int8_t level = iter->i_level;
+
+    MapNode_Collision *node = (MapNode_Collision *)(iter->i_nodes[level]);
+    Py_ssize_t pos = iter->i_pos[level];
+
+    if (pos + 1 >= Py_SIZE(node)) {
+#if !defined(NDEBUG)
+        assert(iter->i_level >= 0);
+        iter->i_nodes[iter->i_level] = NULL;
+#endif
+        iter->i_level--;
+        return map_iterator_next(iter, key, val);
+    }
+
+    *key = node->c_array[pos];
+    *val = node->c_array[pos + 1];
+    iter->i_pos[level] = pos + 2;
+    return I_ITEM;
+}
+
+static map_iter_t
+map_iterator_array_next(MapIteratorState *iter,
+                        PyObject **key, PyObject **val)
+{
+    int8_t level = iter->i_level;
+
+    MapNode_Array *node = (MapNode_Array *)(iter->i_nodes[level]);
+    Py_ssize_t pos = iter->i_pos[level];
+
+    if (pos >= HAMT_ARRAY_NODE_SIZE) {
+#if !defined(NDEBUG)
+        assert(iter->i_level >= 0);
+        iter->i_nodes[iter->i_level] = NULL;
+#endif
+        iter->i_level--;
+        return map_iterator_next(iter, key, val);
+    }
+
+    for (Py_ssize_t i = pos; i < HAMT_ARRAY_NODE_SIZE; i++) {
+        if (node->a_array[i] != NULL) {
+            iter->i_pos[level] = i + 1;
+
+            assert((level + 1) < _Py_HAMT_MAX_TREE_DEPTH);
+            int8_t next_level = (int8_t)(level + 1);
+            iter->i_pos[next_level] = 0;
+            iter->i_nodes[next_level] = node->a_array[i];
+            iter->i_level = next_level;
+
+            return map_iterator_next(iter, key, val);
+        }
+    }
+
+#if !defined(NDEBUG)
+    assert(iter->i_level >= 0);
+    iter->i_nodes[iter->i_level] = NULL;
+#endif
+
+    iter->i_level--;
+    return map_iterator_next(iter, key, val);
+}
+
+static map_iter_t
+map_iterator_next(MapIteratorState *iter, PyObject **key, PyObject **val)
+{
+    if (iter->i_level < 0) {
+        return I_END;
+    }
+
+    assert(iter->i_level < _Py_HAMT_MAX_TREE_DEPTH);
+
+    MapNode *current = iter->i_nodes[iter->i_level];
+
+    if (IS_BITMAP_NODE(current)) {
+        return map_iterator_bitmap_next(iter, key, val);
+    }
+    else if (IS_ARRAY_NODE(current)) {
+        return map_iterator_array_next(iter, key, val);
+    }
+    else {
+        assert(IS_COLLISION_NODE(current));
+        return map_iterator_collision_next(iter, key, val);
+    }
+}
+
+
+/////////////////////////////////// HAMT high-level functions
+
+
+static MapObject *
+map_assoc(MapObject *o, PyObject *key, PyObject *val)
+{
+    int32_t key_hash;
+    int added_leaf = 0;
+    MapNode *new_root;
+    MapObject *new_o;
+
+    key_hash = map_hash(key);
+    if (key_hash == -1) {
+        return NULL;
+    }
+
+    new_root = map_node_assoc(
+        (MapNode *)(o->h_root),
+        0, key_hash, key, val, &added_leaf,
+        0);
+    if (new_root == NULL) {
+        return NULL;
+    }
+
+    if (new_root == o->h_root) {
+        Py_DECREF(new_root);
+        Py_INCREF(o);
+        return o;
+    }
+
+    new_o = map_alloc();
+    if (new_o == NULL) {
+        Py_DECREF(new_root);
+        return NULL;
+    }
+
+    new_o->h_root = new_root;  /* borrow */
+    new_o->h_count = added_leaf ? o->h_count + 1 : o->h_count;
+
+    return new_o;
+}
+
+static MapObject *
+map_without(MapObject *o, PyObject *key)
+{
+    int32_t key_hash = map_hash(key);
+    if (key_hash == -1) {
+        return NULL;
+    }
+
+    MapNode *new_root = NULL;
+
+    map_without_t res = map_node_without(
+        (MapNode *)(o->h_root),
+        0, key_hash, key,
+        &new_root,
+        0);
+
+    switch (res) {
+        case W_ERROR:
+            return NULL;
+        case W_EMPTY:
+            return map_new();
+        case W_NOT_FOUND:
+            PyErr_SetObject(PyExc_KeyError, key);
+            return NULL;
+        case W_NEWNODE: {
+            assert(new_root != NULL);
+
+            MapObject *new_o = map_alloc();
+            if (new_o == NULL) {
+                Py_DECREF(new_root);
+                return NULL;
+            }
+
+            new_o->h_root = new_root;  /* borrow */
+            new_o->h_count = o->h_count - 1;
+            assert(new_o->h_count >= 0);
+            return new_o;
+        }
+        default:
+            abort();
+    }
+}
+
+static map_find_t
+map_find(BaseMapObject *o, PyObject *key, PyObject **val)
+{
+    if (o->b_count == 0) {
+        return F_NOT_FOUND;
+    }
+
+    int32_t key_hash = map_hash(key);
+    if (key_hash == -1) {
+        return F_ERROR;
+    }
+
+    return map_node_find(o->b_root, 0, key_hash, key, val);
+}
+
+static int
+map_eq(BaseMapObject *v, BaseMapObject *w)
+{
+    if (v == w) {
+        return 1;
+    }
+
+    if (v->b_count != w->b_count) {
+        return 0;
+    }
+
+    MapIteratorState iter;
+    map_iter_t iter_res;
+    map_find_t find_res;
+    PyObject *v_key;
+    PyObject *v_val;
+    PyObject *w_val;
+
+    map_iterator_init(&iter, v->b_root);
+
+    do {
+        iter_res = map_iterator_next(&iter, &v_key, &v_val);
+        if (iter_res == I_ITEM) {
+            find_res = map_find(w, v_key, &w_val);
+            switch (find_res) {
+                case F_ERROR:
+                    return -1;
+
+                case F_NOT_FOUND:
+                    return 0;
+
+                case F_FOUND: {
+                    int cmp = PyObject_RichCompareBool(v_val, w_val, Py_EQ);
+                    if (cmp < 0) {
+                        return -1;
+                    }
+                    if (cmp == 0) {
+                        return 0;
+                    }
+                }
+            }
+        }
+    } while (iter_res != I_END);
+
+    return 1;
+}
+
+static Py_ssize_t
+map_len(BaseMapObject *o)
+{
+    return o->b_count;
+}
+
+static MapObject *
+map_alloc(void)
+{
+    MapObject *o;
+    o = PyObject_GC_New(MapObject, &_Map_Type);
+    if (o == NULL) {
+        return NULL;
+    }
+    o->h_weakreflist = NULL;
+    o->h_hash = -1;
+    o->h_count = 0;
+    o->h_root = NULL;
+    PyObject_GC_Track(o);
+    return o;
+}
+
+static MapObject *
+map_new(void)
+{
+    MapObject *o = map_alloc();
+    if (o == NULL) {
+        return NULL;
+    }
+
+    o->h_root = map_node_bitmap_new(0, 0);
+    if (o->h_root == NULL) {
+        Py_DECREF(o);
+        return NULL;
+    }
+
+    return o;
+}
+
+static PyObject *
+map_dump(MapObject *self)
+{
+    _PyUnicodeWriter writer;
+
+    _PyUnicodeWriter_Init(&writer);
+
+    if (_map_dump_format(&writer, "HAMT(len=%zd):\n", self->h_count)) {
+        goto error;
+    }
+
+    if (map_node_dump(self->h_root, &writer, 0)) {
+        goto error;
+    }
+
+    return _PyUnicodeWriter_Finish(&writer);
+
+error:
+    _PyUnicodeWriter_Dealloc(&writer);
+    return NULL;
+}
+
+
+/////////////////////////////////// Iterators: Shared Iterator Implementation
+
+
+static int
+map_baseiter_tp_clear(MapIterator *it)
+{
+    Py_CLEAR(it->mi_obj);
+    return 0;
+}
+
+static void
+map_baseiter_tp_dealloc(MapIterator *it)
+{
+    PyObject_GC_UnTrack(it);
+    (void)map_baseiter_tp_clear(it);
+    PyObject_GC_Del(it);
+}
+
+static int
+map_baseiter_tp_traverse(MapIterator *it, visitproc visit, void *arg)
+{
+    Py_VISIT(it->mi_obj);
+    return 0;
+}
+
+static PyObject *
+map_baseiter_tp_iternext(MapIterator *it)
+{
+    PyObject *key;
+    PyObject *val;
+    map_iter_t res = map_iterator_next(&it->mi_iter, &key, &val);
+
+    switch (res) {
+        case I_END:
+            PyErr_SetNone(PyExc_StopIteration);
+            return NULL;
+
+        case I_ITEM: {
+            return (*(it->mi_yield))(key, val);
+        }
+
+        default: {
+            abort();
+        }
+    }
+}
+
+static int
+map_baseview_tp_clear(MapView *view)
+{
+    Py_CLEAR(view->mv_obj);
+    Py_CLEAR(view->mv_itertype);
+    return 0;
+}
+
+static void
+map_baseview_tp_dealloc(MapView *view)
+{
+    PyObject_GC_UnTrack(view);
+    (void)map_baseview_tp_clear(view);
+    PyObject_GC_Del(view);
+}
+
+static int
+map_baseview_tp_traverse(MapView *view, visitproc visit, void *arg)
+{
+    Py_VISIT(view->mv_obj);
+    return 0;
+}
+
+static Py_ssize_t
+map_baseview_tp_len(MapView *view)
+{
+    return view->mv_obj->h_count;
+}
+
+static PyMappingMethods MapView_as_mapping = {
+    (lenfunc)map_baseview_tp_len,
+};
+
+static PyObject *
+map_baseview_newiter(PyTypeObject *type, binaryfunc yield, MapObject *map)
+{
+    MapIterator *iter = PyObject_GC_New(MapIterator, type);
+    if (iter == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(map);
+    iter->mi_obj = map;
+    iter->mi_yield = yield;
+    map_iterator_init(&iter->mi_iter, map->h_root);
+
+    PyObject_GC_Track(iter);
+    return (PyObject *)iter;
+}
+
+static PyObject *
+map_baseview_iter(MapView *view)
+{
+    return map_baseview_newiter(
+        view->mv_itertype, view->mv_yield, view->mv_obj);
+}
+
+static PyObject *
+map_baseview_new(PyTypeObject *type, binaryfunc yield,
+                 MapObject *o, PyTypeObject *itertype)
+{
+    MapView *view = PyObject_GC_New(MapView, type);
+    if (view == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(o);
+    view->mv_obj = o;
+    view->mv_yield = yield;
+
+    Py_INCREF(itertype);
+    view->mv_itertype = itertype;
+
+    PyObject_GC_Track(view);
+    return (PyObject *)view;
+}
+
+#define ITERATOR_TYPE_SHARED_SLOTS                              \
+    .tp_basicsize = sizeof(MapIterator),                        \
+    .tp_itemsize = 0,                                           \
+    .tp_dealloc = (destructor)map_baseiter_tp_dealloc,          \
+    .tp_getattro = PyObject_GenericGetAttr,                     \
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,        \
+    .tp_traverse = (traverseproc)map_baseiter_tp_traverse,      \
+    .tp_clear = (inquiry)map_baseiter_tp_clear,                 \
+    .tp_iter = PyObject_SelfIter,                               \
+    .tp_iternext = (iternextfunc)map_baseiter_tp_iternext,
+
+
+#define VIEW_TYPE_SHARED_SLOTS                                  \
+    .tp_basicsize = sizeof(MapView),                            \
+    .tp_itemsize = 0,                                           \
+    .tp_as_mapping = &MapView_as_mapping,                       \
+    .tp_dealloc = (destructor)map_baseview_tp_dealloc,          \
+    .tp_getattro = PyObject_GenericGetAttr,                     \
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,        \
+    .tp_traverse = (traverseproc)map_baseview_tp_traverse,      \
+    .tp_clear = (inquiry)map_baseview_tp_clear,                 \
+    .tp_iter = (getiterfunc)map_baseview_iter,                  \
+
+
+/////////////////////////////////// _MapItems_Type
+
+
+PyTypeObject _MapItems_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "items",
+    VIEW_TYPE_SHARED_SLOTS
+};
+
+PyTypeObject _MapItemsIter_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "items_iterator",
+    ITERATOR_TYPE_SHARED_SLOTS
+};
+
+static PyObject *
+map_iter_yield_items(PyObject *key, PyObject *val)
+{
+    return PyTuple_Pack(2, key, val);
+}
+
+static PyObject *
+map_new_items_view(MapObject *o)
+{
+    return map_baseview_new(
+        &_MapItems_Type, map_iter_yield_items, o,
+        &_MapItemsIter_Type);
+}
+
+
+/////////////////////////////////// _MapKeys_Type
+
+
+PyTypeObject _MapKeys_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "keys",
+    VIEW_TYPE_SHARED_SLOTS
+};
+
+PyTypeObject _MapKeysIter_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "keys_iterator",
+    ITERATOR_TYPE_SHARED_SLOTS
+};
+
+static PyObject *
+map_iter_yield_keys(PyObject *key, PyObject *val)
+{
+    Py_INCREF(key);
+    return key;
+}
+
+static PyObject *
+map_new_keys_iter(MapObject *o)
+{
+    return map_baseview_newiter(
+        &_MapKeysIter_Type, map_iter_yield_keys, o);
+}
+
+static PyObject *
+map_new_keys_view(MapObject *o)
+{
+    return map_baseview_new(
+        &_MapKeys_Type, map_iter_yield_keys, o,
+        &_MapKeysIter_Type);
+}
+
+/////////////////////////////////// _MapValues_Type
+
+
+PyTypeObject _MapValues_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "values",
+    VIEW_TYPE_SHARED_SLOTS
+};
+
+PyTypeObject _MapValuesIter_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "values_iterator",
+    ITERATOR_TYPE_SHARED_SLOTS
+};
+
+static PyObject *
+map_iter_yield_values(PyObject *key, PyObject *val)
+{
+    Py_INCREF(val);
+    return val;
+}
+
+static PyObject *
+map_new_values_view(MapObject *o)
+{
+    return map_baseview_new(
+        &_MapValues_Type, map_iter_yield_values, o,
+        &_MapValuesIter_Type);
+}
+
+
+/////////////////////////////////// _Map_Type
+
+
+static PyObject *
+map_dump(MapObject *self);
+
+
+static PyObject *
+map_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    return (PyObject*)map_new();
+}
+
+
+static int
+map_tp_init(MapObject *self, PyObject *args, PyObject *kwds)
+{
+    PyObject *arg = NULL;
+    uint64_t mutid = 0;
+
+    if (!PyArg_UnpackTuple(args, "immutables.Map", 0, 1, &arg)) {
+        return -1;
+    }
+
+    if (arg != NULL) {
+        if (Map_Check(arg)) {
+            MapObject *other = (MapObject *)arg;
+
+            Py_INCREF(other->h_root);
+            Py_SETREF(self->h_root, other->h_root);
+
+            self->h_count = other->h_count;
+            self->h_hash = other->h_hash;
+        }
+        else if (MapMutation_Check(arg)) {
+            PyErr_Format(
+                PyExc_TypeError,
+                "cannot create Maps from MapMutations");
+            return -1;
+        }
+        else {
+            mutid = mutid_counter++;
+            if (map_update_inplace(mutid, (BaseMapObject *)self, arg)) {
+                return -1;
+            }
+        }
+    }
+
+    if (kwds != NULL) {
+        if (!PyArg_ValidateKeywordArguments(kwds)) {
+            return -1;
+        }
+
+        if (!mutid) {
+            mutid = mutid_counter++;
+        }
+
+        if (map_update_inplace(mutid, (BaseMapObject *)self, kwds)) {
+            return -1;
+        }
+    }
+
+    return 0;
+}
+
+
+static int
+map_tp_clear(BaseMapObject *self)
+{
+    Py_CLEAR(self->b_root);
+    return 0;
+}
+
+
+static int
+map_tp_traverse(BaseMapObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->b_root);
+    return 0;
+}
+
+static void
+map_tp_dealloc(BaseMapObject *self)
+{
+    PyObject_GC_UnTrack(self);
+    if (self->b_weakreflist != NULL) {
+        PyObject_ClearWeakRefs((PyObject*)self);
+    }
+    (void)map_tp_clear(self);
+    Py_TYPE(self)->tp_free(self);
+}
+
+
+static PyObject *
+map_tp_richcompare(PyObject *v, PyObject *w, int op)
+{
+    if (!Map_Check(v) || !Map_Check(w) || (op != Py_EQ && op != Py_NE)) {
+        Py_RETURN_NOTIMPLEMENTED;
+    }
+
+    int res = map_eq((BaseMapObject *)v, (BaseMapObject *)w);
+    if (res < 0) {
+        return NULL;
+    }
+
+    if (op == Py_NE) {
+        res = !res;
+    }
+
+    if (res) {
+        Py_RETURN_TRUE;
+    }
+    else {
+        Py_RETURN_FALSE;
+    }
+}
+
+static int
+map_tp_contains(BaseMapObject *self, PyObject *key)
+{
+    PyObject *val;
+    map_find_t res = map_find(self, key, &val);
+    switch (res) {
+        case F_ERROR:
+            return -1;
+        case F_NOT_FOUND:
+            return 0;
+        case F_FOUND:
+            return 1;
+        default:
+            abort();
+    }
+}
+
+static PyObject *
+map_tp_subscript(BaseMapObject *self, PyObject *key)
+{
+    PyObject *val;
+    map_find_t res = map_find(self, key, &val);
+    switch (res) {
+        case F_ERROR:
+            return NULL;
+        case F_FOUND:
+            Py_INCREF(val);
+            return val;
+        case F_NOT_FOUND:
+            PyErr_SetObject(PyExc_KeyError, key);
+            return NULL;
+        default:
+            abort();
+    }
+}
+
+static Py_ssize_t
+map_tp_len(BaseMapObject *self)
+{
+    return map_len(self);
+}
+
+static PyObject *
+map_tp_iter(MapObject *self)
+{
+    return map_new_keys_iter(self);
+}
+
+static PyObject *
+map_py_set(MapObject *self, PyObject *args)
+{
+    PyObject *key;
+    PyObject *val;
+
+    if (!PyArg_UnpackTuple(args, "set", 2, 2, &key, &val)) {
+        return NULL;
+    }
+
+    return (PyObject *)map_assoc(self, key, val);
+}
+
+static PyObject *
+map_py_get(BaseMapObject *self, PyObject *args)
+{
+    PyObject *key;
+    PyObject *def = NULL;
+
+    if (!PyArg_UnpackTuple(args, "get", 1, 2, &key, &def)) {
+        return NULL;
+    }
+
+    PyObject *val = NULL;
+    map_find_t res = map_find(self, key, &val);
+    switch (res) {
+        case F_ERROR:
+            return NULL;
+        case F_FOUND:
+            Py_INCREF(val);
+            return val;
+        case F_NOT_FOUND:
+            if (def == NULL) {
+                Py_RETURN_NONE;
+            }
+            Py_INCREF(def);
+            return def;
+        default:
+            abort();
+    }
+}
+
+static PyObject *
+map_py_delete(MapObject *self, PyObject *key)
+{
+    return (PyObject *)map_without(self, key);
+}
+
+static PyObject *
+map_py_mutate(MapObject *self, PyObject *args)
+{
+
+    MapMutationObject *o;
+    o = PyObject_GC_New(MapMutationObject, &_MapMutation_Type);
+    if (o == NULL) {
+        return NULL;
+    }
+    o->m_weakreflist = NULL;
+    o->m_count = self->h_count;
+
+    Py_INCREF(self->h_root);
+    o->m_root = self->h_root;
+
+    o->m_mutid = mutid_counter++;
+
+    PyObject_GC_Track(o);
+    return (PyObject *)o;
+}
+
+static PyObject *
+map_py_update(MapObject *self, PyObject *args, PyObject *kwds)
+{
+    PyObject *arg = NULL;
+    MapObject *new = NULL;
+    uint64_t mutid = 0;
+
+    if (!PyArg_UnpackTuple(args, "update", 0, 1, &arg)) {
+        return NULL;
+    }
+
+    if (arg != NULL) {
+        mutid = mutid_counter++;
+        new = map_update(mutid, self, arg);
+        if (new == NULL) {
+            return NULL;
+        }
+    }
+    else {
+        Py_INCREF(self);
+        new = self;
+    }
+
+    if (kwds != NULL) {
+        if (!PyArg_ValidateKeywordArguments(kwds)) {
+            Py_DECREF(new);
+            return NULL;
+        }
+
+        if (!mutid) {
+            mutid = mutid_counter++;
+        }
+
+        MapObject *new2 = map_update(mutid, new, kwds);
+        Py_DECREF(new);
+        if (new2 == NULL) {
+            return NULL;
+        }
+        new = new2;
+    }
+
+    return (PyObject *)new;
+}
+
+static PyObject *
+map_py_items(MapObject *self, PyObject *args)
+{
+    return map_new_items_view(self);
+}
+
+static PyObject *
+map_py_values(MapObject *self, PyObject *args)
+{
+    return map_new_values_view(self);
+}
+
+static PyObject *
+map_py_keys(MapObject *self, PyObject *args)
+{
+    return map_new_keys_view(self);
+}
+
+static PyObject *
+map_py_dump(MapObject *self, PyObject *args)
+{
+    return map_dump(self);
+}
+
+
+static PyObject *
+map_py_repr(BaseMapObject *m)
+{
+    Py_ssize_t i;
+    _PyUnicodeWriter writer;
+
+
+    i = Py_ReprEnter((PyObject *)m);
+    if (i != 0) {
+        return i > 0 ? PyUnicode_FromString("{...}") : NULL;
+    }
+
+    _PyUnicodeWriter_Init(&writer);
+
+    if (MapMutation_Check(m)) {
+        if (_PyUnicodeWriter_WriteASCIIString(
+                &writer, "immutables.MapMutation({", 24) < 0)
+        {
+            goto error;
+        }
+    }
+    else {
+        if (_PyUnicodeWriter_WriteASCIIString(
+                &writer, "immutables.Map({", 16) < 0)
+        {
+            goto error;
+        }
+    }
+
+    MapIteratorState iter;
+    map_iter_t iter_res;
+    map_iterator_init(&iter, m->b_root);
+    int second = 0;
+    do {
+        PyObject *v_key;
+        PyObject *v_val;
+
+        iter_res = map_iterator_next(&iter, &v_key, &v_val);
+        if (iter_res == I_ITEM) {
+            if (second) {
+                if (_PyUnicodeWriter_WriteASCIIString(&writer, ", ", 2) < 0) {
+                    goto error;
+                }
+            }
+
+            PyObject *s = PyObject_Repr(v_key);
+            if (s == NULL) {
+                goto error;
+            }
+            if (_PyUnicodeWriter_WriteStr(&writer, s) < 0) {
+                Py_DECREF(s);
+                goto error;
+            }
+            Py_DECREF(s);
+
+            if (_PyUnicodeWriter_WriteASCIIString(&writer, ": ", 2) < 0) {
+                goto error;
+            }
+
+            s = PyObject_Repr(v_val);
+            if (s == NULL) {
+                goto error;
+            }
+            if (_PyUnicodeWriter_WriteStr(&writer, s) < 0) {
+                Py_DECREF(s);
+                goto error;
+            }
+            Py_DECREF(s);
+        }
+
+        second = 1;
+    } while (iter_res != I_END);
+
+    if (_PyUnicodeWriter_WriteASCIIString(&writer, "})", 2) < 0) {
+        goto error;
+    }
+
+    Py_ReprLeave((PyObject *)m);
+    return _PyUnicodeWriter_Finish(&writer);
+
+error:
+    _PyUnicodeWriter_Dealloc(&writer);
+    Py_ReprLeave((PyObject *)m);
+    return NULL;
+}
+
+
+static Py_uhash_t
+_shuffle_bits(Py_uhash_t h)
+{
+    return ((h ^ 89869747UL) ^ (h << 16)) * 3644798167UL;
+}
+
+
+static Py_hash_t
+map_py_hash(MapObject *self)
+{
+    /* Adapted version of frozenset.__hash__: it's important
+       that Map.__hash__ is independant of key/values order.
+
+       Optimization idea: compute and memoize intermediate
+       hash values for HAMT nodes.
+    */
+
+    if (self->h_hash != -1) {
+        return self->h_hash;
+    }
+
+    Py_uhash_t hash = 0;
+
+    MapIteratorState iter;
+    map_iter_t iter_res;
+    map_iterator_init(&iter, self->h_root);
+    do {
+        PyObject *v_key;
+        PyObject *v_val;
+
+        iter_res = map_iterator_next(&iter, &v_key, &v_val);
+        if (iter_res == I_ITEM) {
+            Py_hash_t vh = PyObject_Hash(v_key);
+            if (vh == -1) {
+                return -1;
+            }
+            hash ^= _shuffle_bits((Py_uhash_t)vh);
+
+            vh = PyObject_Hash(v_val);
+            if (vh == -1) {
+                return -1;
+            }
+            hash ^= _shuffle_bits((Py_uhash_t)vh);
+        }
+    } while (iter_res != I_END);
+
+    hash ^= ((Py_uhash_t)self->h_count * 2 + 1) * 1927868237UL;
+
+    hash ^= (hash >> 11) ^ (hash >> 25);
+    hash = hash * 69069U + 907133923UL;
+
+    self->h_hash = (Py_hash_t)hash;
+    if (self->h_hash == -1) {
+        self->h_hash = 1;
+    }
+    return self->h_hash;
+}
+
+static PyObject *
+map_reduce(MapObject *self)
+{
+    MapIteratorState iter;
+    map_iter_t iter_res;
+
+    PyObject *dict = PyDict_New();
+    if (dict == NULL) {
+        return NULL;
+    }
+
+    map_iterator_init(&iter, self->h_root);
+    do {
+        PyObject *key;
+        PyObject *val;
+
+        iter_res = map_iterator_next(&iter, &key, &val);
+        if (iter_res == I_ITEM) {
+            if (PyDict_SetItem(dict, key, val) < 0) {
+                Py_DECREF(dict);
+                return NULL;
+            }
+        }
+    } while (iter_res != I_END);
+
+    PyObject *args = PyTuple_Pack(1, dict);
+    Py_DECREF(dict);
+    if (args == NULL) {
+        return NULL;
+    }
+
+    PyObject *tup = PyTuple_Pack(2, Py_TYPE(self), args);
+    Py_DECREF(args);
+    return tup;
+}
+
+static PyObject *
+map_py_class_getitem(PyObject *type, PyObject *item)
+{
+    Py_INCREF(type);
+    return type;
+}
+
+static PyMethodDef Map_methods[] = {
+    {"set", (PyCFunction)map_py_set, METH_VARARGS, NULL},
+    {"get", (PyCFunction)map_py_get, METH_VARARGS, NULL},
+    {"delete", (PyCFunction)map_py_delete, METH_O, NULL},
+    {"mutate", (PyCFunction)map_py_mutate, METH_NOARGS, NULL},
+    {"items", (PyCFunction)map_py_items, METH_NOARGS, NULL},
+    {"keys", (PyCFunction)map_py_keys, METH_NOARGS, NULL},
+    {"values", (PyCFunction)map_py_values, METH_NOARGS, NULL},
+    {"update", (PyCFunction)map_py_update, METH_VARARGS | METH_KEYWORDS, NULL},
+    {"__reduce__", (PyCFunction)map_reduce, METH_NOARGS, NULL},
+    {"__dump__", (PyCFunction)map_py_dump, METH_NOARGS, NULL},
+    {
+        "__class_getitem__",
+        (PyCFunction)map_py_class_getitem,
+        METH_O|METH_CLASS,
+        NULL
+    },
+    {NULL, NULL}
+};
+
+static PySequenceMethods Map_as_sequence = {
+    0,                                /* sq_length */
+    0,                                /* sq_concat */
+    0,                                /* sq_repeat */
+    0,                                /* sq_item */
+    0,                                /* sq_slice */
+    0,                                /* sq_ass_item */
+    0,                                /* sq_ass_slice */
+    (objobjproc)map_tp_contains,      /* sq_contains */
+    0,                                /* sq_inplace_concat */
+    0,                                /* sq_inplace_repeat */
+};
+
+static PyMappingMethods Map_as_mapping = {
+    (lenfunc)map_tp_len,             /* mp_length */
+    (binaryfunc)map_tp_subscript,    /* mp_subscript */
+};
+
+PyTypeObject _Map_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "immutables._map.Map",
+    sizeof(MapObject),
+    .tp_methods = Map_methods,
+    .tp_as_mapping = &Map_as_mapping,
+    .tp_as_sequence = &Map_as_sequence,
+    .tp_iter = (getiterfunc)map_tp_iter,
+    .tp_dealloc = (destructor)map_tp_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_richcompare = map_tp_richcompare,
+    .tp_traverse = (traverseproc)map_tp_traverse,
+    .tp_clear = (inquiry)map_tp_clear,
+    .tp_new = map_tp_new,
+    .tp_init = (initproc)map_tp_init,
+    .tp_weaklistoffset = offsetof(MapObject, h_weakreflist),
+    .tp_hash = (hashfunc)map_py_hash,
+    .tp_repr = (reprfunc)map_py_repr,
+};
+
+
+/////////////////////////////////// MapMutation
+
+
+static int
+map_node_update_from_map(uint64_t mutid,
+                         MapObject *map,
+                         MapNode *root, Py_ssize_t count,
+                         MapNode **new_root, Py_ssize_t *new_count)
+{
+    assert(Map_Check(map));
+
+    MapIteratorState iter;
+    map_iter_t iter_res;
+
+    MapNode *last_root;
+    Py_ssize_t last_count;
+
+    Py_INCREF(root);
+    last_root = root;
+    last_count = count;
+
+    map_iterator_init(&iter, map->h_root);
+    do {
+        PyObject *key;
+        PyObject *val;
+        int32_t key_hash;
+        int added_leaf;
+
+        iter_res = map_iterator_next(&iter, &key, &val);
+        if (iter_res == I_ITEM) {
+            key_hash = map_hash(key);
+            if (key_hash == -1) {
+                goto err;
+            }
+
+            MapNode *iter_root = map_node_assoc(
+                last_root,
+                0, key_hash, key, val, &added_leaf,
+                mutid);
+
+            if (iter_root == NULL) {
+                goto err;
+            }
+
+            if (added_leaf) {
+                last_count++;
+            }
+
+            Py_SETREF(last_root, iter_root);
+        }
+    } while (iter_res != I_END);
+
+    *new_root = last_root;
+    *new_count = last_count;
+
+    return 0;
+
+err:
+    Py_DECREF(last_root);
+    return -1;
+}
+
+
+static int
+map_node_update_from_dict(uint64_t mutid,
+                          PyObject *dct,
+                          MapNode *root, Py_ssize_t count,
+                          MapNode **new_root, Py_ssize_t *new_count)
+{
+    assert(PyDict_Check(dct));
+
+    PyObject *it = PyObject_GetIter(dct);
+    if (it == NULL) {
+        return -1;
+    }
+
+    MapNode *last_root;
+    Py_ssize_t last_count;
+
+    Py_INCREF(root);
+    last_root = root;
+    last_count = count;
+
+    PyObject *key;
+
+    while ((key = PyIter_Next(it))) {
+        PyObject *val;
+        int added_leaf;
+        int32_t key_hash;
+
+        key_hash = map_hash(key);
+        if (key_hash == -1) {
+            Py_DECREF(key);
+            goto err;
+        }
+
+        val = PyDict_GetItemWithError(dct, key);
+        if (val == NULL) {
+            Py_DECREF(key);
+            goto err;
+        }
+
+        MapNode *iter_root = map_node_assoc(
+            last_root,
+            0, key_hash, key, val, &added_leaf,
+            mutid);
+
+        Py_DECREF(key);
+
+        if (iter_root == NULL) {
+            goto err;
+        }
+
+        if (added_leaf) {
+            last_count++;
+        }
+
+        Py_SETREF(last_root, iter_root);
+    }
+
+    if (key == NULL && PyErr_Occurred()) {
+        goto err;
+    }
+
+    Py_DECREF(it);
+
+    *new_root = last_root;
+    *new_count = last_count;
+
+    return 0;
+
+err:
+    Py_DECREF(it);
+    Py_DECREF(last_root);
+    return -1;
+}
+
+
+static int
+map_node_update_from_seq(uint64_t mutid,
+                         PyObject *seq,
+                         MapNode *root, Py_ssize_t count,
+                         MapNode **new_root, Py_ssize_t *new_count)
+{
+    PyObject *it;
+    Py_ssize_t i;
+    PyObject *item = NULL;
+    PyObject *fast = NULL;
+
+    MapNode *last_root;
+    Py_ssize_t last_count;
+
+    it = PyObject_GetIter(seq);
+    if (it == NULL) {
+        return -1;
+    }
+
+    Py_INCREF(root);
+    last_root = root;
+    last_count = count;
+
+    for (i = 0; ; i++) {
+        PyObject *key, *val;
+        Py_ssize_t n;
+        int32_t key_hash;
+        int added_leaf;
+
+        item = PyIter_Next(it);
+        if (item == NULL) {
+            if (PyErr_Occurred()) {
+                goto err;
+            }
+            break;
+        }
+
+        fast = PySequence_Fast(item, "");
+        if (fast == NULL) {
+            if (PyErr_ExceptionMatches(PyExc_TypeError))
+                PyErr_Format(PyExc_TypeError,
+                    "cannot convert map update "
+                    "sequence element #%zd to a sequence",
+                    i);
+            goto err;
+        }
+
+        n = PySequence_Fast_GET_SIZE(fast);
+        if (n != 2) {
+            PyErr_Format(PyExc_ValueError,
+                         "map update sequence element #%zd "
+                         "has length %zd; 2 is required",
+                         i, n);
+            goto err;
+        }
+
+        key = PySequence_Fast_GET_ITEM(fast, 0);
+        val = PySequence_Fast_GET_ITEM(fast, 1);
+        Py_INCREF(key);
+        Py_INCREF(val);
+
+        key_hash = map_hash(key);
+        if (key_hash == -1) {
+            Py_DECREF(key);
+            Py_DECREF(val);
+            goto err;
+        }
+
+        MapNode *iter_root = map_node_assoc(
+            last_root,
+            0, key_hash, key, val, &added_leaf,
+            mutid);
+
+        Py_DECREF(key);
+        Py_DECREF(val);
+
+        if (iter_root == NULL) {
+            goto err;
+        }
+
+        if (added_leaf) {
+            last_count++;
+        }
+
+        Py_SETREF(last_root, iter_root);
+
+        Py_DECREF(fast);
+        Py_DECREF(item);
+    }
+
+    Py_DECREF(it);
+
+    *new_root = last_root;
+    *new_count = last_count;
+
+    return 0;
+
+err:
+    Py_DECREF(last_root);
+    Py_XDECREF(item);
+    Py_XDECREF(fast);
+    Py_DECREF(it);
+    return -1;
+}
+
+
+static int
+map_node_update(uint64_t mutid,
+                PyObject *src,
+                MapNode *root, Py_ssize_t count,
+                MapNode **new_root, Py_ssize_t *new_count)
+{
+    if (Map_Check(src)) {
+        return map_node_update_from_map(
+            mutid, (MapObject *)src, root, count, new_root, new_count);
+    }
+    else if (PyDict_Check(src)) {
+        return map_node_update_from_dict(
+            mutid, src, root, count, new_root, new_count);
+    }
+    else {
+        return map_node_update_from_seq(
+            mutid, src, root, count, new_root, new_count);
+    }
+}
+
+
+static int
+map_update_inplace(uint64_t mutid, BaseMapObject *o, PyObject *src)
+{
+    MapNode *new_root = NULL;
+    Py_ssize_t new_count;
+
+    int ret = map_node_update(
+        mutid, src,
+        o->b_root, o->b_count,
+        &new_root, &new_count);
+
+    if (ret) {
+        return -1;
+    }
+
+    assert(new_root);
+
+    Py_SETREF(o->b_root, new_root);
+    o->b_count = new_count;
+
+    return 0;
+}
+
+
+static MapObject *
+map_update(uint64_t mutid, MapObject *o, PyObject *src)
+{
+    MapNode *new_root = NULL;
+    Py_ssize_t new_count;
+
+    int ret = map_node_update(
+        mutid, src,
+        o->h_root, o->h_count,
+        &new_root, &new_count);
+
+    if (ret) {
+        return NULL;
+    }
+
+    assert(new_root);
+
+    MapObject *new = map_alloc();
+    if (new == NULL) {
+        Py_DECREF(new_root);
+        return NULL;
+    }
+
+    Py_XSETREF(new->h_root, new_root);
+    new->h_count = new_count;
+
+    return new;
+}
+
+static int
+mapmut_check_finalized(MapMutationObject *o)
+{
+    if (o->m_mutid == 0) {
+        PyErr_Format(
+            PyExc_ValueError,
+            "mutation %R has been finished",
+            o, NULL);
+        return -1;
+    }
+
+    return 0;
+}
+
+static int
+mapmut_delete(MapMutationObject *o, PyObject *key, int32_t key_hash)
+{
+    MapNode *new_root = NULL;
+
+    assert(key_hash != -1);
+    map_without_t res = map_node_without(
+        (MapNode *)(o->m_root),
+        0, key_hash, key,
+        &new_root,
+        o->m_mutid);
+
+    switch (res) {
+        case W_ERROR:
+            return -1;
+
+        case W_EMPTY:
+            new_root = map_node_bitmap_new(0, o->m_mutid);
+            if (new_root == NULL) {
+                return -1;
+            }
+            Py_SETREF(o->m_root, new_root);
+            o->m_count = 0;
+            return 0;
+
+        case W_NOT_FOUND:
+            PyErr_SetObject(PyExc_KeyError, key);
+            return -1;
+
+        case W_NEWNODE: {
+            assert(new_root != NULL);
+            Py_SETREF(o->m_root, new_root);
+            o->m_count--;
+            return 0;
+        }
+
+        default:
+            abort();
+    }
+}
+
+static int
+mapmut_set(MapMutationObject *o, PyObject *key, int32_t key_hash,
+           PyObject *val)
+{
+    int added_leaf = 0;
+
+    assert(key_hash != -1);
+    MapNode *new_root = map_node_assoc(
+        (MapNode *)(o->m_root),
+        0, key_hash, key, val, &added_leaf,
+        o->m_mutid);
+    if (new_root == NULL) {
+        return -1;
+    }
+
+    if (added_leaf) {
+        o->m_count++;
+    }
+
+    if (new_root == o->m_root) {
+        Py_DECREF(new_root);
+        return 0;
+    }
+
+    Py_SETREF(o->m_root, new_root);
+    return 0;
+}
+
+static int
+mapmut_finish(MapMutationObject *o)
+{
+    o->m_mutid = 0;
+    return 0;
+}
+
+static PyObject *
+mapmut_py_set(MapMutationObject *o, PyObject *args)
+{
+    PyObject *key;
+    PyObject *val;
+
+    if (!PyArg_UnpackTuple(args, "set", 2, 2, &key, &val)) {
+        return NULL;
+    }
+
+    if (mapmut_check_finalized(o)) {
+        return NULL;
+    }
+
+    int32_t key_hash = map_hash(key);
+    if (key_hash == -1) {
+        return NULL;
+    }
+
+    if (mapmut_set(o, key, key_hash, val)) {
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+mapmut_tp_richcompare(PyObject *v, PyObject *w, int op)
+{
+    if (!MapMutation_Check(v) || !MapMutation_Check(w) ||
+            (op != Py_EQ && op != Py_NE))
+    {
+        Py_RETURN_NOTIMPLEMENTED;
+    }
+
+    int res = map_eq((BaseMapObject *)v, (BaseMapObject *)w);
+    if (res < 0) {
+        return NULL;
+    }
+
+    if (op == Py_NE) {
+        res = !res;
+    }
+
+    if (res) {
+        Py_RETURN_TRUE;
+    }
+    else {
+        Py_RETURN_FALSE;
+    }
+}
+
+static PyObject *
+mapmut_py_update(MapMutationObject *self, PyObject *args, PyObject *kwds)
+{
+    PyObject *arg = NULL;
+
+    if (!PyArg_UnpackTuple(args, "update", 0, 1, &arg)) {
+        return NULL;
+    }
+
+    if (mapmut_check_finalized(self)) {
+        return NULL;
+    }
+
+    if (arg != NULL) {
+        if (map_update_inplace(self->m_mutid, (BaseMapObject *)self, arg)) {
+            return NULL;
+        }
+    }
+
+    if (kwds != NULL) {
+        if (!PyArg_ValidateKeywordArguments(kwds)) {
+            return NULL;
+        }
+
+        if (map_update_inplace(self->m_mutid, (BaseMapObject *)self, kwds)) {
+            return NULL;
+        }
+    }
+
+    Py_RETURN_NONE;
+}
+
+
+static PyObject *
+mapmut_py_finish(MapMutationObject *self, PyObject *args)
+{
+    if (mapmut_finish(self)) {
+        return NULL;
+    }
+
+    MapObject *o = map_alloc();
+    if (o == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(self->m_root);
+    o->h_root = self->m_root;
+    o->h_count = self->m_count;
+
+    return (PyObject *)o;
+}
+
+static PyObject *
+mapmut_py_enter(MapMutationObject *self, PyObject *args)
+{
+    Py_INCREF(self);
+    return (PyObject *)self;
+}
+
+static PyObject *
+mapmut_py_exit(MapMutationObject *self, PyObject *args)
+{
+    if (mapmut_finish(self)) {
+        return NULL;
+    }
+    Py_RETURN_FALSE;
+}
+
+static int
+mapmut_tp_ass_sub(MapMutationObject *self, PyObject *key, PyObject *val)
+{
+    if (mapmut_check_finalized(self)) {
+        return -1;
+    }
+
+    int32_t key_hash = map_hash(key);
+    if (key_hash == -1) {
+        return -1;
+    }
+
+    if (val == NULL) {
+        return mapmut_delete(self, key, key_hash);
+    }
+    else {
+        return mapmut_set(self, key, key_hash, val);
+    }
+}
+
+static PyObject *
+mapmut_py_pop(MapMutationObject *self, PyObject *args)
+{
+    PyObject *key, *deflt = NULL, *val = NULL;
+
+    if(!PyArg_UnpackTuple(args, "pop", 1, 2, &key, &deflt)) {
+        return NULL;
+    }
+
+    if (mapmut_check_finalized(self)) {
+        return NULL;
+    }
+
+    if (!self->m_count) {
+        goto not_found;
+    }
+
+    int32_t key_hash = map_hash(key);
+    if (key_hash == -1) {
+        return NULL;
+    }
+
+    map_find_t find_res = map_node_find(self->m_root, 0, key_hash, key, &val);
+
+    switch (find_res) {
+        case F_ERROR:
+            return NULL;
+
+        case F_NOT_FOUND:
+            goto not_found;
+
+        case F_FOUND:
+            break;
+
+        default:
+            abort();
+    }
+
+    Py_INCREF(val);
+
+    if (mapmut_delete(self, key, key_hash)) {
+        Py_DECREF(val);
+        return NULL;
+    }
+
+    return val;
+
+not_found:
+    if (deflt) {
+        Py_INCREF(deflt);
+        return deflt;
+    }
+
+    PyErr_SetObject(PyExc_KeyError, key);
+    return NULL;
+}
+
+
+static PyMethodDef MapMutation_methods[] = {
+    {"set", (PyCFunction)mapmut_py_set, METH_VARARGS, NULL},
+    {"get", (PyCFunction)map_py_get, METH_VARARGS, NULL},
+    {"pop", (PyCFunction)mapmut_py_pop, METH_VARARGS, NULL},
+    {"finish", (PyCFunction)mapmut_py_finish, METH_NOARGS, NULL},
+    {"update", (PyCFunction)mapmut_py_update,
+        METH_VARARGS | METH_KEYWORDS, NULL},
+    {"__enter__", (PyCFunction)mapmut_py_enter, METH_NOARGS, NULL},
+    {"__exit__", (PyCFunction)mapmut_py_exit, METH_VARARGS, NULL},
+    {NULL, NULL}
+};
+
+static PySequenceMethods MapMutation_as_sequence = {
+    0,                                /* sq_length */
+    0,                                /* sq_concat */
+    0,                                /* sq_repeat */
+    0,                                /* sq_item */
+    0,                                /* sq_slice */
+    0,                                /* sq_ass_item */
+    0,                                /* sq_ass_slice */
+    (objobjproc)map_tp_contains,      /* sq_contains */
+    0,                                /* sq_inplace_concat */
+    0,                                /* sq_inplace_repeat */
+};
+
+static PyMappingMethods MapMutation_as_mapping = {
+    (lenfunc)map_tp_len,              /* mp_length */
+    (binaryfunc)map_tp_subscript,     /* mp_subscript */
+    (objobjargproc)mapmut_tp_ass_sub, /* mp_subscript */
+};
+
+PyTypeObject _MapMutation_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "immutables._map.MapMutation",
+    sizeof(MapMutationObject),
+    .tp_methods = MapMutation_methods,
+    .tp_as_mapping = &MapMutation_as_mapping,
+    .tp_as_sequence = &MapMutation_as_sequence,
+    .tp_dealloc = (destructor)map_tp_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)map_tp_traverse,
+    .tp_richcompare = mapmut_tp_richcompare,
+    .tp_clear = (inquiry)map_tp_clear,
+    .tp_weaklistoffset = offsetof(MapMutationObject, m_weakreflist),
+    .tp_repr = (reprfunc)map_py_repr,
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+
+/////////////////////////////////// Tree Node Types
+
+
+PyTypeObject _Map_ArrayNode_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "map_array_node",
+    sizeof(MapNode_Array),
+    0,
+    .tp_dealloc = (destructor)map_node_array_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)map_node_array_traverse,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+PyTypeObject _Map_BitmapNode_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "map_bitmap_node",
+    sizeof(MapNode_Bitmap) - sizeof(PyObject *),
+    sizeof(PyObject *),
+    .tp_dealloc = (destructor)map_node_bitmap_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)map_node_bitmap_traverse,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+PyTypeObject _Map_CollisionNode_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "map_collision_node",
+    sizeof(MapNode_Collision) - sizeof(PyObject *),
+    sizeof(PyObject *),
+    .tp_dealloc = (destructor)map_node_collision_dealloc,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)map_node_collision_traverse,
+    .tp_free = PyObject_GC_Del,
+    .tp_hash = PyObject_HashNotImplemented,
+};
+
+
+static void
+module_free(void *m)
+{
+    Py_CLEAR(_empty_bitmap_node);
+}
+
+
+static struct PyModuleDef _mapmodule = {
+    PyModuleDef_HEAD_INIT,      /* m_base */
+    "_map",                     /* m_name */
+    NULL,                       /* m_doc */
+    -1,                         /* m_size */
+    NULL,                       /* m_methods */
+    NULL,                       /* m_slots */
+    NULL,                       /* m_traverse */
+    NULL,                       /* m_clear */
+    module_free,                /* m_free */
+};
+
+
+PyMODINIT_FUNC
+PyInit__map(void)
+{
+    PyObject *m = PyModule_Create(&_mapmodule);
+
+    if ((PyType_Ready(&_Map_Type) < 0) ||
+        (PyType_Ready(&_MapMutation_Type) < 0) ||
+        (PyType_Ready(&_Map_ArrayNode_Type) < 0) ||
+        (PyType_Ready(&_Map_BitmapNode_Type) < 0) ||
+        (PyType_Ready(&_Map_CollisionNode_Type) < 0) ||
+        (PyType_Ready(&_MapKeys_Type) < 0) ||
+        (PyType_Ready(&_MapValues_Type) < 0) ||
+        (PyType_Ready(&_MapItems_Type) < 0) ||
+        (PyType_Ready(&_MapKeysIter_Type) < 0) ||
+        (PyType_Ready(&_MapValuesIter_Type) < 0) ||
+        (PyType_Ready(&_MapItemsIter_Type) < 0))
+    {
+        return 0;
+    }
+
+    Py_INCREF(&_Map_Type);
+    if (PyModule_AddObject(m, "Map", (PyObject *)&_Map_Type) < 0) {
+        Py_DECREF(&_Map_Type);
+        return NULL;
+    }
+
+    return m;
+}
diff --git a/immutables/_map.h b/immutables/_map.h
new file mode 100644
index 0000000..28142ce
--- /dev/null
+++ b/immutables/_map.h
@@ -0,0 +1,107 @@
+#ifndef IMMUTABLES_MAP_H
+#define IMMUTABLES_MAP_H
+
+#include <stdint.h>
+#include "Python.h"
+
+#define _Py_HAMT_MAX_TREE_DEPTH 8
+
+
+#define Map_Check(o) (Py_TYPE(o) == &_Map_Type)
+#define MapMutation_Check(o) (Py_TYPE(o) == &_MapMutation_Type)
+
+
+/* Abstract tree node. */
+typedef struct {
+    PyObject_HEAD
+} MapNode;
+
+
+#define _MapCommonFields(pref)          \
+    PyObject_HEAD                       \
+    MapNode *pref##_root;               \
+    PyObject *pref##_weakreflist;       \
+    Py_ssize_t pref##_count;
+
+
+/* Base mapping struct; used in methods shared between
+   MapObject and MapMutationObject types. */
+typedef struct {
+    _MapCommonFields(b)
+} BaseMapObject;
+
+
+/* An HAMT immutable mapping collection. */
+typedef struct {
+    _MapCommonFields(h)
+    Py_hash_t h_hash;
+} MapObject;
+
+
+/* MapMutation object (returned from `map.mutate()`.) */
+typedef struct {
+    _MapCommonFields(m)
+    uint64_t m_mutid;
+} MapMutationObject;
+
+
+/* A struct to hold the state of depth-first traverse of the tree.
+
+   HAMT is an immutable collection.  Iterators will hold a strong reference
+   to it, and every node in the HAMT has strong references to its children.
+
+   So for iterators, we can implement zero allocations and zero reference
+   inc/dec depth-first iteration.
+
+   - i_nodes: an array of seven pointers to tree nodes
+   - i_level: the current node in i_nodes
+   - i_pos: an array of positions within nodes in i_nodes.
+*/
+typedef struct {
+    MapNode *i_nodes[_Py_HAMT_MAX_TREE_DEPTH];
+    Py_ssize_t i_pos[_Py_HAMT_MAX_TREE_DEPTH];
+    int8_t i_level;
+} MapIteratorState;
+
+
+/* Base iterator object.
+
+   Contains the iteration state, a pointer to the HAMT tree,
+   and a pointer to the 'yield function'.  The latter is a simple
+   function that returns a key/value tuple for the 'Items' iterator,
+   just a key for the 'Keys' iterator, and a value for the 'Values'
+   iterator.
+*/
+
+typedef struct {
+    PyObject_HEAD
+    MapObject *mv_obj;
+    binaryfunc mv_yield;
+    PyTypeObject *mv_itertype;
+} MapView;
+
+typedef struct {
+    PyObject_HEAD
+    MapObject *mi_obj;
+    binaryfunc mi_yield;
+    MapIteratorState mi_iter;
+} MapIterator;
+
+
+/* PyTypes */
+
+
+PyTypeObject _Map_Type;
+PyTypeObject _MapMutation_Type;
+PyTypeObject _Map_ArrayNode_Type;
+PyTypeObject _Map_BitmapNode_Type;
+PyTypeObject _Map_CollisionNode_Type;
+PyTypeObject _MapKeys_Type;
+PyTypeObject _MapValues_Type;
+PyTypeObject _MapItems_Type;
+PyTypeObject _MapKeysIter_Type;
+PyTypeObject _MapValuesIter_Type;
+PyTypeObject _MapItemsIter_Type;
+
+
+#endif
diff --git a/immutables/_map.pyi b/immutables/_map.pyi
new file mode 100644
index 0000000..4590cd4
--- /dev/null
+++ b/immutables/_map.pyi
@@ -0,0 +1,73 @@
+from typing import Any
+from typing import Dict
+from typing import Generic
+from typing import Iterable
+from typing import Iterator
+from typing import Mapping
+from typing import Optional
+from typing import Tuple
+from typing import Type
+from typing import Union
+from typing import overload
+
+from ._protocols import IterableItems
+from ._protocols import MapItems
+from ._protocols import MapKeys
+from ._protocols import MapMutation
+from ._protocols import MapValues
+from ._protocols import HT
+from ._protocols import KT
+from ._protocols import T
+from ._protocols import VT_co
+
+
+class Map(Mapping[KT, VT_co]):
+    @overload
+    def __init__(self) -> None: ...
+    @overload
+    def __init__(self: Map[str, VT_co], **kw: VT_co) -> None: ...
+    @overload
+    def __init__(
+        self, __col: Union[IterableItems[KT, VT_co], Iterable[Tuple[KT, VT_co]]]
+    ) -> None: ...
+    @overload
+    def __init__(
+        self: Map[Union[KT, str], VT_co],
+        __col: Union[IterableItems[KT, VT_co], Iterable[Tuple[KT, VT_co]]],
+        **kw: VT_co
+    ) -> None: ...
+    def __reduce__(self) -> Tuple[Type[Map[KT, VT_co]], Tuple[Dict[KT, VT_co]]]: ...
+    def __len__(self) -> int: ...
+    def __eq__(self, other: Any) -> bool: ...
+    @overload
+    def update(
+        self,
+        __col: Union[IterableItems[KT, VT_co], Iterable[Tuple[KT, VT_co]]]
+    ) -> Map[KT, VT_co]: ...
+    @overload
+    def update(
+        self: Map[Union[HT, str], Any],
+        __col: Union[IterableItems[KT, VT_co], Iterable[Tuple[KT, VT_co]]],
+        **kw: VT_co  # type: ignore[misc]
+    ) -> Map[KT, VT_co]: ...
+    @overload
+    def update(
+        self: Map[Union[HT, str], Any],
+        **kw: VT_co  # type: ignore[misc]
+    ) -> Map[KT, VT_co]: ...
+    def mutate(self) -> MapMutation[KT, VT_co]: ...
+    def set(self, key: KT, val: VT_co) -> Map[KT, VT_co]: ...  # type: ignore[misc]
+    def delete(self, key: KT) -> Map[KT, VT_co]: ...
+    @overload
+    def get(self, key: KT) -> Optional[VT_co]: ...
+    @overload
+    def get(self, key: KT, default: Union[VT_co, T]) -> Union[VT_co, T]: ...
+    def __getitem__(self, key: KT) -> VT_co: ...
+    def __contains__(self, key: Any) -> bool: ...
+    def __iter__(self) -> Iterator[KT]: ...
+    def keys(self) -> MapKeys[KT]: ...  # type: ignore[override]
+    def values(self) -> MapValues[VT_co]: ...  # type: ignore[override]
+    def items(self) -> MapItems[KT, VT_co]: ...  # type: ignore[override]
+    def __hash__(self) -> int: ...
+    def __dump__(self) -> str: ...
+    def __class_getitem__(cls, item: Any) -> Type[Map[Any, Any]]: ...
diff --git a/immutables/_protocols.py b/immutables/_protocols.py
new file mode 100644
index 0000000..de87d23
--- /dev/null
+++ b/immutables/_protocols.py
@@ -0,0 +1,85 @@
+import sys
+from typing import Any
+from typing import Hashable
+from typing import Iterable
+from typing import Iterator
+from typing import NoReturn
+from typing import Optional
+from typing import Tuple
+from typing import TypeVar
+from typing import Union
+from typing import overload
+
+if sys.version_info >= (3, 8):
+    from typing import Protocol
+    from typing import TYPE_CHECKING
+else:
+    from typing_extensions import Protocol
+    from typing_extensions import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from ._map import Map
+
+HT = TypeVar('HT', bound=Hashable)
+KT = TypeVar('KT', bound=Hashable)
+KT_co = TypeVar('KT_co', covariant=True)
+MM = TypeVar('MM', bound='MapMutation[Any, Any]')
+T = TypeVar('T')
+VT = TypeVar('VT')
+VT_co = TypeVar('VT_co', covariant=True)
+
+
+class MapKeys(Protocol[KT_co]):
+    def __len__(self) -> int: ...
+    def __iter__(self) -> Iterator[KT_co]: ...
+
+
+class MapValues(Protocol[VT_co]):
+    def __len__(self) -> int: ...
+    def __iter__(self) -> Iterator[VT_co]: ...
+
+
+class MapItems(Protocol[KT_co, VT_co]):
+    def __len__(self) -> int: ...
+    def __iter__(self) -> Iterator[Tuple[KT_co, VT_co]]: ...
+
+
+class IterableItems(Protocol[KT_co, VT_co]):
+    def items(self) -> Iterable[Tuple[KT_co, VT_co]]: ...
+
+
+class MapMutation(Protocol[KT, VT]):
+    def set(self, key: KT, val: VT) -> None: ...
+    def __enter__(self: MM) -> MM: ...
+    def __exit__(self, *exc: Any) -> bool: ...
+    def __iter__(self) -> NoReturn: ...
+    def __delitem__(self, key: KT) -> None: ...
+    def __setitem__(self, key: KT, val: VT) -> None: ...
+    @overload
+    def pop(self, __key: KT) -> VT: ...
+    @overload
+    def pop(self, __key: KT, __default: T) -> Union[VT, T]: ...
+    @overload
+    def get(self, key: KT) -> Optional[VT]: ...
+    @overload
+    def get(self, key: KT, default: Union[VT, T]) -> Union[VT, T]: ...
+    def __getitem__(self, key: KT) -> VT: ...
+    def __contains__(self, key: object) -> bool: ...
+
+    @overload
+    def update(
+        self,
+        __col: Union[IterableItems[KT, VT], Iterable[Tuple[KT, VT]]]
+    ) -> None: ...
+
+    @overload
+    def update(
+        self: 'MapMutation[Union[HT, str], Any]',
+        __col: Union[IterableItems[KT, VT], Iterable[Tuple[KT, VT]]],
+        **kw: VT
+    ) -> None: ...
+    @overload
+    def update(self: 'MapMutation[Union[HT, str], Any]', **kw: VT) -> None: ...
+    def finish(self) -> 'Map[KT, VT]': ...
+    def __len__(self) -> int: ...
+    def __eq__(self, other: Any) -> bool: ...
diff --git a/immutables/_testutils.py b/immutables/_testutils.py
new file mode 100644
index 0000000..3f174b2
--- /dev/null
+++ b/immutables/_testutils.py
@@ -0,0 +1,80 @@
+class HashKey:
+    _crasher = None
+
+    def __init__(self, hash, name, *, error_on_eq_to=None):
+        assert hash != -1
+        self.name = name
+        self.hash = hash
+        self.error_on_eq_to = error_on_eq_to
+
+    def __repr__(self):
+        if self._crasher is not None and self._crasher.error_on_repr:
+            raise ReprError
+        return '<Key name:{} hash:{}>'.format(self.name, self.hash)
+
+    def __hash__(self):
+        if self._crasher is not None and self._crasher.error_on_hash:
+            raise HashingError
+
+        return self.hash
+
+    def __eq__(self, other):
+        if not isinstance(other, HashKey):
+            return NotImplemented
+
+        if self._crasher is not None and self._crasher.error_on_eq:
+            raise EqError
+
+        if self.error_on_eq_to is not None and self.error_on_eq_to is other:
+            raise ValueError('cannot compare {!r} to {!r}'.format(self, other))
+        if other.error_on_eq_to is not None and other.error_on_eq_to is self:
+            raise ValueError('cannot compare {!r} to {!r}'.format(other, self))
+
+        return (self.name, self.hash) == (other.name, other.hash)
+
+
+class KeyStr(str):
+
+    def __hash__(self):
+        if HashKey._crasher is not None and HashKey._crasher.error_on_hash:
+            raise HashingError
+        return super().__hash__()
+
+    def __eq__(self, other):
+        if HashKey._crasher is not None and HashKey._crasher.error_on_eq:
+            raise EqError
+        return super().__eq__(other)
+
+    def __repr__(self, other):
+        if HashKey._crasher is not None and HashKey._crasher.error_on_repr:
+            raise ReprError
+        return super().__eq__(other)
+
+
+class HashKeyCrasher:
+
+    def __init__(self, *, error_on_hash=False, error_on_eq=False,
+                 error_on_repr=False):
+        self.error_on_hash = error_on_hash
+        self.error_on_eq = error_on_eq
+        self.error_on_repr = error_on_repr
+
+    def __enter__(self):
+        if HashKey._crasher is not None:
+            raise RuntimeError('cannot nest crashers')
+        HashKey._crasher = self
+
+    def __exit__(self, *exc):
+        HashKey._crasher = None
+
+
+class HashingError(Exception):
+    pass
+
+
+class EqError(Exception):
+    pass
+
+
+class ReprError(Exception):
+    pass
diff --git a/immutables/_version.py b/immutables/_version.py
new file mode 100644
index 0000000..b8cc76f
--- /dev/null
+++ b/immutables/_version.py
@@ -0,0 +1,13 @@
+# This file MUST NOT contain anything but the __version__ assignment.
+#
+# When making a release, change the value of __version__
+# to an appropriate value, and open a pull request against
+# the correct branch (master if making a new feature release).
+# The commit message MUST contain a properly formatted release
+# log, and the commit must be signed.
+#
+# The release automation will: build and test the packages for the
+# supported platforms, publish the packages on PyPI, merge the PR
+# to the target branch, create a Git tag pointing to the commit.
+
+__version__ = '0.18'
diff --git a/immutables/map.py b/immutables/map.py
new file mode 100644
index 0000000..0ad2858
--- /dev/null
+++ b/immutables/map.py
@@ -0,0 +1,855 @@
+import collections.abc
+import itertools
+import reprlib
+import sys
+
+
+__all__ = ('Map',)
+
+
+# Thread-safe counter.
+_mut_id = itertools.count(1).__next__
+
+
+# Python version of _map.c.  The topmost comment there explains
+# all datastructures and algorithms.
+# The code here follows C code closely on purpose to make
+# debugging and testing easier.
+
+
+def map_hash(o):
+    x = hash(o)
+    if sys.hash_info.width > 32:
+        return (x & 0xffffffff) ^ ((x >> 32) & 0xffffffff)
+    else:
+        return x
+
+
+def map_mask(hash, shift):
+    return (hash >> shift) & 0x01f
+
+
+def map_bitpos(hash, shift):
+    return 1 << map_mask(hash, shift)
+
+
+def map_bitcount(v):
+    v = v - ((v >> 1) & 0x55555555)
+    v = (v & 0x33333333) + ((v >> 2) & 0x33333333)
+    v = (v & 0x0F0F0F0F) + ((v >> 4) & 0x0F0F0F0F)
+    v = v + (v >> 8)
+    v = (v + (v >> 16)) & 0x3F
+    return v
+
+
+def map_bitindex(bitmap, bit):
+    return map_bitcount(bitmap & (bit - 1))
+
+
+W_EMPTY, W_NEWNODE, W_NOT_FOUND = range(3)
+void = object()
+
+
+class _Unhashable:
+    __slots__ = ()
+    __hash__ = None
+
+
+_NULL = _Unhashable()
+del _Unhashable
+
+
+class BitmapNode:
+
+    def __init__(self, size, bitmap, array, mutid):
+        self.size = size
+        self.bitmap = bitmap
+        assert isinstance(array, list) and len(array) == size
+        self.array = array
+        self.mutid = mutid
+
+    def clone(self, mutid):
+        return BitmapNode(self.size, self.bitmap, self.array.copy(), mutid)
+
+    def assoc(self, shift, hash, key, val, mutid):
+        bit = map_bitpos(hash, shift)
+        idx = map_bitindex(self.bitmap, bit)
+
+        if self.bitmap & bit:
+            key_idx = 2 * idx
+            val_idx = key_idx + 1
+
+            key_or_null = self.array[key_idx]
+            val_or_node = self.array[val_idx]
+
+            if key_or_null is _NULL:
+                sub_node, added = val_or_node.assoc(
+                    shift + 5, hash, key, val, mutid)
+                if val_or_node is sub_node:
+                    return self, added
+
+                if mutid and mutid == self.mutid:
+                    self.array[val_idx] = sub_node
+                    return self, added
+                else:
+                    ret = self.clone(mutid)
+                    ret.array[val_idx] = sub_node
+                    return ret, added
+
+            if key == key_or_null:
+                if val is val_or_node:
+                    return self, False
+
+                if mutid and mutid == self.mutid:
+                    self.array[val_idx] = val
+                    return self, False
+                else:
+                    ret = self.clone(mutid)
+                    ret.array[val_idx] = val
+                    return ret, False
+
+            existing_key_hash = map_hash(key_or_null)
+            if existing_key_hash == hash:
+                sub_node = CollisionNode(
+                    4, hash, [key_or_null, val_or_node, key, val], mutid)
+            else:
+                sub_node = BitmapNode(0, 0, [], mutid)
+                sub_node, _ = sub_node.assoc(
+                    shift + 5, existing_key_hash,
+                    key_or_null, val_or_node,
+                    mutid)
+                sub_node, _ = sub_node.assoc(
+                    shift + 5, hash, key, val,
+                    mutid)
+
+            if mutid and mutid == self.mutid:
+                self.array[key_idx] = _NULL
+                self.array[val_idx] = sub_node
+                return self, True
+            else:
+                ret = self.clone(mutid)
+                ret.array[key_idx] = _NULL
+                ret.array[val_idx] = sub_node
+                return ret, True
+
+        else:
+            key_idx = 2 * idx
+            val_idx = key_idx + 1
+
+            n = map_bitcount(self.bitmap)
+
+            new_array = self.array[:key_idx]
+            new_array.append(key)
+            new_array.append(val)
+            new_array.extend(self.array[key_idx:])
+
+            if mutid and mutid == self.mutid:
+                self.size = 2 * (n + 1)
+                self.bitmap |= bit
+                self.array = new_array
+                return self, True
+            else:
+                return BitmapNode(
+                    2 * (n + 1), self.bitmap | bit, new_array, mutid), True
+
+    def find(self, shift, hash, key):
+        bit = map_bitpos(hash, shift)
+
+        if not (self.bitmap & bit):
+            raise KeyError
+
+        idx = map_bitindex(self.bitmap, bit)
+        key_idx = idx * 2
+        val_idx = key_idx + 1
+
+        key_or_null = self.array[key_idx]
+        val_or_node = self.array[val_idx]
+
+        if key_or_null is _NULL:
+            return val_or_node.find(shift + 5, hash, key)
+
+        if key == key_or_null:
+            return val_or_node
+
+        raise KeyError(key)
+
+    def without(self, shift, hash, key, mutid):
+        bit = map_bitpos(hash, shift)
+        if not (self.bitmap & bit):
+            return W_NOT_FOUND, None
+
+        idx = map_bitindex(self.bitmap, bit)
+        key_idx = 2 * idx
+        val_idx = key_idx + 1
+
+        key_or_null = self.array[key_idx]
+        val_or_node = self.array[val_idx]
+
+        if key_or_null is _NULL:
+            res, sub_node = val_or_node.without(shift + 5, hash, key, mutid)
+
+            if res is W_EMPTY:
+                raise RuntimeError('unreachable code')  # pragma: no cover
+
+            elif res is W_NEWNODE:
+                if (type(sub_node) is BitmapNode and
+                        sub_node.size == 2 and
+                        sub_node.array[0] is not _NULL):
+
+                    if mutid and mutid == self.mutid:
+                        self.array[key_idx] = sub_node.array[0]
+                        self.array[val_idx] = sub_node.array[1]
+                        return W_NEWNODE, self
+                    else:
+                        clone = self.clone(mutid)
+                        clone.array[key_idx] = sub_node.array[0]
+                        clone.array[val_idx] = sub_node.array[1]
+                        return W_NEWNODE, clone
+
+                if mutid and mutid == self.mutid:
+                    self.array[val_idx] = sub_node
+                    return W_NEWNODE, self
+                else:
+                    clone = self.clone(mutid)
+                    clone.array[val_idx] = sub_node
+                    return W_NEWNODE, clone
+
+            else:
+                assert sub_node is None
+                return res, None
+
+        else:
+            if key == key_or_null:
+                if self.size == 2:
+                    return W_EMPTY, None
+
+                new_array = self.array[:key_idx]
+                new_array.extend(self.array[val_idx + 1:])
+
+                if mutid and mutid == self.mutid:
+                    self.size -= 2
+                    self.bitmap &= ~bit
+                    self.array = new_array
+                    return W_NEWNODE, self
+                else:
+                    new_node = BitmapNode(
+                        self.size - 2, self.bitmap & ~bit, new_array, mutid)
+                    return W_NEWNODE, new_node
+
+            else:
+                return W_NOT_FOUND, None
+
+    def keys(self):
+        for i in range(0, self.size, 2):
+            key_or_null = self.array[i]
+
+            if key_or_null is _NULL:
+                val_or_node = self.array[i + 1]
+                yield from val_or_node.keys()
+            else:
+                yield key_or_null
+
+    def values(self):
+        for i in range(0, self.size, 2):
+            key_or_null = self.array[i]
+            val_or_node = self.array[i + 1]
+
+            if key_or_null is _NULL:
+                yield from val_or_node.values()
+            else:
+                yield val_or_node
+
+    def items(self):
+        for i in range(0, self.size, 2):
+            key_or_null = self.array[i]
+            val_or_node = self.array[i + 1]
+
+            if key_or_null is _NULL:
+                yield from val_or_node.items()
+            else:
+                yield key_or_null, val_or_node
+
+    def dump(self, buf, level):  # pragma: no cover
+        buf.append(
+            '    ' * (level + 1) +
+            'BitmapNode(size={} count={} bitmap={} id={:0x}):'.format(
+                self.size, self.size / 2, bin(self.bitmap), id(self)))
+
+        for i in range(0, self.size, 2):
+            key_or_null = self.array[i]
+            val_or_node = self.array[i + 1]
+
+            pad = '    ' * (level + 2)
+
+            if key_or_null is _NULL:
+                buf.append(pad + 'NULL:')
+                val_or_node.dump(buf, level + 2)
+            else:
+                buf.append(pad + '{!r}: {!r}'.format(key_or_null, val_or_node))
+
+
+class CollisionNode:
+
+    def __init__(self, size, hash, array, mutid):
+        self.size = size
+        self.hash = hash
+        self.array = array
+        self.mutid = mutid
+
+    def find_index(self, key):
+        for i in range(0, self.size, 2):
+            if self.array[i] == key:
+                return i
+        return -1
+
+    def find(self, shift, hash, key):
+        for i in range(0, self.size, 2):
+            if self.array[i] == key:
+                return self.array[i + 1]
+        raise KeyError(key)
+
+    def assoc(self, shift, hash, key, val, mutid):
+        if hash == self.hash:
+            key_idx = self.find_index(key)
+
+            if key_idx == -1:
+                new_array = self.array.copy()
+                new_array.append(key)
+                new_array.append(val)
+
+                if mutid and mutid == self.mutid:
+                    self.size += 2
+                    self.array = new_array
+                    return self, True
+                else:
+                    new_node = CollisionNode(
+                        self.size + 2, hash, new_array, mutid)
+                    return new_node, True
+
+            val_idx = key_idx + 1
+            if self.array[val_idx] is val:
+                return self, False
+
+            if mutid and mutid == self.mutid:
+                self.array[val_idx] = val
+                return self, False
+            else:
+                new_array = self.array.copy()
+                new_array[val_idx] = val
+                return CollisionNode(self.size, hash, new_array, mutid), False
+
+        else:
+            new_node = BitmapNode(
+                2, map_bitpos(self.hash, shift), [_NULL, self], mutid)
+            return new_node.assoc(shift, hash, key, val, mutid)
+
+    def without(self, shift, hash, key, mutid):
+        if hash != self.hash:
+            return W_NOT_FOUND, None
+
+        key_idx = self.find_index(key)
+        if key_idx == -1:
+            return W_NOT_FOUND, None
+
+        new_size = self.size - 2
+        if new_size == 0:
+            # Shouldn't be ever reachable
+            return W_EMPTY, None  # pragma: no cover
+
+        if new_size == 2:
+            if key_idx == 0:
+                new_array = [self.array[2], self.array[3]]
+            else:
+                assert key_idx == 2
+                new_array = [self.array[0], self.array[1]]
+
+            new_node = BitmapNode(
+                2, map_bitpos(hash, shift), new_array, mutid)
+            return W_NEWNODE, new_node
+
+        new_array = self.array[:key_idx]
+        new_array.extend(self.array[key_idx + 2:])
+        if mutid and mutid == self.mutid:
+            self.array = new_array
+            self.size -= 2
+            return W_NEWNODE, self
+        else:
+            new_node = CollisionNode(
+                self.size - 2, self.hash, new_array, mutid)
+            return W_NEWNODE, new_node
+
+    def keys(self):
+        for i in range(0, self.size, 2):
+            yield self.array[i]
+
+    def values(self):
+        for i in range(1, self.size, 2):
+            yield self.array[i]
+
+    def items(self):
+        for i in range(0, self.size, 2):
+            yield self.array[i], self.array[i + 1]
+
+    def dump(self, buf, level):  # pragma: no cover
+        pad = '    ' * (level + 1)
+        buf.append(
+            pad + 'CollisionNode(size={} id={:0x}):'.format(
+                self.size, id(self)))
+
+        pad = '    ' * (level + 2)
+        for i in range(0, self.size, 2):
+            key = self.array[i]
+            val = self.array[i + 1]
+
+            buf.append('{}{!r}: {!r}'.format(pad, key, val))
+
+
+class MapKeys:
+
+    def __init__(self, c, m):
+        self.__count = c
+        self.__root = m
+
+    def __len__(self):
+        return self.__count
+
+    def __iter__(self):
+        return iter(self.__root.keys())
+
+
+class MapValues:
+
+    def __init__(self, c, m):
+        self.__count = c
+        self.__root = m
+
+    def __len__(self):
+        return self.__count
+
+    def __iter__(self):
+        return iter(self.__root.values())
+
+
+class MapItems:
+
+    def __init__(self, c, m):
+        self.__count = c
+        self.__root = m
+
+    def __len__(self):
+        return self.__count
+
+    def __iter__(self):
+        return iter(self.__root.items())
+
+
+class Map:
+
+    def __init__(self, *args, **kw):
+        if not args:
+            col = None
+        elif len(args) == 1:
+            col = args[0]
+        else:
+            raise TypeError(
+                "immutables.Map expected at most 1 arguments, "
+                "got {}".format(len(args))
+            )
+
+        self.__count = 0
+        self.__root = BitmapNode(0, 0, [], 0)
+        self.__hash = -1
+
+        if isinstance(col, Map):
+            self.__count = col.__count
+            self.__root = col.__root
+            self.__hash = col.__hash
+            col = None
+        elif isinstance(col, MapMutation):
+            raise TypeError('cannot create Maps from MapMutations')
+
+        if col or kw:
+            init = self.update(col, **kw)
+            self.__count = init.__count
+            self.__root = init.__root
+
+    @classmethod
+    def _new(cls, count, root):
+        m = Map.__new__(Map)
+        m.__count = count
+        m.__root = root
+        m.__hash = -1
+        return m
+
+    def __reduce__(self):
+        return (type(self), (dict(self.items()),))
+
+    def __len__(self):
+        return self.__count
+
+    def __eq__(self, other):
+        if not isinstance(other, Map):
+            return NotImplemented
+
+        if len(self) != len(other):
+            return False
+
+        for key, val in self.__root.items():
+            try:
+                oval = other.__root.find(0, map_hash(key), key)
+            except KeyError:
+                return False
+            else:
+                if oval != val:
+                    return False
+
+        return True
+
+    def update(self, *args, **kw):
+        if not args:
+            col = None
+        elif len(args) == 1:
+            col = args[0]
+        else:
+            raise TypeError(
+                "update expected at most 1 arguments, got {}".format(len(args))
+            )
+
+        it = None
+
+        if col is not None:
+            if hasattr(col, 'items'):
+                it = iter(col.items())
+            else:
+                it = iter(col)
+
+        if it is not None:
+            if kw:
+                it = iter(itertools.chain(it, kw.items()))
+        else:
+            if kw:
+                it = iter(kw.items())
+
+        if it is None:
+
+            return self
+
+        mutid = _mut_id()
+        root = self.__root
+        count = self.__count
+
+        i = 0
+        while True:
+            try:
+                tup = next(it)
+            except StopIteration:
+                break
+
+            try:
+                tup = tuple(tup)
+            except TypeError:
+                raise TypeError(
+                    'cannot convert map update '
+                    'sequence element #{} to a sequence'.format(i)) from None
+            key, val, *r = tup
+            if r:
+                raise ValueError(
+                    'map update sequence element #{} has length '
+                    '{}; 2 is required'.format(i, len(r) + 2))
+
+            root, added = root.assoc(0, map_hash(key), key, val, mutid)
+            if added:
+                count += 1
+
+            i += 1
+
+        return Map._new(count, root)
+
+    def mutate(self):
+        return MapMutation(self.__count, self.__root)
+
+    def set(self, key, val):
+        new_count = self.__count
+        new_root, added = self.__root.assoc(0, map_hash(key), key, val, 0)
+
+        if new_root is self.__root:
+            assert not added
+            return self
+
+        if added:
+            new_count += 1
+
+        return Map._new(new_count, new_root)
+
+    def delete(self, key):
+        res, node = self.__root.without(0, map_hash(key), key, 0)
+        if res is W_EMPTY:
+            return Map()
+        elif res is W_NOT_FOUND:
+            raise KeyError(key)
+        else:
+            return Map._new(self.__count - 1, node)
+
+    def get(self, key, default=None):
+        try:
+            return self.__root.find(0, map_hash(key), key)
+        except KeyError:
+            return default
+
+    def __getitem__(self, key):
+        return self.__root.find(0, map_hash(key), key)
+
+    def __contains__(self, key):
+        try:
+            self.__root.find(0, map_hash(key), key)
+        except KeyError:
+            return False
+        else:
+            return True
+
+    def __iter__(self):
+        yield from self.__root.keys()
+
+    def keys(self):
+        return MapKeys(self.__count, self.__root)
+
+    def values(self):
+        return MapValues(self.__count, self.__root)
+
+    def items(self):
+        return MapItems(self.__count, self.__root)
+
+    def __hash__(self):
+        if self.__hash != -1:
+            return self.__hash
+
+        MAX = sys.maxsize
+        MASK = 2 * MAX + 1
+
+        h = 1927868237 * (self.__count * 2 + 1)
+        h &= MASK
+
+        for key, value in self.__root.items():
+            hx = hash(key)
+            h ^= (hx ^ (hx << 16) ^ 89869747) * 3644798167
+            h &= MASK
+
+            hx = hash(value)
+            h ^= (hx ^ (hx << 16) ^ 89869747) * 3644798167
+            h &= MASK
+
+        h = h * 69069 + 907133923
+        h &= MASK
+
+        if h > MAX:
+            h -= MASK + 1  # pragma: no cover
+        if h == -1:
+            h = 590923713  # pragma: no cover
+
+        self.__hash = h
+        return h
+
+    @reprlib.recursive_repr("{...}")
+    def __repr__(self):
+        items = []
+        for key, val in self.items():
+            items.append("{!r}: {!r}".format(key, val))
+        return 'immutables.Map({{{}}})'.format(', '.join(items))
+
+    def __dump__(self):  # pragma: no cover
+        buf = []
+        self.__root.dump(buf, 0)
+        return '\n'.join(buf)
+
+    def __class_getitem__(cls, item):
+        return cls
+
+
+class MapMutation:
+
+    def __init__(self, count, root):
+        self.__count = count
+        self.__root = root
+        self.__mutid = _mut_id()
+
+    def set(self, key, val):
+        self[key] = val
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        self.finish()
+        return False
+
+    def __iter__(self):
+        raise TypeError('{} is not iterable'.format(type(self)))
+
+    def __delitem__(self, key):
+        if self.__mutid == 0:
+            raise ValueError('mutation {!r} has been finished'.format(self))
+
+        res, new_root = self.__root.without(
+            0, map_hash(key), key, self.__mutid)
+        if res is W_EMPTY:
+            self.__count = 0
+            self.__root = BitmapNode(0, 0, [], self.__mutid)
+        elif res is W_NOT_FOUND:
+            raise KeyError(key)
+        else:
+            self.__root = new_root
+            self.__count -= 1
+
+    def __setitem__(self, key, val):
+        if self.__mutid == 0:
+            raise ValueError('mutation {!r} has been finished'.format(self))
+
+        self.__root, added = self.__root.assoc(
+            0, map_hash(key), key, val, self.__mutid)
+
+        if added:
+            self.__count += 1
+
+    def pop(self, key, *args):
+        if self.__mutid == 0:
+            raise ValueError('mutation {!r} has been finished'.format(self))
+
+        if len(args) > 1:
+            raise TypeError(
+                'pop() accepts 1 to 2 positional arguments, '
+                'got {}'.format(len(args) + 1))
+        elif len(args) == 1:
+            default = args[0]
+        else:
+            default = void
+
+        val = self.get(key, default)
+
+        try:
+            del self[key]
+        except KeyError:
+            if val is void:
+                raise
+            return val
+        else:
+            assert val is not void
+            return val
+
+    def get(self, key, default=None):
+        try:
+            return self.__root.find(0, map_hash(key), key)
+        except KeyError:
+            return default
+
+    def __getitem__(self, key):
+        return self.__root.find(0, map_hash(key), key)
+
+    def __contains__(self, key):
+        try:
+            self.__root.find(0, map_hash(key), key)
+        except KeyError:
+            return False
+        else:
+            return True
+
+    def update(self, *args, **kw):
+        if not args:
+            col = None
+        elif len(args) == 1:
+            col = args[0]
+        else:
+            raise TypeError(
+                "update expected at most 1 arguments, got {}".format(len(args))
+            )
+
+        if self.__mutid == 0:
+            raise ValueError('mutation {!r} has been finished'.format(self))
+
+        it = None
+        if col is not None:
+            if hasattr(col, 'items'):
+                it = iter(col.items())
+            else:
+                it = iter(col)
+
+        if it is not None:
+            if kw:
+                it = iter(itertools.chain(it, kw.items()))
+        else:
+            if kw:
+                it = iter(kw.items())
+
+        if it is None:
+            return
+
+        root = self.__root
+        count = self.__count
+
+        i = 0
+        while True:
+            try:
+                tup = next(it)
+            except StopIteration:
+                break
+
+            try:
+                tup = tuple(tup)
+            except TypeError:
+                raise TypeError(
+                    'cannot convert map update '
+                    'sequence element #{} to a sequence'.format(i)) from None
+            key, val, *r = tup
+            if r:
+                raise ValueError(
+                    'map update sequence element #{} has length '
+                    '{}; 2 is required'.format(i, len(r) + 2))
+
+            root, added = root.assoc(0, map_hash(key), key, val, self.__mutid)
+            if added:
+                count += 1
+
+            i += 1
+
+        self.__root = root
+        self.__count = count
+
+    def finish(self):
+        self.__mutid = 0
+        return Map._new(self.__count, self.__root)
+
+    @reprlib.recursive_repr("{...}")
+    def __repr__(self):
+        items = []
+        for key, val in self.__root.items():
+            items.append("{!r}: {!r}".format(key, val))
+        return 'immutables.MapMutation({{{}}})'.format(', '.join(items))
+
+    def __len__(self):
+        return self.__count
+
+    def __reduce__(self):
+        raise TypeError("can't pickle {} objects".format(type(self).__name__))
+
+    def __hash__(self):
+        raise TypeError('unhashable type: {}'.format(type(self).__name__))
+
+    def __eq__(self, other):
+        if not isinstance(other, MapMutation):
+            return NotImplemented
+
+        if len(self) != len(other):
+            return False
+
+        for key, val in self.__root.items():
+            try:
+                oval = other.__root.find(0, map_hash(key), key)
+            except KeyError:
+                return False
+            else:
+                if oval != val:
+                    return False
+
+        return True
+
+
+collections.abc.Mapping.register(Map)
diff --git a/immutables/py.typed b/immutables/py.typed
new file mode 100644
index 0000000..b648ac9
--- /dev/null
+++ b/immutables/py.typed
@@ -0,0 +1 @@
+partial
diff --git a/immutables/pythoncapi_compat.h b/immutables/pythoncapi_compat.h
new file mode 100644
index 0000000..2837467
--- /dev/null
+++ b/immutables/pythoncapi_compat.h
@@ -0,0 +1,446 @@
+// Header file providing new C API functions to old Python versions.
+//
+// File distributed under the Zero Clause BSD (0BSD) license.
+// Copyright Contributors to the pythoncapi_compat project.
+//
+// Homepage:
+// https://github.com/python/pythoncapi_compat
+//
+// Latest version:
+// https://raw.githubusercontent.com/python/pythoncapi_compat/master/pythoncapi_compat.h
+//
+// SPDX-License-Identifier: 0BSD
+
+#ifndef PYTHONCAPI_COMPAT
+#define PYTHONCAPI_COMPAT
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <Python.h>
+#include "frameobject.h"          // PyFrameObject, PyFrame_GetBack()
+
+
+// Compatibility with Visual Studio 2013 and older which don't support
+// the inline keyword in C (only in C++): use __inline instead.
+#if (defined(_MSC_VER) && _MSC_VER < 1900 \
+     && !defined(__cplusplus) && !defined(inline))
+#  define PYCAPI_COMPAT_INLINE(TYPE static __inline TYPE
+#else
+#  define PYCAPI_COMPAT_STATIC_INLINE(TYPE) static inline TYPE
+#endif
+
+
+// C++ compatibility
+#ifdef __cplusplus
+#  define PYCAPI_COMPAT_CAST(TYPE, EXPR) reinterpret_cast<TYPE>(EXPR)
+#  define PYCAPI_COMPAT_NULL nullptr
+#else
+#  define PYCAPI_COMPAT_CAST(TYPE, EXPR) ((TYPE)(EXPR))
+#  define PYCAPI_COMPAT_NULL NULL
+#endif
+
+// Cast argument to PyObject* type.
+#ifndef _PyObject_CAST
+#  define _PyObject_CAST(op) PYCAPI_COMPAT_CAST(PyObject*, op)
+#endif
+#ifndef _PyObject_CAST_CONST
+#  define _PyObject_CAST_CONST(op) PYCAPI_COMPAT_CAST(const PyObject*, op)
+#endif
+
+
+// bpo-42262 added Py_NewRef() to Python 3.10.0a3
+#if PY_VERSION_HEX < 0x030A00A3 && !defined(Py_NewRef)
+PYCAPI_COMPAT_STATIC_INLINE(PyObject*)
+_Py_NewRef(PyObject *obj)
+{
+    Py_INCREF(obj);
+    return obj;
+}
+#define Py_NewRef(obj) _Py_NewRef(_PyObject_CAST(obj))
+#endif
+
+
+// bpo-42262 added Py_XNewRef() to Python 3.10.0a3
+#if PY_VERSION_HEX < 0x030A00A3 && !defined(Py_XNewRef)
+PYCAPI_COMPAT_STATIC_INLINE(PyObject*)
+_Py_XNewRef(PyObject *obj)
+{
+    Py_XINCREF(obj);
+    return obj;
+}
+#define Py_XNewRef(obj) _Py_XNewRef(_PyObject_CAST(obj))
+#endif
+
+
+// See https://bugs.python.org/issue42522
+#if !defined(_Py_StealRef)
+PYCAPI_COMPAT_STATIC_INLINE(PyObject*)
+__Py_StealRef(PyObject *obj)
+{
+    Py_DECREF(obj);
+    return obj;
+}
+#define _Py_StealRef(obj) __Py_StealRef(_PyObject_CAST(obj))
+#endif
+
+
+// See https://bugs.python.org/issue42522
+#if !defined(_Py_XStealRef)
+PYCAPI_COMPAT_STATIC_INLINE(PyObject*)
+__Py_XStealRef(PyObject *obj)
+{
+    Py_XDECREF(obj);
+    return obj;
+}
+#define _Py_XStealRef(obj) __Py_XStealRef(_PyObject_CAST(obj))
+#endif
+
+
+// bpo-39573 added Py_SET_REFCNT() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_REFCNT)
+PYCAPI_COMPAT_STATIC_INLINE(void)
+_Py_SET_REFCNT(PyObject *ob, Py_ssize_t refcnt)
+{
+    ob->ob_refcnt = refcnt;
+}
+#define Py_SET_REFCNT(ob, refcnt) _Py_SET_REFCNT(_PyObject_CAST(ob), refcnt)
+#endif
+
+
+// Py_SETREF() and Py_XSETREF() were added to Python 3.5.2.
+// It is excluded from the limited C API.
+#if (PY_VERSION_HEX < 0x03050200 && !defined(Py_SETREF)) && !defined(Py_LIMITED_API)
+#define Py_SETREF(op, op2)                      \
+    do {                                        \
+        PyObject *_py_tmp = _PyObject_CAST(op); \
+        (op) = (op2);                           \
+        Py_DECREF(_py_tmp);                     \
+    } while (0)
+
+#define Py_XSETREF(op, op2)                     \
+    do {                                        \
+        PyObject *_py_tmp = _PyObject_CAST(op); \
+        (op) = (op2);                           \
+        Py_XDECREF(_py_tmp);                    \
+    } while (0)
+#endif
+
+
+// bpo-43753 added Py_Is(), Py_IsNone(), Py_IsTrue() and Py_IsFalse()
+// to Python 3.10.0b1.
+#if PY_VERSION_HEX < 0x030A00B1 && !defined(Py_Is)
+#  define Py_Is(x, y) ((x) == (y))
+#endif
+#if PY_VERSION_HEX < 0x030A00B1 && !defined(Py_IsNone)
+#  define Py_IsNone(x) Py_Is(x, Py_None)
+#endif
+#if PY_VERSION_HEX < 0x030A00B1 && !defined(Py_IsTrue)
+#  define Py_IsTrue(x) Py_Is(x, Py_True)
+#endif
+#if PY_VERSION_HEX < 0x030A00B1 && !defined(Py_IsFalse)
+#  define Py_IsFalse(x) Py_Is(x, Py_False)
+#endif
+
+
+// bpo-39573 added Py_SET_TYPE() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_TYPE)
+PYCAPI_COMPAT_STATIC_INLINE(void)
+_Py_SET_TYPE(PyObject *ob, PyTypeObject *type)
+{
+    ob->ob_type = type;
+}
+#define Py_SET_TYPE(ob, type) _Py_SET_TYPE(_PyObject_CAST(ob), type)
+#endif
+
+
+// bpo-39573 added Py_SET_SIZE() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_SIZE)
+PYCAPI_COMPAT_STATIC_INLINE(void)
+_Py_SET_SIZE(PyVarObject *ob, Py_ssize_t size)
+{
+    ob->ob_size = size;
+}
+#define Py_SET_SIZE(ob, size) _Py_SET_SIZE((PyVarObject*)(ob), size)
+#endif
+
+
+// bpo-40421 added PyFrame_GetCode() to Python 3.9.0b1
+#if PY_VERSION_HEX < 0x030900B1
+PYCAPI_COMPAT_STATIC_INLINE(PyCodeObject*)
+PyFrame_GetCode(PyFrameObject *frame)
+{
+    assert(frame != PYCAPI_COMPAT_NULL);
+    assert(frame->f_code != PYCAPI_COMPAT_NULL);
+    return PYCAPI_COMPAT_CAST(PyCodeObject*, Py_NewRef(frame->f_code));
+}
+#endif
+
+PYCAPI_COMPAT_STATIC_INLINE(PyCodeObject*)
+_PyFrame_GetCodeBorrow(PyFrameObject *frame)
+{
+    return PYCAPI_COMPAT_CAST(PyCodeObject *,
+                              _Py_StealRef(PyFrame_GetCode(frame)));
+}
+
+
+// bpo-40421 added PyFrame_GetCode() to Python 3.9.0b1
+#if PY_VERSION_HEX < 0x030900B1 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(PyFrameObject*)
+PyFrame_GetBack(PyFrameObject *frame)
+{
+    assert(frame != PYCAPI_COMPAT_NULL);
+    return PYCAPI_COMPAT_CAST(PyFrameObject*, Py_XNewRef(frame->f_back));
+}
+#endif
+
+#if !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(PyFrameObject*)
+_PyFrame_GetBackBorrow(PyFrameObject *frame)
+{
+    return PYCAPI_COMPAT_CAST(PyFrameObject *,
+                              _Py_XStealRef(PyFrame_GetBack(frame)));
+}
+#endif
+
+
+// bpo-39947 added PyThreadState_GetInterpreter() to Python 3.9.0a5
+#if PY_VERSION_HEX < 0x030900A5
+PYCAPI_COMPAT_STATIC_INLINE(PyInterpreterState *)
+PyThreadState_GetInterpreter(PyThreadState *tstate)
+{
+    assert(tstate != PYCAPI_COMPAT_NULL);
+    return tstate->interp;
+}
+#endif
+
+
+// bpo-40429 added PyThreadState_GetFrame() to Python 3.9.0b1
+#if PY_VERSION_HEX < 0x030900B1 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(PyFrameObject*)
+PyThreadState_GetFrame(PyThreadState *tstate)
+{
+    assert(tstate != PYCAPI_COMPAT_NULL);
+    return PYCAPI_COMPAT_CAST(PyFrameObject *, Py_XNewRef(tstate->frame));
+}
+#endif
+
+#if !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(PyFrameObject*)
+_PyThreadState_GetFrameBorrow(PyThreadState *tstate)
+{
+    return PYCAPI_COMPAT_CAST(PyFrameObject*,
+                              _Py_XStealRef(PyThreadState_GetFrame(tstate)));
+}
+#endif
+
+
+// bpo-39947 added PyInterpreterState_Get() to Python 3.9.0a5
+#if PY_VERSION_HEX < 0x030900A5
+PYCAPI_COMPAT_STATIC_INLINE(PyInterpreterState*)
+PyInterpreterState_Get(void)
+{
+    PyThreadState *tstate;
+    PyInterpreterState *interp;
+
+    tstate = PyThreadState_GET();
+    if (tstate == PYCAPI_COMPAT_NULL) {
+        Py_FatalError("GIL released (tstate is NULL)");
+    }
+    interp = tstate->interp;
+    if (interp == PYCAPI_COMPAT_NULL) {
+        Py_FatalError("no current interpreter");
+    }
+    return interp;
+}
+#endif
+
+
+// bpo-39947 added PyInterpreterState_Get() to Python 3.9.0a6
+#if 0x030700A1 <= PY_VERSION_HEX && PY_VERSION_HEX < 0x030900A6 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(uint64_t)
+PyThreadState_GetID(PyThreadState *tstate)
+{
+    assert(tstate != PYCAPI_COMPAT_NULL);
+    return tstate->id;
+}
+#endif
+
+// bpo-43760 added PyThreadState_EnterTracing() to Python 3.11.0a2
+#if PY_VERSION_HEX < 0x030B00A2 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(void)
+PyThreadState_EnterTracing(PyThreadState *tstate)
+{
+    tstate->tracing++;
+#if PY_VERSION_HEX >= 0x030A00A1
+    tstate->cframe->use_tracing = 0;
+#else
+    tstate->use_tracing = 0;
+#endif
+}
+#endif
+
+// bpo-43760 added PyThreadState_LeaveTracing() to Python 3.11.0a2
+#if PY_VERSION_HEX < 0x030B00A2 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(void)
+PyThreadState_LeaveTracing(PyThreadState *tstate)
+{
+    int use_tracing = (tstate->c_tracefunc != PYCAPI_COMPAT_NULL
+                       || tstate->c_profilefunc != PYCAPI_COMPAT_NULL);
+    tstate->tracing--;
+#if PY_VERSION_HEX >= 0x030A00A1
+    tstate->cframe->use_tracing = use_tracing;
+#else
+    tstate->use_tracing = use_tracing;
+#endif
+}
+#endif
+
+
+// bpo-37194 added PyObject_CallNoArgs() to Python 3.9.0a1
+#if PY_VERSION_HEX < 0x030900A1
+PYCAPI_COMPAT_STATIC_INLINE(PyObject*)
+PyObject_CallNoArgs(PyObject *func)
+{
+    return PyObject_CallFunctionObjArgs(func, NULL);
+}
+#endif
+
+
+// bpo-39245 made PyObject_CallOneArg() public (previously called
+// _PyObject_CallOneArg) in Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4
+PYCAPI_COMPAT_STATIC_INLINE(PyObject*)
+PyObject_CallOneArg(PyObject *func, PyObject *arg)
+{
+    return PyObject_CallFunctionObjArgs(func, arg, NULL);
+}
+#endif
+
+
+// bpo-1635741 added PyModule_AddObjectRef() to Python 3.10.0a3
+#if PY_VERSION_HEX < 0x030A00A3
+PYCAPI_COMPAT_STATIC_INLINE(int)
+PyModule_AddObjectRef(PyObject *mod, const char *name, PyObject *value)
+{
+    int res;
+    Py_XINCREF(value);
+    res = PyModule_AddObject(mod, name, value);
+    if (res < 0) {
+        Py_XDECREF(value);
+    }
+    return res;
+}
+#endif
+
+
+// bpo-40024 added PyModule_AddType() to Python 3.9.0a5
+#if PY_VERSION_HEX < 0x030900A5
+PYCAPI_COMPAT_STATIC_INLINE(int)
+PyModule_AddType(PyObject *mod, PyTypeObject *type)
+{
+    const char *name, *dot;
+
+    if (PyType_Ready(type) < 0) {
+        return -1;
+    }
+
+    // inline _PyType_Name()
+    name = type->tp_name;
+    assert(name != PYCAPI_COMPAT_NULL);
+    dot = strrchr(name, '.');
+    if (dot != PYCAPI_COMPAT_NULL) {
+        name = dot + 1;
+    }
+
+    return PyModule_AddObjectRef(mod, name, _PyObject_CAST(type));
+}
+#endif
+
+
+// bpo-40241 added PyObject_GC_IsTracked() to Python 3.9.0a6.
+// bpo-4688 added _PyObject_GC_IS_TRACKED() to Python 2.7.0a2.
+#if PY_VERSION_HEX < 0x030900A6 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(int)
+PyObject_GC_IsTracked(PyObject* obj)
+{
+    return (PyObject_IS_GC(obj) && _PyObject_GC_IS_TRACKED(obj));
+}
+#endif
+
+// bpo-40241 added PyObject_GC_IsFinalized() to Python 3.9.0a6.
+// bpo-18112 added _PyGCHead_FINALIZED() to Python 3.4.0 final.
+#if PY_VERSION_HEX < 0x030900A6 && PY_VERSION_HEX >= 0x030400F0 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(int)
+PyObject_GC_IsFinalized(PyObject *obj)
+{
+    PyGC_Head *gc = PYCAPI_COMPAT_CAST(PyGC_Head *, obj) - 1;
+    return (PyObject_IS_GC(obj) && _PyGCHead_FINALIZED(gc));
+}
+#endif
+
+
+// bpo-39573 added Py_IS_TYPE() to Python 3.9.0a4
+#if PY_VERSION_HEX < 0x030900A4 && !defined(Py_IS_TYPE)
+PYCAPI_COMPAT_STATIC_INLINE(int)
+_Py_IS_TYPE(const PyObject *ob, const PyTypeObject *type) {
+    return ob->ob_type == type;
+}
+#define Py_IS_TYPE(ob, type) _Py_IS_TYPE(_PyObject_CAST_CONST(ob), type)
+#endif
+
+
+// bpo-46906 added PyFloat_Pack2() and PyFloat_Unpack2() to Python 3.11a7.
+// bpo-11734 added _PyFloat_Pack2() and _PyFloat_Unpack2() to Python 3.6.0b1.
+// Python 3.11a2 moved _PyFloat_Pack2() and _PyFloat_Unpack2() to the internal
+// C API: Python 3.11a2-3.11a6 versions are not supported.
+#if 0x030600B1 <= PY_VERSION_HEX && PY_VERSION_HEX <= 0x030B00A1 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(int)
+PyFloat_Pack2(double x, char *p, int le)
+{ return _PyFloat_Pack2(x, (unsigned char*)p, le); }
+
+PYCAPI_COMPAT_STATIC_INLINE(double)
+PyFloat_Unpack2(const char *p, int le)
+{ return _PyFloat_Unpack2((const unsigned char *)p, le); }
+#endif
+
+
+// bpo-46906 added PyFloat_Pack4(), PyFloat_Pack8(), PyFloat_Unpack4() and
+// PyFloat_Unpack8() to Python 3.11a7.
+// Python 3.11a2 moved _PyFloat_Pack4(), _PyFloat_Pack8(), _PyFloat_Unpack4()
+// and _PyFloat_Unpack8() to the internal C API: Python 3.11a2-3.11a6 versions
+// are not supported.
+#if PY_VERSION_HEX <= 0x030B00A1 && !defined(PYPY_VERSION)
+PYCAPI_COMPAT_STATIC_INLINE(int)
+PyFloat_Pack4(double x, char *p, int le)
+{ return _PyFloat_Pack4(x, (unsigned char*)p, le); }
+
+PYCAPI_COMPAT_STATIC_INLINE(int)
+PyFloat_Pack8(double x, char *p, int le)
+{ return _PyFloat_Pack8(x, (unsigned char*)p, le); }
+
+PYCAPI_COMPAT_STATIC_INLINE(double)
+PyFloat_Unpack4(const char *p, int le)
+{ return _PyFloat_Unpack4((const unsigned char *)p, le); }
+
+PYCAPI_COMPAT_STATIC_INLINE(double)
+PyFloat_Unpack8(const char *p, int le)
+{ return _PyFloat_Unpack8((const unsigned char *)p, le); }
+#endif
+
+
+// Py_UNUSED() was added to Python 3.4.0b2.
+#if PY_VERSION_HEX < 0x030400B2 && !defined(Py_UNUSED)
+#  if defined(__GNUC__) || defined(__clang__)
+#    define Py_UNUSED(name) _unused_ ## name __attribute__((unused))
+#  else
+#    define Py_UNUSED(name) _unused_ ## name
+#  endif
+#endif
+
+
+#ifdef __cplusplus
+}
+#endif
+#endif  // PYTHONCAPI_COMPAT
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..e9018e7
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,25 @@
+[project]
+name = 'immutables'
+requires-python = ">=3.6"
+
+[build-system]
+requires = ["setuptools>=42", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[tool.pytest.ini_options]
+minversion = "6.0"
+addopts = "--capture=no --assert=plain --strict-markers --tb=native --import-mode=importlib"
+testpaths = "tests"
+filterwarnings = "default"
+
+[tool.mypy]
+incremental = true
+strict = true
+
+[[tool.mypy.overrides]]
+module = "immutables.map"
+ignore_errors = true
+
+[[tool.mypy.overrides]]
+module = "immutables._testutils"
+ignore_errors = true
diff --git a/setup.py b/setup.py
new file mode 100644
index 0000000..09b8de4
--- /dev/null
+++ b/setup.py
@@ -0,0 +1,93 @@
+import os
+import platform
+import setuptools
+
+
+# Minimal dependencies required to test immutables.
+TEST_DEPENDENCIES = [
+    # pycodestyle is a dependency of flake8, but it must be frozen because
+    # their combination breaks too often
+    # (example breakage: https://gitlab.com/pycqa/flake8/issues/427)
+    'flake8~=3.8.4',
+    'pycodestyle~=2.6.0',
+    'mypy==0.942',
+    'pytest~=6.2.4',
+]
+
+EXTRA_DEPENDENCIES = {
+    'test': TEST_DEPENDENCIES,
+}
+
+CFLAGS = ['-O2']
+if platform.uname().system != 'Windows':
+    CFLAGS.extend(['-std=c99', '-fsigned-char', '-Wall',
+                   '-Wsign-compare', '-Wconversion'])
+
+
+with open(os.path.join(
+        os.path.dirname(__file__), 'immutables', '_version.py')) as f:
+    for line in f:
+        if line.startswith('__version__ ='):
+            _, _, version = line.partition('=')
+            VERSION = version.strip(" \n'\"")
+            break
+    else:
+        raise RuntimeError(
+            'unable to read the version from immutables/_version.py')
+
+
+if platform.python_implementation() == 'CPython':
+    if os.environ.get("DEBUG_IMMUTABLES") == '1':
+        define_macros = []
+        undef_macros = ['NDEBUG']
+    else:
+        define_macros = [('NDEBUG', '1')]
+        undef_macros = []
+
+    ext_modules = [
+        setuptools.Extension(
+            "immutables._map",
+            ["immutables/_map.c"],
+            extra_compile_args=CFLAGS,
+            define_macros=define_macros,
+            undef_macros=undef_macros)
+    ]
+else:
+    ext_modules = []
+
+
+with open(os.path.join(os.path.dirname(__file__), 'README.rst')) as f:
+    readme = f.read()
+
+
+setuptools.setup(
+    name='immutables',
+    version=VERSION,
+    description='Immutable Collections',
+    long_description=readme,
+    python_requires='>=3.6',
+    classifiers=[
+        'License :: OSI Approved :: Apache Software License',
+        'Intended Audience :: Developers',
+        'Programming Language :: Python :: 3 :: Only',
+        'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
+        'Programming Language :: Python :: 3.10',
+        'Operating System :: POSIX',
+        'Operating System :: MacOS :: MacOS X',
+        'Operating System :: Microsoft :: Windows',
+    ],
+    author='MagicStack Inc',
+    author_email='hello@magic.io',
+    url='https://github.com/MagicStack/immutables',
+    license='Apache License, Version 2.0',
+    packages=['immutables'],
+    package_data={"immutables": ["py.typed", "*.pyi"]},
+    provides=['immutables'],
+    include_package_data=True,
+    ext_modules=ext_modules,
+    install_requires=['typing-extensions>=3.7.4.3;python_version<"3.8"'],
+    extras_require=EXTRA_DEPENDENCIES,
+)
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..450c573
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1,16 @@
+import os.path
+import sys
+import unittest
+
+
+def suite():
+    test_loader = unittest.TestLoader()
+    test_suite = test_loader.discover(os.path.dirname(__file__),
+                                      pattern='test_*.py')
+    return test_suite
+
+
+if __name__ == '__main__':
+    runner = unittest.runner.TextTestRunner()
+    result = runner.run(suite())
+    sys.exit(not result.wasSuccessful())
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..9eb9e61
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,13 @@
+# We need the mypy pytest plugin to do the test collection for our
+# typing tests.
+
+# mypy demands that its test-data be present for mypy.test.config to be
+# imported, so thwart that check. mypy PR #10919 fixes this.
+import unittest.mock
+with unittest.mock.patch('os.path.isdir') as isdir:
+    isdir.return_value = True
+    import mypy.test.config  # noqa
+
+pytest_plugins = [
+    'mypy.test.data',
+]
diff --git a/tests/test-data/check-immu.test b/tests/test-data/check-immu.test
new file mode 100644
index 0000000..4998542
--- /dev/null
+++ b/tests/test-data/check-immu.test
@@ -0,0 +1,73 @@
+[case testMypyImmu]
+# cmd: mypy test.py
+[file test.py]
+from immutables import Map
+from typing import Dict, Union, Any, cast
+
+def init() -> None:
+    def thing(m: Map[str, Union[str, int]]) -> None:
+        ...
+
+    thing(Map(foo=1))
+    thing(Map(foo='bar', baz=1))
+    thing(Map([('foo', 'bar'), ('bar', 1)]))
+    thing(Map(Map(foo=1), bar='foo'))
+    m = Map({1: 2})
+    thing(m)  # E: Argument 1 to "thing" has incompatible type "Map[int, int]"; expected "Map[str, Union[str, int]]"
+
+def assignments() -> None:
+    m_int__str = Map[int, str]()
+    m_str__str = Map[str, str]()
+    m_int_str__str = Map[Union[int, str], str]()
+    m_str__int_str = Map[str, Union[int, str]]()
+
+    m_int__str = m_str__str  # E: Incompatible types in assignment (expression has type "Map[str, str]", variable has type "Map[int, str]")
+    m_int__str = m_int_str__str  # E: Incompatible types in assignment (expression has type "Map[Union[int, str], str]", variable has type "Map[int, str]")
+    m_int__str = m_str__int_str  # E: Incompatible types in assignment (expression has type "Map[str, Union[int, str]]", variable has type "Map[int, str]")
+
+    m_str__str = m_int__str  # E: Incompatible types in assignment (expression has type "Map[int, str]", variable has type "Map[str, str]")
+    m_str__str = m_int_str__str  # E: Incompatible types in assignment (expression has type "Map[Union[int, str], str]", variable has type "Map[str, str]")
+    m_str__str = m_str__int_str  # E: Incompatible types in assignment (expression has type "Map[str, Union[int, str]]", variable has type "Map[str, str]")
+
+    m_int_str__str = m_int__str  # E: Incompatible types in assignment (expression has type "Map[int, str]", variable has type "Map[Union[int, str], str]")
+    m_int_str__str = m_str__str  # E: Incompatible types in assignment (expression has type "Map[str, str]", variable has type "Map[Union[int, str], str]")
+    m_int_str__str = m_str__int_str  # E: Incompatible types in assignment (expression has type "Map[str, Union[int, str]]", variable has type "Map[Union[int, str], str]")
+
+    m_str__int_str = m_int__str  # E: Incompatible types in assignment (expression has type "Map[int, str]", variable has type "Map[str, Union[int, str]]")
+    m_str__int_str = m_int_str__str  # E: Incompatible types in assignment (expression has type "Map[Union[int, str], str]", variable has type "Map[str, Union[int, str]]")
+    m_str__int_str = m_str__str
+
+def update() -> None:
+    m_int__str: Map[int, str] = Map()
+    m_str__str: Map[str, str] = Map()
+    m_int_str__str: Map[Union[int, str], str] = Map()
+    m_str__int_str: Map[str, Union[int, str]] = Map()
+
+    m_int__str.update({1: '2'})
+    m_int__str.update({1: '2'}, three='4')  # E: Unexpected keyword argument "three" for "update" of "Map"
+    m_int__str.update({1: 2})  # E: Argument 1 to "update" of "Map" has incompatible type "Dict[int, int]"; expected "Union[IterableItems[int, str], Iterable[Tuple[int, str]]]"
+
+    m_str__str.update({'1': '2'})
+    m_str__str.update({'1': '2'}, three='4')
+    m_str__str.update({'1': 2})  # E: Argument 1 to "update" of "Map" has incompatible type "Dict[str, int]"; expected "Union[IterableItems[str, str], Iterable[Tuple[str, str]]]"
+
+    m_int_str__str.update(cast(Dict[Union[int, str], str], {1: '2', '3': '4'}))
+    m_int_str__str.update({1: '2'}, three='4')
+    m_int_str__str.update({'1': 2})  # E: Argument 1 to "update" of "Map" has incompatible type "Dict[str, int]"; expected "Union[IterableItems[Union[int, str], str], Iterable[Tuple[Union[int, str], str]]]"
+
+    m_str__int_str.update({'1': 2, '2': 3})
+    m_str__int_str.update({'1': 2, '2': 3}, four='5')
+    m_str__int_str.update({1: 2})  # E: Argument 1 to "update" of "Map" has incompatible type "Dict[int, int]"; expected "Union[IterableItems[str, Union[int, str]], Iterable[Tuple[str, Union[int, str]]]]"
+
+def mutate() -> None:
+    m = Map[str, str]()
+
+    with m.mutate() as mm:
+        mm[0] = '1' # E: Invalid index type "int" for "MapMutation[str, str]"; expected type "str"
+        mm['1'] = 0  # E: Incompatible types in assignment (expression has type "int", target has type "str")
+        mm['1'] = '2'
+        del mm['1']
+        mm.set('3', '4')
+        m2 = mm.finish()
+
+    reveal_type(m2)  # N: Revealed type is "immutables._map.Map[builtins.str*, builtins.str*]"
diff --git a/tests/test_issue24.py b/tests/test_issue24.py
new file mode 100644
index 0000000..7d51e34
--- /dev/null
+++ b/tests/test_issue24.py
@@ -0,0 +1,156 @@
+import unittest
+
+from immutables.map import Map as PyMap, map_bitcount
+
+
+class CollisionKey:
+    def __hash__(self):
+        return 0
+
+
+class Issue24Base:
+    Map = None
+
+    def test_issue24(self):
+        keys = range(27)
+        new_entries = dict.fromkeys(keys, True)
+        m = self.Map(new_entries)
+        self.assertTrue(17 in m)
+        with m.mutate() as mm:
+            for i in keys:
+                del mm[i]
+            self.assertEqual(len(mm), 0)
+
+    def dump_check_node_kind(self, header, kind):
+        header = header.strip()
+        self.assertTrue(header.strip().startswith(kind))
+
+    def dump_check_node_size(self, header, size):
+        node_size = header.split('size=', 1)[1]
+        node_size = int(node_size.split(maxsplit=1)[0])
+        self.assertEqual(node_size, size)
+
+    def dump_check_bitmap_count(self, header, count):
+        header = header.split('bitmap=')[1]
+        bitmap = int(header.split(maxsplit=1)[0], 0)
+        self.assertEqual(map_bitcount(bitmap), count)
+
+    def dump_check_bitmap_node_count(self, header, count):
+        self.dump_check_node_kind(header, 'Bitmap')
+        self.dump_check_node_size(header, count * 2)
+        self.dump_check_bitmap_count(header, count)
+
+    def dump_check_collision_node_count(self, header, count):
+        self.dump_check_node_kind(header, 'Collision')
+        self.dump_check_node_size(header, 2 * count)
+
+    def test_bitmap_node_update_in_place_count(self):
+        keys = range(7)
+        new_entries = dict.fromkeys(keys, True)
+        m = self.Map(new_entries)
+        d = m.__dump__().splitlines()
+        self.assertTrue(d)
+        if d[0].startswith('HAMT'):
+            header = d[1]  # skip _map.Map.__dump__() header
+        else:
+            header = d[0]
+        self.dump_check_bitmap_node_count(header, 7)
+
+    def test_bitmap_node_delete_in_place_count(self):
+        keys = range(7)
+        new_entries = dict.fromkeys(keys, True)
+        m = self.Map(new_entries)
+        with m.mutate() as mm:
+            del mm[0], mm[2], mm[3]
+            m2 = mm.finish()
+        d = m2.__dump__().splitlines()
+        self.assertTrue(d)
+        if d[0].startswith('HAMT'):
+            header = d[1]  # skip _map.Map.__dump__() header
+        else:
+            header = d[0]
+        self.dump_check_bitmap_node_count(header, 4)
+
+    def test_collision_node_update_in_place_count(self):
+        keys = (CollisionKey() for i in range(7))
+        new_entries = dict.fromkeys(keys, True)
+        m = self.Map(new_entries)
+        d = m.__dump__().splitlines()
+        self.assertTrue(len(d) > 3)
+        # get node headers
+        if d[0].startswith('HAMT'):
+            h1, h2 = d[1], d[3]  # skip _map.Map.__dump__() header
+        else:
+            h1, h2 = d[0], d[2]
+        self.dump_check_node_kind(h1, 'Bitmap')
+        self.dump_check_collision_node_count(h2, 7)
+
+    def test_collision_node_delete_in_place_count(self):
+        keys = [CollisionKey() for i in range(7)]
+        new_entries = dict.fromkeys(keys, True)
+        m = self.Map(new_entries)
+        with m.mutate() as mm:
+            del mm[keys[0]], mm[keys[2]], mm[keys[3]]
+            m2 = mm.finish()
+        d = m2.__dump__().splitlines()
+        self.assertTrue(len(d) > 3)
+        # get node headers
+        if d[0].startswith('HAMT'):
+            h1, h2 = d[1], d[3]  # skip _map.Map.__dump__() header
+        else:
+            h1, h2 = d[0], d[2]
+        self.dump_check_node_kind(h1, 'Bitmap')
+        self.dump_check_collision_node_count(h2, 4)
+
+
+try:
+    from immutables._map import Map as CMap
+except ImportError:
+    CMap = None
+
+
+class Issue24PyTest(Issue24Base, unittest.TestCase):
+    Map = PyMap
+
+
+@unittest.skipIf(CMap is None, 'C Map is not available')
+class Issue24CTest(Issue24Base, unittest.TestCase):
+    Map = CMap
+
+    def hamt_dump_check_first_return_second(self, m):
+        d = m.__dump__().splitlines()
+        self.assertTrue(len(d) > 2)
+        self.assertTrue(d[0].startswith('HAMT'))
+        return d[1]
+
+    def test_array_node_update_in_place_count(self):
+        keys = range(27)
+        new_entries = dict.fromkeys(keys, True)
+        m = self.Map(new_entries)
+        header = self.hamt_dump_check_first_return_second(m)
+        self.dump_check_node_kind(header, 'Array')
+        for i in range(2, 18):
+            m = m.delete(i)
+        header = self.hamt_dump_check_first_return_second(m)
+        self.dump_check_bitmap_node_count(header, 11)
+
+    def test_array_node_delete_in_place_count(self):
+        keys = range(27)
+        new_entries = dict.fromkeys(keys, True)
+        m = self.Map(new_entries)
+        header = self.hamt_dump_check_first_return_second(m)
+        self.dump_check_node_kind(header, 'Array')
+        with m.mutate() as mm:
+            for i in range(5):
+                del mm[i]
+            m2 = mm.finish()
+        header = self.hamt_dump_check_first_return_second(m2)
+        self.dump_check_node_kind(header, 'Array')
+        for i in range(6, 17):
+            m2 = m2.delete(i)
+        header = self.hamt_dump_check_first_return_second(m2)
+        self.dump_check_bitmap_node_count(header, 11)
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/tests/test_map.py b/tests/test_map.py
new file mode 100644
index 0000000..4640029
--- /dev/null
+++ b/tests/test_map.py
@@ -0,0 +1,1417 @@
+import collections.abc
+import gc
+import pickle
+import random
+import sys
+import unittest
+import weakref
+
+from immutables.map import Map as PyMap
+from immutables._testutils import EqError
+from immutables._testutils import HashKey
+from immutables._testutils import HashKeyCrasher
+from immutables._testutils import HashingError
+from immutables._testutils import KeyStr
+from immutables._testutils import ReprError
+
+
+class BaseMapTest:
+
+    Map = None
+
+    def test_hashkey_helper_1(self):
+        k1 = HashKey(10, 'aaa')
+        k2 = HashKey(10, 'bbb')
+
+        self.assertNotEqual(k1, k2)
+        self.assertEqual(hash(k1), hash(k2))
+
+        d = dict()
+        d[k1] = 'a'
+        d[k2] = 'b'
+
+        self.assertEqual(d[k1], 'a')
+        self.assertEqual(d[k2], 'b')
+
+    def test_map_basics_1(self):
+        h = self.Map()
+        h = None  # NoQA
+
+    def test_map_basics_2(self):
+        h = self.Map()
+        self.assertEqual(len(h), 0)
+
+        h2 = h.set('a', 'b')
+        self.assertIsNot(h, h2)
+        self.assertEqual(len(h), 0)
+        self.assertEqual(len(h2), 1)
+
+        self.assertIsNone(h.get('a'))
+        self.assertEqual(h.get('a', 42), 42)
+
+        self.assertEqual(h2.get('a'), 'b')
+
+        h3 = h2.set('b', 10)
+        self.assertIsNot(h2, h3)
+        self.assertEqual(len(h), 0)
+        self.assertEqual(len(h2), 1)
+        self.assertEqual(len(h3), 2)
+        self.assertEqual(h3.get('a'), 'b')
+        self.assertEqual(h3.get('b'), 10)
+
+        self.assertIsNone(h.get('b'))
+        self.assertIsNone(h2.get('b'))
+
+        self.assertIsNone(h.get('a'))
+        self.assertEqual(h2.get('a'), 'b')
+
+        h = h2 = h3 = None
+
+    def test_map_basics_3(self):
+        h = self.Map()
+        o = object()
+        h1 = h.set('1', o)
+        h2 = h1.set('1', o)
+        self.assertIs(h1, h2)
+
+    def test_map_basics_4(self):
+        h = self.Map()
+        h1 = h.set('key', [])
+        h2 = h1.set('key', [])
+        self.assertIsNot(h1, h2)
+        self.assertEqual(len(h1), 1)
+        self.assertEqual(len(h2), 1)
+        self.assertIsNot(h1.get('key'), h2.get('key'))
+
+    def test_map_collision_1(self):
+        k1 = HashKey(10, 'aaa')
+        k2 = HashKey(10, 'bbb')
+        k3 = HashKey(10, 'ccc')
+
+        h = self.Map()
+        h2 = h.set(k1, 'a')
+        h3 = h2.set(k2, 'b')
+
+        self.assertEqual(h.get(k1), None)
+        self.assertEqual(h.get(k2), None)
+
+        self.assertEqual(h2.get(k1), 'a')
+        self.assertEqual(h2.get(k2), None)
+
+        self.assertEqual(h3.get(k1), 'a')
+        self.assertEqual(h3.get(k2), 'b')
+
+        h4 = h3.set(k2, 'cc')
+        h5 = h4.set(k3, 'aa')
+
+        self.assertEqual(h3.get(k1), 'a')
+        self.assertEqual(h3.get(k2), 'b')
+        self.assertEqual(h4.get(k1), 'a')
+        self.assertEqual(h4.get(k2), 'cc')
+        self.assertEqual(h4.get(k3), None)
+        self.assertEqual(h5.get(k1), 'a')
+        self.assertEqual(h5.get(k2), 'cc')
+        self.assertEqual(h5.get(k2), 'cc')
+        self.assertEqual(h5.get(k3), 'aa')
+
+        self.assertEqual(len(h), 0)
+        self.assertEqual(len(h2), 1)
+        self.assertEqual(len(h3), 2)
+        self.assertEqual(len(h4), 2)
+        self.assertEqual(len(h5), 3)
+
+    def test_map_collision_2(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(0b011000011100000100, 'C')
+        D = HashKey(0b011000011100000100, 'D')
+        E = HashKey(0b1011000011100000100, 'E')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+
+        # BitmapNode(size=6 bitmap=0b100110000):
+        #     NULL:
+        #         BitmapNode(size=4 bitmap=0b1000000000000000000001000):
+        #             <Key name:A hash:100>: 'a'
+        #             NULL:
+        #                 CollisionNode(size=4 id=0x108572410):
+        #                     <Key name:C hash:100100>: 'c'
+        #                     <Key name:D hash:100100>: 'd'
+        #     <Key name:B hash:101>: 'b'
+
+        h = h.set(E, 'e')
+
+        # BitmapNode(size=4 count=2.0 bitmap=0b110000 id=10b8ea5c0):
+        #     None:
+        #         BitmapNode(size=4 count=2.0
+        #                    bitmap=0b1000000000000000000001000 id=10b8ea518):
+        #             <Key name:A hash:100>: 'a'
+        #             None:
+        #                 BitmapNode(size=2 count=1.0 bitmap=0b10
+        #                            id=10b8ea4a8):
+        #                     None:
+        #                         BitmapNode(size=4 count=2.0
+        #                                    bitmap=0b100000001000
+        #                                    id=10b8ea4e0):
+        #                             None:
+        #                                 CollisionNode(size=4 id=10b8ea470):
+        #                                     <Key name:C hash:100100>: 'c'
+        #                                     <Key name:D hash:100100>: 'd'
+        #                             <Key name:E hash:362244>: 'e'
+        #     <Key name:B hash:101>: 'b'
+
+    def test_map_stress_01(self):
+        COLLECTION_SIZE = 7000
+        TEST_ITERS_EVERY = 647
+        CRASH_HASH_EVERY = 97
+        CRASH_EQ_EVERY = 11
+        RUN_XTIMES = 3
+
+        for _ in range(RUN_XTIMES):
+            h = self.Map()
+            d = dict()
+
+            for i in range(COLLECTION_SIZE):
+                key = KeyStr(i)
+
+                if not (i % CRASH_HASH_EVERY):
+                    with HashKeyCrasher(error_on_hash=True):
+                        with self.assertRaises(HashingError):
+                            h.set(key, i)
+
+                h = h.set(key, i)
+
+                if not (i % CRASH_EQ_EVERY):
+                    with HashKeyCrasher(error_on_eq=True):
+                        with self.assertRaises(EqError):
+                            h.get(KeyStr(i))  # really trigger __eq__
+
+                d[key] = i
+                self.assertEqual(len(d), len(h))
+
+                if not (i % TEST_ITERS_EVERY):
+                    self.assertEqual(set(h.items()), set(d.items()))
+                    self.assertEqual(len(h.items()), len(d.items()))
+
+            self.assertEqual(len(h), COLLECTION_SIZE)
+
+            for key in range(COLLECTION_SIZE):
+                self.assertEqual(h.get(KeyStr(key), 'not found'), key)
+
+            keys_to_delete = list(range(COLLECTION_SIZE))
+            random.shuffle(keys_to_delete)
+            for iter_i, i in enumerate(keys_to_delete):
+                key = KeyStr(i)
+
+                if not (iter_i % CRASH_HASH_EVERY):
+                    with HashKeyCrasher(error_on_hash=True):
+                        with self.assertRaises(HashingError):
+                            h.delete(key)
+
+                if not (iter_i % CRASH_EQ_EVERY):
+                    with HashKeyCrasher(error_on_eq=True):
+                        with self.assertRaises(EqError):
+                            h.delete(KeyStr(i))
+
+                h = h.delete(key)
+                self.assertEqual(h.get(key, 'not found'), 'not found')
+                del d[key]
+                self.assertEqual(len(d), len(h))
+
+                if iter_i == COLLECTION_SIZE // 2:
+                    hm = h
+                    dm = d.copy()
+
+                if not (iter_i % TEST_ITERS_EVERY):
+                    self.assertEqual(set(h.keys()), set(d.keys()))
+                    self.assertEqual(len(h.keys()), len(d.keys()))
+
+            self.assertEqual(len(d), 0)
+            self.assertEqual(len(h), 0)
+
+            # ============
+
+            for key in dm:
+                self.assertEqual(hm.get(str(key)), dm[key])
+            self.assertEqual(len(dm), len(hm))
+
+            for i, key in enumerate(keys_to_delete):
+                if str(key) in dm:
+                    hm = hm.delete(str(key))
+                    dm.pop(str(key))
+                self.assertEqual(hm.get(str(key), 'not found'), 'not found')
+                self.assertEqual(len(d), len(h))
+
+                if not (i % TEST_ITERS_EVERY):
+                    self.assertEqual(set(h.values()), set(d.values()))
+                    self.assertEqual(len(h.values()), len(d.values()))
+
+            self.assertEqual(len(d), 0)
+            self.assertEqual(len(h), 0)
+            self.assertEqual(list(h.items()), [])
+
+    def test_map_collision_3(self):
+        # Test that iteration works with the deepest tree possible.
+
+        C = HashKey(0b10000000_00000000_00000000_00000000, 'C')
+        D = HashKey(0b10000000_00000000_00000000_00000000, 'D')
+
+        E = HashKey(0b00000000_00000000_00000000_00000000, 'E')
+
+        h = self.Map()
+        h = h.set(C, 'C')
+        h = h.set(D, 'D')
+        h = h.set(E, 'E')
+
+        # BitmapNode(size=2 count=1 bitmap=0b1):
+        #   NULL:
+        #     BitmapNode(size=2 count=1 bitmap=0b1):
+        #       NULL:
+        #         BitmapNode(size=2 count=1 bitmap=0b1):
+        #           NULL:
+        #             BitmapNode(size=2 count=1 bitmap=0b1):
+        #               NULL:
+        #                 BitmapNode(size=2 count=1 bitmap=0b1):
+        #                   NULL:
+        #                     BitmapNode(size=2 count=1 bitmap=0b1):
+        #                       NULL:
+        #                         BitmapNode(size=4 count=2 bitmap=0b101):
+        #                           <Key name:E hash:0>: 'E'
+        #                           NULL:
+        #                             CollisionNode(size=4 id=0x107a24520):
+        #                               <Key name:C hash:2147483648>: 'C'
+        #                               <Key name:D hash:2147483648>: 'D'
+
+        self.assertEqual({k.name for k in h.keys()}, {'C', 'D', 'E'})
+
+    def test_map_stress_02(self):
+        COLLECTION_SIZE = 20000
+        TEST_ITERS_EVERY = 647
+        CRASH_HASH_EVERY = 97
+        DELETE_EVERY = 3
+        CRASH_EQ_EVERY = 11
+
+        h = self.Map()
+        d = dict()
+
+        for i in range(COLLECTION_SIZE // 2):
+            key = KeyStr(i)
+
+            if not (i % CRASH_HASH_EVERY):
+                with HashKeyCrasher(error_on_hash=True):
+                    with self.assertRaises(HashingError):
+                        h.set(key, i)
+
+            h = h.set(key, i)
+
+            if not (i % CRASH_EQ_EVERY):
+                with HashKeyCrasher(error_on_eq=True):
+                    with self.assertRaises(EqError):
+                        h.get(KeyStr(i))  # really trigger __eq__
+
+            d[key] = i
+            self.assertEqual(len(d), len(h))
+
+            if not (i % TEST_ITERS_EVERY):
+                self.assertEqual(set(h.items()), set(d.items()))
+                self.assertEqual(len(h.items()), len(d.items()))
+
+        with h.mutate() as m:
+            for i in range(COLLECTION_SIZE // 2, COLLECTION_SIZE):
+                key = KeyStr(i)
+
+                if not (i % CRASH_HASH_EVERY):
+                    with HashKeyCrasher(error_on_hash=True):
+                        with self.assertRaises(HashingError):
+                            m[key] = i
+
+                m[key] = i
+
+                if not (i % CRASH_EQ_EVERY):
+                    with HashKeyCrasher(error_on_eq=True):
+                        with self.assertRaises(EqError):
+                            m[KeyStr(i)]
+
+                d[key] = i
+                self.assertEqual(len(d), len(m))
+
+                if not (i % DELETE_EVERY):
+                    del m[key]
+                    del d[key]
+
+                self.assertEqual(len(d), len(m))
+
+            h = m.finish()
+
+        self.assertEqual(len(h), len(d))
+        self.assertEqual(set(h.items()), set(d.items()))
+
+        with h.mutate() as m:
+            for key in list(d):
+                del d[key]
+                del m[key]
+                self.assertEqual(len(m), len(d))
+            h = m.finish()
+
+        self.assertEqual(len(h), len(d))
+        self.assertEqual(set(h.items()), set(d.items()))
+
+    def test_map_delete_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(102, 'C')
+        D = HashKey(103, 'D')
+        E = HashKey(104, 'E')
+        Z = HashKey(-100, 'Z')
+
+        Er = HashKey(103, 'Er', error_on_eq_to=D)
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        orig_len = len(h)
+
+        # BitmapNode(size=10 bitmap=0b111110000 id=0x10eadc618):
+        #     <Key name:A hash:100>: 'a'
+        #     <Key name:B hash:101>: 'b'
+        #     <Key name:C hash:102>: 'c'
+        #     <Key name:D hash:103>: 'd'
+        #     <Key name:E hash:104>: 'e'
+
+        h = h.delete(C)
+        self.assertEqual(len(h), orig_len - 1)
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h.delete(Er)
+
+        h = h.delete(D)
+        self.assertEqual(len(h), orig_len - 2)
+
+        with self.assertRaises(KeyError) as ex:
+            h.delete(Z)
+        self.assertIs(ex.exception.args[0], Z)
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 3)
+
+        self.assertEqual(h.get(A, 42), 42)
+        self.assertEqual(h.get(B), 'b')
+        self.assertEqual(h.get(E), 'e')
+
+    def test_map_delete_2(self):
+        A = HashKey(100, 'A')
+        B = HashKey(201001, 'B')
+        C = HashKey(101001, 'C')
+        BLike = HashKey(201001, 'B-like')
+        D = HashKey(103, 'D')
+        E = HashKey(104, 'E')
+        Z = HashKey(-100, 'Z')
+
+        Er = HashKey(201001, 'Er', error_on_eq_to=B)
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        h = h.set(B, 'b')  # trigger branch in BitmapNode.assoc
+
+        with self.assertRaises(KeyError):
+            h.delete(BLike)    # trigger branch in BitmapNode.without
+
+        orig_len = len(h)
+
+        # BitmapNode(size=8 bitmap=0b1110010000):
+        #     <Key name:A hash:100>: 'a'
+        #     <Key name:D hash:103>: 'd'
+        #     <Key name:E hash:104>: 'e'
+        #     NULL:
+        #         BitmapNode(size=4 bitmap=0b100000000001000000000):
+        #             <Key name:B hash:201001>: 'b'
+        #             <Key name:C hash:101001>: 'c'
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h.delete(Er)
+
+        with self.assertRaises(KeyError) as ex:
+            h.delete(Z)
+        self.assertIs(ex.exception.args[0], Z)
+        self.assertEqual(len(h), orig_len)
+
+        h = h.delete(C)
+        self.assertEqual(len(h), orig_len - 1)
+
+        h = h.delete(B)
+        self.assertEqual(len(h), orig_len - 2)
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 3)
+
+        self.assertEqual(h.get(D), 'd')
+        self.assertEqual(h.get(E), 'e')
+
+        with self.assertRaises(KeyError):
+            h = h.delete(A)
+        with self.assertRaises(KeyError):
+            h = h.delete(B)
+        h = h.delete(D)
+        h = h.delete(E)
+        self.assertEqual(len(h), 0)
+
+    def test_map_delete_3(self):
+        A = HashKey(0b00000000001100100, 'A')
+        B = HashKey(0b00000000001100101, 'B')
+
+        C = HashKey(0b11000011100000100, 'C')
+        D = HashKey(0b11000011100000100, 'D')
+        X = HashKey(0b01000011100000100, 'Z')
+        Y = HashKey(0b11000011100000100, 'Y')
+
+        E = HashKey(0b00000000001101000, 'E')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        self.assertEqual(len(h), 5)
+        h = h.set(C, 'c')  # trigger branch in CollisionNode.assoc
+        self.assertEqual(len(h), 5)
+
+        orig_len = len(h)
+
+        with self.assertRaises(KeyError):
+            h.delete(X)
+        with self.assertRaises(KeyError):
+            h.delete(Y)
+
+        # BitmapNode(size=6 bitmap=0b100110000):
+        #     NULL:
+        #         BitmapNode(size=4 bitmap=0b1000000000000000000001000):
+        #             <Key name:A hash:100>: 'a'
+        #             NULL:
+        #                 CollisionNode(size=4 id=0x108572410):
+        #                     <Key name:C hash:100100>: 'c'
+        #                     <Key name:D hash:100100>: 'd'
+        #     <Key name:B hash:101>: 'b'
+        #     <Key name:E hash:104>: 'e'
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 1)
+
+        h = h.delete(E)
+        self.assertEqual(len(h), orig_len - 2)
+
+        self.assertEqual(h.get(C), 'c')
+        self.assertEqual(h.get(B), 'b')
+
+        h2 = h.delete(C)
+        self.assertEqual(len(h2), orig_len - 3)
+
+        h2 = h.delete(D)
+        self.assertEqual(len(h2), orig_len - 3)
+
+        self.assertEqual(len(h), orig_len - 2)
+
+    def test_map_delete_4(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(100100, 'E')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+
+        orig_len = len(h)
+
+        # BitmapNode(size=4 bitmap=0b110000):
+        #     NULL:
+        #         BitmapNode(size=4 bitmap=0b1000000000000000000001000):
+        #             <Key name:A hash:100>: 'a'
+        #             NULL:
+        #                 CollisionNode(size=6 id=0x10515ef30):
+        #                     <Key name:C hash:100100>: 'c'
+        #                     <Key name:D hash:100100>: 'd'
+        #                     <Key name:E hash:100100>: 'e'
+        #     <Key name:B hash:101>: 'b'
+
+        h = h.delete(D)
+        self.assertEqual(len(h), orig_len - 1)
+
+        h = h.delete(E)
+        self.assertEqual(len(h), orig_len - 2)
+
+        h = h.delete(C)
+        self.assertEqual(len(h), orig_len - 3)
+
+        h = h.delete(A)
+        self.assertEqual(len(h), orig_len - 4)
+
+        h = h.delete(B)
+        self.assertEqual(len(h), 0)
+
+    def test_map_delete_5(self):
+        h = self.Map()
+
+        keys = []
+        for i in range(17):
+            key = HashKey(i, str(i))
+            keys.append(key)
+            h = h.set(key, 'val-{}'.format(i))
+
+        collision_key16 = HashKey(16, '18')
+        h = h.set(collision_key16, 'collision')
+
+        # ArrayNode(id=0x10f8b9318):
+        #     0::
+        #     BitmapNode(size=2 count=1 bitmap=0b1):
+        #         <Key name:0 hash:0>: 'val-0'
+        #
+        # ... 14 more BitmapNodes ...
+        #
+        #     15::
+        #     BitmapNode(size=2 count=1 bitmap=0b1):
+        #         <Key name:15 hash:15>: 'val-15'
+        #
+        #     16::
+        #     BitmapNode(size=2 count=1 bitmap=0b1):
+        #         NULL:
+        #             CollisionNode(size=4 id=0x10f2f5af8):
+        #                 <Key name:16 hash:16>: 'val-16'
+        #                 <Key name:18 hash:16>: 'collision'
+
+        self.assertEqual(len(h), 18)
+
+        h = h.delete(keys[2])
+        self.assertEqual(len(h), 17)
+
+        h = h.delete(collision_key16)
+        self.assertEqual(len(h), 16)
+        h = h.delete(keys[16])
+        self.assertEqual(len(h), 15)
+
+        h = h.delete(keys[1])
+        self.assertEqual(len(h), 14)
+        with self.assertRaises(KeyError) as ex:
+            h.delete(keys[1])
+        self.assertIs(ex.exception.args[0], keys[1])
+        self.assertEqual(len(h), 14)
+
+        for key in keys:
+            if key in h:
+                h = h.delete(key)
+        self.assertEqual(len(h), 0)
+
+    def test_map_delete_6(self):
+        h = self.Map()
+        h = h.set(1, 1)
+        h = h.delete(1)
+        self.assertEqual(len(h), 0)
+        self.assertEqual(h, self.Map())
+
+    def test_map_items_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(201001, 'B')
+        C = HashKey(101001, 'C')
+        D = HashKey(103, 'D')
+        E = HashKey(104, 'E')
+        F = HashKey(110, 'F')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+        h = h.set(F, 'f')
+
+        it = h.items()
+        self.assertEqual(
+            set(list(it)),
+            {(A, 'a'), (B, 'b'), (C, 'c'), (D, 'd'), (E, 'e'), (F, 'f')})
+
+    def test_map_items_2(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(100100, 'E')
+        F = HashKey(110, 'F')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+        h = h.set(F, 'f')
+
+        it = h.items()
+        self.assertEqual(
+            set(list(it)),
+            {(A, 'a'), (B, 'b'), (C, 'c'), (D, 'd'), (E, 'e'), (F, 'f')})
+
+    def test_map_items_3(self):
+        h = self.Map()
+        self.assertEqual(len(h.items()), 0)
+        self.assertEqual(list(h.items()), [])
+
+    def test_map_items_4(self):
+        h = self.Map(a=1, b=2, c=3)
+        k = h.items()
+        self.assertEqual(set(k), {('a', 1), ('b', 2), ('c', 3)})
+        self.assertEqual(set(k), {('a', 1), ('b', 2), ('c', 3)})
+
+    def test_map_keys_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(100100, 'E')
+        F = HashKey(110, 'F')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+        h = h.set(F, 'f')
+
+        self.assertEqual(set(list(h.keys())), {A, B, C, D, E, F})
+        self.assertEqual(set(list(h)), {A, B, C, D, E, F})
+
+    def test_map_keys_2(self):
+        h = self.Map(a=1, b=2, c=3)
+        k = h.keys()
+        self.assertEqual(set(k), {'a', 'b', 'c'})
+        self.assertEqual(set(k), {'a', 'b', 'c'})
+
+    def test_map_values_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(100100, 'E')
+        F = HashKey(110, 'F')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(B, 'b')
+        h = h.set(C, 'c')
+        h = h.set(D, 'd')
+        h = h.set(E, 'e')
+        h = h.set(F, 'f')
+
+        self.assertEqual(set(list(h.values())), {'a', 'b', 'c', 'd', 'e', 'f'})
+
+    def test_map_values_2(self):
+        h = self.Map(a=1, b=2, c=3)
+        k = h.values()
+        self.assertEqual(set(k), {1, 2, 3})
+        self.assertEqual(set(k), {1, 2, 3})
+
+    def test_map_eq_1(self):
+        A = HashKey(100, 'A')
+        B = HashKey(101, 'B')
+        C = HashKey(100100, 'C')
+        D = HashKey(100100, 'D')
+        E = HashKey(120, 'E')
+
+        h1 = self.Map()
+        h1 = h1.set(A, 'a')
+        h1 = h1.set(B, 'b')
+        h1 = h1.set(C, 'c')
+        h1 = h1.set(D, 'd')
+
+        h2 = self.Map()
+        h2 = h2.set(A, 'a')
+
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(B, 'b')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(C, 'c')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(D, 'd2')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(D, 'd')
+        self.assertTrue(h1 == h2)
+        self.assertFalse(h1 != h2)
+
+        h2 = h2.set(E, 'e')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.delete(D)
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+        h2 = h2.set(E, 'd')
+        self.assertFalse(h1 == h2)
+        self.assertTrue(h1 != h2)
+
+    def test_map_eq_2(self):
+        A = HashKey(100, 'A')
+        Er = HashKey(100, 'Er', error_on_eq_to=A)
+
+        h1 = self.Map()
+        h1 = h1.set(A, 'a')
+
+        h2 = self.Map()
+        h2 = h2.set(Er, 'a')
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h1 == h2
+
+        with self.assertRaisesRegex(ValueError, 'cannot compare'):
+            h1 != h2
+
+    def test_map_eq_3(self):
+        self.assertNotEqual(self.Map(), 1)
+
+    def test_map_gc_1(self):
+        A = HashKey(100, 'A')
+
+        h = self.Map()
+        h = h.set(0, 0)  # empty Map node is memoized in _map.c
+        ref = weakref.ref(h)
+
+        a = []
+        a.append(a)
+        a.append(h)
+        b = []
+        a.append(b)
+        b.append(a)
+        h = h.set(A, b)
+
+        del h, a, b
+
+        gc.collect()
+        gc.collect()
+        gc.collect()
+
+        self.assertIsNone(ref())
+
+    def test_map_gc_2(self):
+        A = HashKey(100, 'A')
+
+        h = self.Map()
+        h = h.set(A, 'a')
+        h = h.set(A, h)
+
+        ref = weakref.ref(h)
+        hi = iter(h.items())
+        next(hi)
+
+        del h, hi
+
+        gc.collect()
+        gc.collect()
+        gc.collect()
+
+        self.assertIsNone(ref())
+
+    def test_map_in_1(self):
+        A = HashKey(100, 'A')
+        AA = HashKey(100, 'A')
+
+        B = HashKey(101, 'B')
+
+        h = self.Map()
+        h = h.set(A, 1)
+
+        self.assertTrue(A in h)
+        self.assertFalse(B in h)
+
+        with self.assertRaises(EqError):
+            with HashKeyCrasher(error_on_eq=True):
+                AA in h
+
+        with self.assertRaises(HashingError):
+            with HashKeyCrasher(error_on_hash=True):
+                AA in h
+
+    def test_map_getitem_1(self):
+        A = HashKey(100, 'A')
+        AA = HashKey(100, 'A')
+
+        B = HashKey(101, 'B')
+
+        h = self.Map()
+        h = h.set(A, 1)
+
+        self.assertEqual(h[A], 1)
+        self.assertEqual(h[AA], 1)
+
+        with self.assertRaises(KeyError):
+            h[B]
+
+        with self.assertRaises(EqError):
+            with HashKeyCrasher(error_on_eq=True):
+                h[AA]
+
+        with self.assertRaises(HashingError):
+            with HashKeyCrasher(error_on_hash=True):
+                h[AA]
+
+    def test_repr_1(self):
+        h = self.Map()
+        self.assertEqual(repr(h), 'immutables.Map({})')
+
+        h = h.set(1, 2).set(2, 3).set(3, 4)
+        self.assertEqual(repr(h), 'immutables.Map({1: 2, 2: 3, 3: 4})')
+
+    def test_repr_2(self):
+        h = self.Map()
+        A = HashKey(100, 'A')
+
+        with self.assertRaises(ReprError):
+            with HashKeyCrasher(error_on_repr=True):
+                repr(h.set(1, 2).set(A, 3).set(3, 4))
+
+        with self.assertRaises(ReprError):
+            with HashKeyCrasher(error_on_repr=True):
+                repr(h.set(1, 2).set(2, A).set(3, 4))
+
+    def test_repr_3(self):
+        class Key:
+            def __init__(self):
+                self.val = None
+
+            def __hash__(self):
+                return 123
+
+            def __repr__(self):
+                return repr(self.val)
+
+        h = self.Map()
+        k = Key()
+        h = h.set(k, 1)
+        k.val = h
+
+        self.assertEqual(repr(h), 'immutables.Map({{...}: 1})')
+
+    def test_hash_1(self):
+        h = self.Map()
+        self.assertNotEqual(hash(h), -1)
+        self.assertEqual(hash(h), hash(h))
+
+        h = h.set(1, 2).set('a', 'b')
+        self.assertNotEqual(hash(h), -1)
+        self.assertEqual(hash(h), hash(h))
+
+        self.assertEqual(
+            hash(h.set(1, 2).set('a', 'b')),
+            hash(h.set('a', 'b').set(1, 2)))
+
+    def test_hash_2(self):
+        h = self.Map()
+        A = HashKey(100, 'A')
+
+        m = h.set(1, 2).set(A, 3).set(3, 4)
+        with self.assertRaises(HashingError):
+            with HashKeyCrasher(error_on_hash=True):
+                hash(m)
+
+        m = h.set(1, 2).set(2, A).set(3, 4)
+        with self.assertRaises(HashingError):
+            with HashKeyCrasher(error_on_hash=True):
+                hash(m)
+
+    def test_abc_1(self):
+        self.assertTrue(issubclass(self.Map, collections.abc.Mapping))
+
+    def test_map_mut_1(self):
+        h = self.Map()
+        h = h.set('a', 1)
+
+        hm1 = h.mutate()
+        hm2 = h.mutate()
+
+        self.assertFalse(isinstance(hm1, self.Map))
+
+        self.assertIsNot(hm1, hm2)
+        self.assertEqual(hm1['a'], 1)
+        self.assertEqual(hm2['a'], 1)
+
+        hm1.set('b', 2)
+        hm1.set('c', 3)
+
+        hm2.set('x', 100)
+        hm2.set('a', 1000)
+
+        self.assertEqual(hm1['a'], 1)
+        self.assertEqual(hm1.get('x', -1), -1)
+
+        self.assertEqual(hm2['a'], 1000)
+        self.assertTrue('x' in hm2)
+
+        h1 = hm1.finish()
+        h2 = hm2.finish()
+
+        self.assertTrue(isinstance(h1, self.Map))
+
+        self.assertEqual(dict(h.items()), {'a': 1})
+        self.assertEqual(dict(h1.items()), {'a': 1, 'b': 2, 'c': 3})
+        self.assertEqual(dict(h2.items()), {'a': 1000, 'x': 100})
+
+    def test_map_mut_2(self):
+        h = self.Map()
+        h = h.set('a', 1)
+
+        hm1 = h.mutate()
+        hm1.set('a', 2)
+        hm1.set('a', 3)
+        hm1.set('a', 4)
+        h2 = hm1.finish()
+
+        self.assertEqual(dict(h.items()), {'a': 1})
+        self.assertEqual(dict(h2.items()), {'a': 4})
+
+    def test_map_mut_3(self):
+        h = self.Map()
+        h = h.set('a', 1)
+        hm1 = h.mutate()
+
+        self.assertEqual(repr(hm1), "immutables.MapMutation({'a': 1})")
+
+        with self.assertRaisesRegex(TypeError, 'unhashable type'):
+            hash(hm1)
+
+    def test_map_mut_4(self):
+        h = self.Map()
+        h = h.set('a', 1)
+        h = h.set('b', 2)
+
+        hm1 = h.mutate()
+        hm2 = h.mutate()
+
+        self.assertEqual(hm1, hm2)
+
+        hm1.set('a', 10)
+        self.assertNotEqual(hm1, hm2)
+
+        hm2.set('a', 10)
+        self.assertEqual(hm1, hm2)
+
+        self.assertEqual(hm2.pop('a'), 10)
+        self.assertNotEqual(hm1, hm2)
+
+    def test_map_mut_5(self):
+        h = self.Map({'a': 1, 'b': 2}, z=100)
+        self.assertTrue(isinstance(h, self.Map))
+        self.assertEqual(dict(h.items()), {'a': 1, 'b': 2, 'z': 100})
+
+        h2 = h.update(z=200, y=-1)
+        self.assertEqual(dict(h.items()), {'a': 1, 'b': 2, 'z': 100})
+        self.assertEqual(dict(h2.items()), {'a': 1, 'b': 2, 'z': 200, 'y': -1})
+
+        h3 = h2.update([(1, 2), (3, 4)])
+        self.assertEqual(dict(h.items()), {'a': 1, 'b': 2, 'z': 100})
+        self.assertEqual(dict(h2.items()), {'a': 1, 'b': 2, 'z': 200, 'y': -1})
+        self.assertEqual(dict(h3.items()),
+                         {'a': 1, 'b': 2, 'z': 200, 'y': -1, 1: 2, 3: 4})
+
+        h4 = h3.update()
+        self.assertIs(h4, h3)
+
+        h5 = h4.update(self.Map({'zzz': 'yyz'}))
+
+        self.assertEqual(dict(h5.items()),
+                         {'a': 1, 'b': 2, 'z': 200, 'y': -1, 1: 2, 3: 4,
+                          'zzz': 'yyz'})
+
+    def test_map_mut_6(self):
+        h = self.Map({'a': 1, 'b': 2}, z=100)
+        self.assertEqual(dict(h.items()), {'a': 1, 'b': 2, 'z': 100})
+
+        with self.assertRaisesRegex(TypeError, 'not iterable'):
+            h.update(1)
+
+        with self.assertRaisesRegex(ValueError, 'map update sequence element'):
+            h.update([(1, 2), (3, 4, 5)])
+
+        with self.assertRaisesRegex(TypeError, 'cannot convert map update'):
+            h.update([(1, 2), 1])
+
+        self.assertEqual(dict(h.items()), {'a': 1, 'b': 2, 'z': 100})
+
+    def test_map_mut_7(self):
+        key = HashKey(123, 'aaa')
+
+        h = self.Map({'a': 1, 'b': 2}, z=100)
+        self.assertEqual(dict(h.items()), {'a': 1, 'b': 2, 'z': 100})
+
+        upd = {key: 1}
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                h.update(upd)
+
+        upd = self.Map({key: 'zzz'})
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                h.update(upd)
+
+        upd = [(1, 2), (key, 'zzz')]
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                h.update(upd)
+
+        self.assertEqual(dict(h.items()), {'a': 1, 'b': 2, 'z': 100})
+
+    def test_map_mut_8(self):
+        key1 = HashKey(123, 'aaa')
+        key2 = HashKey(123, 'bbb')
+
+        h = self.Map({key1: 123})
+        self.assertEqual(dict(h.items()), {key1: 123})
+
+        upd = {key2: 1}
+        with HashKeyCrasher(error_on_eq=True):
+            with self.assertRaises(EqError):
+                h.update(upd)
+
+        upd = self.Map({key2: 'zzz'})
+        with HashKeyCrasher(error_on_eq=True):
+            with self.assertRaises(EqError):
+                h.update(upd)
+
+        upd = [(1, 2), (key2, 'zzz')]
+        with HashKeyCrasher(error_on_eq=True):
+            with self.assertRaises(EqError):
+                h.update(upd)
+
+        self.assertEqual(dict(h.items()), {key1: 123})
+
+    def test_map_mut_9(self):
+        key1 = HashKey(123, 'aaa')
+
+        src = {key1: 123}
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                self.Map(src)
+
+        src = [(1, 2), (key1, 123)]
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                self.Map(src)
+
+    def test_map_mut_10(self):
+        key1 = HashKey(123, 'aaa')
+
+        m = self.Map({key1: 123})
+
+        mm = m.mutate()
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                del mm[key1]
+
+        mm = m.mutate()
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                mm.pop(key1, None)
+
+        mm = m.mutate()
+        with HashKeyCrasher(error_on_hash=True):
+            with self.assertRaises(HashingError):
+                mm.set(key1, 123)
+
+    def test_map_mut_11(self):
+        m = self.Map({'a': 1, 'b': 2})
+
+        mm = m.mutate()
+        self.assertEqual(mm.pop('a', 1), 1)
+        self.assertEqual(mm.finish(), self.Map({'b': 2}))
+
+        mm = m.mutate()
+        self.assertEqual(mm.pop('b', 1), 2)
+        self.assertEqual(mm.finish(), self.Map({'a': 1}))
+
+        mm = m.mutate()
+        self.assertEqual(mm.pop('b', 1), 2)
+        del mm['a']
+        self.assertEqual(mm.finish(), self.Map())
+
+    def test_map_mut_12(self):
+        m = self.Map({'a': 1, 'b': 2})
+
+        mm = m.mutate()
+        mm.finish()
+
+        with self.assertRaisesRegex(ValueError, 'has been finished'):
+            mm.pop('a')
+
+        with self.assertRaisesRegex(ValueError, 'has been finished'):
+            del mm['a']
+
+        with self.assertRaisesRegex(ValueError, 'has been finished'):
+            mm.set('a', 'b')
+
+        with self.assertRaisesRegex(ValueError, 'has been finished'):
+            mm['a'] = 'b'
+
+        with self.assertRaisesRegex(ValueError, 'has been finished'):
+            mm.update(a='b')
+
+    def test_map_mut_13(self):
+        key1 = HashKey(123, 'aaa')
+        key2 = HashKey(123, 'aaa')
+
+        m = self.Map({key1: 123})
+
+        mm = m.mutate()
+        with HashKeyCrasher(error_on_eq=True):
+            with self.assertRaises(EqError):
+                del mm[key2]
+
+        mm = m.mutate()
+        with HashKeyCrasher(error_on_eq=True):
+            with self.assertRaises(EqError):
+                mm.pop(key2, None)
+
+        mm = m.mutate()
+        with HashKeyCrasher(error_on_eq=True):
+            with self.assertRaises(EqError):
+                mm.set(key2, 123)
+
+    def test_map_mut_14(self):
+        m = self.Map(a=1, b=2)
+
+        with m.mutate() as mm:
+            mm['z'] = 100
+            del mm['a']
+
+        self.assertEqual(mm.finish(), self.Map(z=100, b=2))
+
+    def test_map_mut_15(self):
+        m = self.Map(a=1, b=2)
+
+        with self.assertRaises(ZeroDivisionError):
+            with m.mutate() as mm:
+                mm['z'] = 100
+                del mm['a']
+                1 / 0
+
+        self.assertEqual(mm.finish(), self.Map(z=100, b=2))
+        self.assertEqual(m, self.Map(a=1, b=2))
+
+    def test_map_mut_16(self):
+        m = self.Map(a=1, b=2)
+        hash(m)
+
+        m2 = self.Map(m)
+        m3 = self.Map(m, c=3)
+
+        self.assertEqual(m, m2)
+        self.assertEqual(len(m), len(m2))
+        self.assertEqual(hash(m), hash(m2))
+
+        self.assertIsNot(m, m2)
+        self.assertEqual(m3, self.Map(a=1, b=2, c=3))
+
+    def test_map_mut_17(self):
+        m = self.Map(a=1)
+        with m.mutate() as mm:
+            with self.assertRaisesRegex(
+                    TypeError, 'cannot create Maps from MapMutations'):
+                self.Map(mm)
+
+    def test_map_mut_18(self):
+        m = self.Map(a=1, b=2)
+        with m.mutate() as mm:
+            mm.update(self.Map(x=1), z=2)
+            mm.update(c=3)
+            mm.update({'n': 100, 'a': 20})
+            m2 = mm.finish()
+
+        expected = self.Map(
+            {'b': 2, 'c': 3, 'n': 100, 'z': 2, 'x': 1, 'a': 20})
+
+        self.assertEqual(len(m2), 6)
+        self.assertEqual(m2, expected)
+        self.assertEqual(m, self.Map(a=1, b=2))
+
+    def test_map_mut_19(self):
+        m = self.Map(a=1, b=2)
+        m2 = m.update({'a': 20})
+        self.assertEqual(len(m2), 2)
+
+    def test_map_mut_20(self):
+        # Issue 24:
+
+        h = self.Map()
+
+        for i in range(19):
+            # Create more than 16 keys to trigger the root bitmap
+            # node to be converted into an array node
+            h = h.set(HashKey(i, i), i)
+
+        h = h.set(HashKey(18, '18-collision'), 18)
+
+        with h.mutate() as m:
+            del m[HashKey(18, 18)]
+            del m[HashKey(18, '18-collision')]
+
+            # The pre-issue-24 code failed to update the number of array
+            # node element, so at this point it would be greater than it
+            # actually is.
+            h = m.finish()
+
+        # Any of the below operations shouldn't crash the debug build.
+        with h.mutate() as m:
+            for i in range(18):
+                del m[HashKey(i, i)]
+            h = m.finish()
+        h = h.set(HashKey(21, 21), 21)
+        h = h.set(HashKey(22, 22), 22)
+
+    def test_map_mut_21(self):
+        # Issue 24:
+        # Array nodes, while in mutation, failed to increment the
+        # internal count of elements when adding a new key to it.
+        # Because the internal count
+
+        h = self.Map()
+
+        for i in range(18):
+            # Create more than 16 keys to trigger the root bitmap
+            # node to be converted into an array node
+            h = h.set(HashKey(i, i), i)
+
+        with h.mutate() as m:
+            # Add one new key to the array node
+            m[HashKey(18, 18)] = 18
+            # Add another key -- after this the old code failed
+            # to increment the number of elements in the mutated
+            # array node.
+            m[HashKey(19, 19)] = 19
+            h = m.finish()
+
+        for i in range(20):
+            # Start deleting keys one by one. Because array node
+            # element count was accounted incorrectly (smaller by 1
+            # than it actually is, the mutation for "del h[18]" would
+            # create an empty array node, clipping the "19" key).
+            # Before the issue #24 fix, the below line would crash
+            # on i=19.
+            h = h.delete(HashKey(i, i))
+
+    def test_map_mut_stress(self):
+        COLLECTION_SIZE = 7000
+        TEST_ITERS_EVERY = 647
+        RUN_XTIMES = 3
+
+        for _ in range(RUN_XTIMES):
+            h = self.Map()
+            d = dict()
+
+            for i in range(COLLECTION_SIZE // TEST_ITERS_EVERY):
+
+                hm = h.mutate()
+                for j in range(TEST_ITERS_EVERY):
+                    key = random.randint(1, 100000)
+                    key = HashKey(key % 271, str(key))
+
+                    hm.set(key, key)
+                    d[key] = key
+
+                    self.assertEqual(len(hm), len(d))
+
+                h2 = hm.finish()
+                self.assertEqual(dict(h2.items()), d)
+                h = h2
+
+            self.assertEqual(dict(h.items()), d)
+            self.assertEqual(len(h), len(d))
+
+            it = iter(tuple(d.keys()))
+            for i in range(COLLECTION_SIZE // TEST_ITERS_EVERY):
+
+                hm = h.mutate()
+                for j in range(TEST_ITERS_EVERY):
+                    try:
+                        key = next(it)
+                    except StopIteration:
+                        break
+
+                    del d[key]
+                    del hm[key]
+
+                    self.assertEqual(len(hm), len(d))
+
+                h2 = hm.finish()
+                self.assertEqual(dict(h2.items()), d)
+                h = h2
+
+            self.assertEqual(dict(h.items()), d)
+            self.assertEqual(len(h), len(d))
+
+    def test_map_pickle(self):
+        h = self.Map(a=1, b=2)
+        for proto in range(pickle.HIGHEST_PROTOCOL):
+            p = pickle.dumps(h, proto)
+            uh = pickle.loads(p)
+
+            self.assertTrue(isinstance(uh, self.Map))
+            self.assertEqual(h, uh)
+
+        with self.assertRaisesRegex(TypeError, "can('t|not) pickle"):
+            pickle.dumps(h.mutate())
+
+    @unittest.skipIf(
+        sys.version_info < (3, 7, 0), "__class_getitem__ is not available"
+    )
+    def test_map_is_subscriptable(self):
+        self.assertIs(self.Map[int, str], self.Map)
+
+    def test_kwarg_named_col(self):
+        self.assertEqual(dict(self.Map(col=0)), {"col": 0})
+        self.assertEqual(dict(self.Map(a=0, col=1)), {"a": 0, "col": 1})
+        self.assertEqual(dict(self.Map({"a": 0}, col=1)), {"a": 0, "col": 1})
+
+
+class PyMapTest(BaseMapTest, unittest.TestCase):
+
+    Map = PyMap
+
+
+try:
+    from immutables._map import Map as CMap
+except ImportError:
+    CMap = None
+
+
+@unittest.skipIf(CMap is None, 'C Map is not available')
+class CMapTest(BaseMapTest, unittest.TestCase):
+
+    Map = CMap
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/tests/test_mypy.py b/tests/test_mypy.py
new file mode 100644
index 0000000..fcdc11c
--- /dev/null
+++ b/tests/test_mypy.py
@@ -0,0 +1,26 @@
+import os
+
+try:
+    import mypy.test.testcmdline
+    from mypy.test.helpers import normalize_error_messages
+except (ImportError, AssertionError):
+    if os.environ.get('IMMU_SKIP_MYPY_TESTS'):
+        pass
+    else:
+        raise
+else:
+    # I'm upset. There's no other way to deal with the little 'defined here'
+    # notes that mypy emits when passing an unexpected keyword argument
+    # and at no other time.
+    def renormalize_error_messages(messages):
+        messages = [x for x in messages if not x.endswith(' defined here')]
+        return normalize_error_messages(messages)
+
+    mypy.test.testcmdline.normalize_error_messages = renormalize_error_messages
+
+    this_file_dir = os.path.dirname(os.path.realpath(__file__))
+    test_data_prefix = os.path.join(this_file_dir, 'test-data')
+
+    class ImmuMypyTest(mypy.test.testcmdline.PythonCmdlineSuite):
+        data_prefix = test_data_prefix
+        files = ['check-immu.test']
diff --git a/tests/test_none_keys.py b/tests/test_none_keys.py
new file mode 100644
index 0000000..26d4220
--- /dev/null
+++ b/tests/test_none_keys.py
@@ -0,0 +1,515 @@
+import ctypes
+import unittest
+
+from immutables.map import map_hash, map_mask, Map as PyMap
+from immutables._testutils import HashKey
+
+
+none_hash = map_hash(None)
+assert(none_hash != 1)
+assert(none_hash.bit_length() <= 32)
+
+none_hash_u = ctypes.c_size_t(none_hash).value
+not_collision = 0xffffffff & (~none_hash_u)
+
+mask = 0x7ffffffff
+none_collisions = [none_hash_u & (mask >> shift)
+                   for shift in reversed(range(0, 32, 5))]
+assert(len(none_collisions) == 7)
+none_collisions = [
+    ctypes.c_ssize_t(h | (not_collision & (mask << shift))).value
+    for shift, h in zip(range(5, 37, 5), none_collisions)
+]
+
+
+class NoneCollision(HashKey):
+    def __init__(self, name, level):
+        if name is None:
+            raise ValueError("Can't have a NoneCollision with a None value")
+        super().__init__(none_collisions[level], name)
+
+    def __eq__(self, other):
+        if other is None:
+            return False
+        return super().__eq__(other)
+
+    __hash__ = HashKey.__hash__
+
+
+class BaseNoneTest:
+    Map = None
+
+    def test_none_collisions(self):
+        collisions = [NoneCollision('a', level) for level in range(7)]
+        indices = [map_mask(none_hash, shift) for shift in range(0, 32, 5)]
+
+        for i, c in enumerate(collisions[:-1], 1):
+            self.assertNotEqual(c, None)
+            c_hash = map_hash(c)
+            self.assertNotEqual(c_hash, none_hash)
+            for j, idx in enumerate(indices[:i]):
+                self.assertEqual(map_mask(c_hash, j*5), idx)
+            for j, idx in enumerate(indices[i:], i):
+                self.assertNotEqual(map_mask(c_hash, j*5), idx)
+
+        c = collisions[-1]
+        self.assertNotEqual(c, None)
+        c_hash = map_hash(c)
+        self.assertEqual(c_hash, none_hash)
+        for i, idx in enumerate(indices):
+            self.assertEqual(map_mask(c_hash, i*5), idx)
+
+    def test_none_as_key(self):
+        m = self.Map({None: 1})
+
+        self.assertEqual(len(m), 1)
+        self.assertTrue(None in m)
+        self.assertEqual(m[None], 1)
+        self.assertEqual(repr(m), 'immutables.Map({None: 1})')
+
+        for level in range(7):
+            key = NoneCollision('a', level)
+            self.assertFalse(key in m)
+            with self.assertRaises(KeyError):
+                m.delete(key)
+
+        m = m.delete(None)
+        self.assertEqual(len(m), 0)
+        self.assertFalse(None in m)
+        self.assertEqual(repr(m), 'immutables.Map({})')
+
+        self.assertEqual(m, self.Map())
+
+        with self.assertRaises(KeyError):
+            m.delete(None)
+
+    def test_none_set(self):
+        m = self.Map().set(None, 2)
+
+        self.assertEqual(len(m), 1)
+        self.assertTrue(None in m)
+        self.assertEqual(m[None], 2)
+
+        m = m.set(None, 1)
+
+        self.assertEqual(len(m), 1)
+        self.assertTrue(None in m)
+        self.assertEqual(m[None], 1)
+
+        m = m.delete(None)
+
+        self.assertEqual(len(m), 0)
+        self.assertEqual(m, self.Map())
+        self.assertFalse(None in m)
+
+        with self.assertRaises(KeyError):
+            m.delete(None)
+
+    def test_none_collision_1(self):
+        for level in range(7):
+            key = NoneCollision('a', level)
+            m = self.Map({None: 1, key: 2})
+
+            self.assertEqual(len(m), 2)
+            self.assertTrue(None in m)
+            self.assertEqual(m[None], 1)
+            self.assertTrue(key in m)
+            self.assertEqual(m[key], 2)
+
+            m2 = m.delete(None)
+            self.assertEqual(len(m2), 1)
+            self.assertTrue(key in m2)
+            self.assertEqual(m2[key], 2)
+            self.assertFalse(None in m2)
+            with self.assertRaises(KeyError):
+                m2.delete(None)
+
+            m3 = m2.delete(key)
+            self.assertEqual(len(m3), 0)
+            self.assertFalse(None in m3)
+            self.assertFalse(key in m3)
+            self.assertEqual(m3, self.Map())
+            self.assertEqual(repr(m3), 'immutables.Map({})')
+            with self.assertRaises(KeyError):
+                m3.delete(None)
+            with self.assertRaises(KeyError):
+                m3.delete(key)
+
+            m2 = m.delete(key)
+            self.assertEqual(len(m2), 1)
+            self.assertTrue(None in m2)
+            self.assertEqual(m2[None], 1)
+            self.assertFalse(key in m2)
+            with self.assertRaises(KeyError):
+                m2.delete(key)
+
+            m4 = m2.delete(None)
+            self.assertEqual(len(m4), 0)
+            self.assertFalse(None in m4)
+            self.assertFalse(key in m4)
+            self.assertEqual(m4, self.Map())
+            self.assertEqual(repr(m4), 'immutables.Map({})')
+            with self.assertRaises(KeyError):
+                m4.delete(None)
+            with self.assertRaises(KeyError):
+                m4.delete(key)
+
+            self.assertEqual(m3, m4)
+
+    def test_none_collision_2(self):
+        key = HashKey(not_collision, 'a')
+        m = self.Map().set(None, 1).set(key, 2)
+
+        self.assertEqual(len(m), 2)
+        self.assertTrue(key in m)
+        self.assertTrue(None in m)
+        self.assertEqual(m[key], 2)
+        self.assertEqual
+
+        m = m.set(None, 0)
+        self.assertEqual(len(m), 2)
+        self.assertTrue(key in m)
+        self.assertTrue(None in m)
+
+        for level in range(7):
+            key2 = NoneCollision('b', level)
+            self.assertFalse(key2 in m)
+            m2 = m.set(key2, 1)
+
+            self.assertEqual(len(m2), 3)
+            self.assertTrue(key in m2)
+            self.assertTrue(None in m2)
+            self.assertTrue(key2 in m2)
+            self.assertEqual(m2[key], 2)
+            self.assertEqual(m2[None], 0)
+            self.assertEqual(m2[key2], 1)
+
+            m2 = m2.set(None, 1)
+            self.assertEqual(len(m2), 3)
+            self.assertTrue(key in m2)
+            self.assertTrue(None in m2)
+            self.assertTrue(key2 in m2)
+            self.assertEqual(m2[key], 2)
+            self.assertEqual(m2[None], 1)
+            self.assertEqual(m2[key2], 1)
+
+            m2 = m2.set(None, 2)
+            self.assertEqual(len(m2), 3)
+            self.assertTrue(key in m2)
+            self.assertTrue(None in m2)
+            self.assertTrue(key2 in m2)
+            self.assertEqual(m2[key], 2)
+            self.assertEqual(m2[None], 2)
+            self.assertEqual(m2[key2], 1)
+
+            m3 = m2.delete(key)
+            self.assertEqual(len(m3), 2)
+            self.assertTrue(None in m3)
+            self.assertTrue(key2 in m3)
+            self.assertFalse(key in m3)
+            self.assertEqual(m3[None], 2)
+            self.assertEqual(m3[key2], 1)
+            with self.assertRaises(KeyError):
+                m3.delete(key)
+
+            m3 = m2.delete(key2)
+            self.assertEqual(len(m3), 2)
+            self.assertTrue(None in m3)
+            self.assertTrue(key in m3)
+            self.assertFalse(key2 in m3)
+            self.assertEqual(m3[None], 2)
+            self.assertEqual(m3[key], 2)
+            with self.assertRaises(KeyError):
+                m3.delete(key2)
+
+            m3 = m2.delete(None)
+            self.assertEqual(len(m3), 2)
+            self.assertTrue(key in m3)
+            self.assertTrue(key2 in m3)
+            self.assertFalse(None in m3)
+            self.assertEqual(m3[key], 2)
+            self.assertEqual(m3[key2], 1)
+            with self.assertRaises(KeyError):
+                m3.delete(None)
+
+        m2 = m.delete(None)
+        self.assertEqual(len(m2), 1)
+        self.assertFalse(None in m2)
+        self.assertTrue(key in m2)
+        self.assertEqual(m2[key], 2)
+        with self.assertRaises(KeyError):
+            m2.delete(None)
+
+        m2 = m.delete(key)
+        self.assertEqual(len(m2), 1)
+        self.assertFalse(key in m2)
+        self.assertTrue(None in m2)
+        self.assertEqual(m2[None], 0)
+        with self.assertRaises(KeyError):
+            m2.delete(key)
+
+    def test_none_collision_3(self):
+        for level in range(7):
+            key = NoneCollision('a', level)
+            m = self.Map({key: 2})
+
+            self.assertEqual(len(m), 1)
+            self.assertFalse(None in m)
+            self.assertTrue(key in m)
+            self.assertEqual(m[key], 2)
+            with self.assertRaises(KeyError):
+                m.delete(None)
+
+            m = m.set(None, 1)
+            self.assertEqual(len(m), 2)
+            self.assertTrue(key in m)
+            self.assertEqual(m[key], 2)
+            self.assertTrue(None in m)
+            self.assertEqual(m[None], 1)
+
+            m = m.set(None, 0)
+            self.assertEqual(len(m), 2)
+            self.assertTrue(key in m)
+            self.assertEqual(m[key], 2)
+            self.assertTrue(None in m)
+            self.assertEqual(m[None], 0)
+
+            m2 = m.delete(key)
+            self.assertEqual(len(m2), 1)
+            self.assertTrue(None in m2)
+            self.assertEqual(m2[None], 0)
+            self.assertFalse(key in m2)
+            with self.assertRaises(KeyError):
+                m2.delete(key)
+
+            m2 = m.delete(None)
+            self.assertEqual(len(m2), 1)
+            self.assertTrue(key in m2)
+            self.assertEqual(m2[key], 2)
+            self.assertFalse(None in m2)
+            with self.assertRaises(KeyError):
+                m2.delete(None)
+
+    def test_collision_4(self):
+        key2 = NoneCollision('a', 2)
+        key4 = NoneCollision('b', 4)
+        m = self.Map({key2: 2, key4: 4})
+
+        self.assertEqual(len(m), 2)
+        self.assertTrue(key2 in m)
+        self.assertTrue(key4 in m)
+        self.assertEqual(m[key2], 2)
+        self.assertEqual(m[key4], 4)
+        self.assertFalse(None in m)
+
+        m2 = m.set(None, 9)
+
+        self.assertEqual(len(m2), 3)
+        self.assertTrue(key2 in m2)
+        self.assertTrue(key4 in m2)
+        self.assertTrue(None in m2)
+        self.assertEqual(m2[key2], 2)
+        self.assertEqual(m2[key4], 4)
+        self.assertEqual(m2[None], 9)
+
+        m3 = m2.set(None, 0)
+        self.assertEqual(len(m3), 3)
+        self.assertTrue(key2 in m3)
+        self.assertTrue(key4 in m3)
+        self.assertTrue(None in m3)
+        self.assertEqual(m3[key2], 2)
+        self.assertEqual(m3[key4], 4)
+        self.assertEqual(m3[None], 0)
+
+        m3 = m2.set(key2, 0)
+        self.assertEqual(len(m3), 3)
+        self.assertTrue(key2 in m3)
+        self.assertTrue(key4 in m3)
+        self.assertTrue(None in m3)
+        self.assertEqual(m3[key2], 0)
+        self.assertEqual(m3[key4], 4)
+        self.assertEqual(m3[None], 9)
+
+        m3 = m2.set(key4, 0)
+        self.assertEqual(len(m3), 3)
+        self.assertTrue(key2 in m3)
+        self.assertTrue(key4 in m3)
+        self.assertTrue(None in m3)
+        self.assertEqual(m3[key2], 2)
+        self.assertEqual(m3[key4], 0)
+        self.assertEqual(m3[None], 9)
+
+        m3 = m2.delete(None)
+        self.assertEqual(m3, m)
+        self.assertEqual(len(m3), 2)
+        self.assertTrue(key2 in m3)
+        self.assertTrue(key4 in m3)
+        self.assertEqual(m3[key2], 2)
+        self.assertEqual(m3[key4], 4)
+        self.assertFalse(None in m3)
+        with self.assertRaises(KeyError):
+            m3.delete(None)
+
+        m3 = m2.delete(key2)
+        self.assertEqual(len(m3), 2)
+        self.assertTrue(None in m3)
+        self.assertTrue(key4 in m3)
+        self.assertEqual(m3[None], 9)
+        self.assertEqual(m3[key4], 4)
+        self.assertFalse(key2 in m3)
+        with self.assertRaises(KeyError):
+            m3.delete(key2)
+
+        m3 = m2.delete(key4)
+        self.assertEqual(len(m3), 2)
+        self.assertTrue(None in m3)
+        self.assertTrue(key2 in m3)
+        self.assertEqual(m3[None], 9)
+        self.assertEqual(m3[key2], 2)
+        self.assertFalse(key4 in m3)
+        with self.assertRaises(KeyError):
+            m3.delete(key4)
+
+    def test_none_mutation(self):
+        key2 = NoneCollision('a', 2)
+        key4 = NoneCollision('b', 4)
+        key = NoneCollision('c', -1)
+        m = self.Map({key: -1, key2: 2, key4: 4, None: 9})
+
+        with m.mutate() as mm:
+            self.assertEqual(len(mm), 4)
+            self.assertTrue(key in mm)
+            self.assertTrue(key2 in mm)
+            self.assertTrue(key4 in mm)
+            self.assertTrue(None in mm)
+            self.assertEqual(mm[key2], 2)
+            self.assertEqual(mm[key4], 4)
+            self.assertEqual(mm[key], -1)
+            self.assertEqual(mm[None], 9)
+
+            for k in m:
+                mm[k] = -mm[k]
+
+            self.assertEqual(len(mm), 4)
+            self.assertTrue(key in mm)
+            self.assertTrue(key2 in mm)
+            self.assertTrue(key4 in mm)
+            self.assertTrue(None in mm)
+            self.assertEqual(mm[key2], -2)
+            self.assertEqual(mm[key4], -4)
+            self.assertEqual(mm[key], 1)
+            self.assertEqual(mm[None], -9)
+
+            for k in m:
+                del mm[k]
+                self.assertEqual(len(mm), 3)
+                self.assertFalse(k in mm)
+                for n in m:
+                    if n != k:
+                        self.assertTrue(n in mm)
+                        self.assertEqual(mm[n], -m[n])
+                with self.assertRaises(KeyError):
+                    del mm[k]
+                mm[k] = -m[k]
+                self.assertEqual(len(mm), 4)
+                self.assertTrue(k in mm)
+                self.assertEqual(mm[k], -m[k])
+
+            for k in m:
+                mm[k] = -mm[k]
+
+            self.assertEqual(len(mm), 4)
+            self.assertTrue(key in mm)
+            self.assertTrue(key2 in mm)
+            self.assertTrue(key4 in mm)
+            self.assertTrue(None in mm)
+            self.assertEqual(mm[key2], 2)
+            self.assertEqual(mm[key4], 4)
+            self.assertEqual(mm[key], -1)
+            self.assertEqual(mm[None], 9)
+
+            for k in m:
+                mm[k] = -mm[k]
+
+            self.assertEqual(len(mm), 4)
+            self.assertTrue(key in mm)
+            self.assertTrue(key2 in mm)
+            self.assertTrue(key4 in mm)
+            self.assertTrue(None in mm)
+            self.assertEqual(mm[key2], -2)
+            self.assertEqual(mm[key4], -4)
+            self.assertEqual(mm[key], 1)
+            self.assertEqual(mm[None], -9)
+
+            m2 = mm.finish()
+
+        self.assertEqual(set(m), set(m2))
+        self.assertEqual(len(m2), 4)
+        self.assertTrue(key in m2)
+        self.assertTrue(key2 in m2)
+        self.assertTrue(key4 in m2)
+        self.assertTrue(None in m2)
+        self.assertEqual(m2[key2], -2)
+        self.assertEqual(m2[key4], -4)
+        self.assertEqual(m2[key], 1)
+        self.assertEqual(m2[None], -9)
+
+        for k, v in m.items():
+            self.assertTrue(k in m2)
+            self.assertEqual(m2[k], -v)
+
+    def test_iterators(self):
+        key2 = NoneCollision('a', 2)
+        key4 = NoneCollision('b', 4)
+        key = NoneCollision('c', -1)
+        m = self.Map({key: -1, key2: 2, key4: 4, None: 9})
+
+        self.assertEqual(len(m), 4)
+        self.assertTrue(key in m)
+        self.assertTrue(key2 in m)
+        self.assertTrue(key4 in m)
+        self.assertTrue(None in m)
+        self.assertEqual(m[key2], 2)
+        self.assertEqual(m[key4], 4)
+        self.assertEqual(m[key], -1)
+        self.assertEqual(m[None], 9)
+
+        s = set(m)
+        self.assertEqual(len(s), 4)
+        self.assertEqual(s, set([None, key, key2, key4]))
+
+        sk = set(m.keys())
+        self.assertEqual(s, sk)
+
+        sv = set(m.values())
+        self.assertEqual(len(sv), 4)
+        self.assertEqual(sv, set([-1, 2, 4, 9]))
+
+        si = set(m.items())
+        self.assertEqual(len(si), 4)
+        self.assertEqual(si,
+                         set([(key, -1), (key2, 2), (key4, 4), (None, 9)]))
+
+        d = {key: -1, key2: 2, key4: 4, None: 9}
+        self.assertEqual(dict(m.items()), d)
+
+
+class PyMapNoneTest(BaseNoneTest, unittest.TestCase):
+
+    Map = PyMap
+
+
+try:
+    from immutables._map import Map as CMap
+except ImportError:
+    CMap = None
+
+
+@unittest.skipIf(CMap is None, 'C Map is not available')
+class CMapNoneTest(BaseNoneTest, unittest.TestCase):
+
+    Map = CMap
+
+
+if __name__ == "__main__":
+    unittest.main()