/**
 * C hashtable implementation
 *
 * Copyright (C) 2018-2021 Wojtek Kosior
 * Redistribution terms are gathered in the `copyright' file.
 */

/*
 * https://git.koszko.org/C-hashtable
 * Note that this version is likely to be more up-to-date than the one linked.
 *
 * This is a separate chaining hashtable for general use. It's not
 * universal: it uses malloc() and free(), so it requires a standard
 * library to function and it's for single-threaded use only. It does,
 * however, have one advantage: it rehashes automatically, both when
 * it grows in size and when it shrinks, while retaining O(1) access
 * time. A normal hashtable with rehashing would have amortized O(1)
 * access time, but there would be single access with O(n) time
 * complexity for each rehashing. In this hashtable, rehashing is done
 * in parts. For example, a ht_add(), aside from adding an entry,
 * might also rehash 4 other entries from old table to the new one and
 * leave the rest unrehashed.
 * Of course, it is assumed that a good hash function is provided
 * by the programmer. If not, accesses may still degenerate to O(n).
 * Hence, this hashtable is not secure against DoS attacks.
 */

#ifndef HASHTABLE_H
#define HASHTABLE_H

#include <sys/types.h> /* for ssize_t */

/* These are possible return values of some ht_ functions (see below). */
#define HT_OK           0
#define HT_NO_MEM      -1
#define HT_KEY_PRESENT -2
#define HT_KEY_ABSENT  -3

typedef struct
{
  /* All members are considered implementation details, except for "entries",
   * which can be read, but should not be modified by external code.
   */
  size_t entries;

  /*
   * tab[] is where entries (chains of entries) are stored.
   * When rehashing, newtab[] is also used.
   */
  struct ht_node **tab, **newtab;

  /* sizes of tab[] and newtab[], obviously */
  size_t tab_size, new_size;

  size_t (*hashfunc)(const void* key);
  int (*cmpfunc)(const void* key1, const void *key2);

  /*
   * When no rehashing is taking place, rehashing_position is -1 (#define'd as
   * NOT_REHASHING in hashtable.c). At any other time, rehashing_position is the
   * lowest not yet rehashed position in the smaller table.
   */
  ssize_t rehashing_position;
} hashtable_t;

/*
 * All int functions return 0 (#define'd as HT_OK) on success and in
 * case of failure they return error codes, as described below.
 */

/* May fail with HT_NO_MEM. */
int ht_init(hashtable_t *ht,
	    size_t (*hash)(const void* key),
	    int (*cmp)(const void* key1, const void *key2));

/* May fail with HT_NO_MEM and HT_KEY_PRESENT. */
int ht_add(hashtable_t *ht, const void *key, const void *val);

/*
 * May fail with HT_NO_MEM. If key was not yet present in hashtable, *oldkey and
 * *oldval are not modified. Otherwise, just-replaced pair is stored in them.
 */
int ht_set(hashtable_t *ht, const void *key, const void *val,
	   void **oldkey, void **oldval);

/*
 * If present, the looked for pair is stored in *storedkey and *val. Otherwise,
 * they're not modified and HT_KEY_ABSENT is returned. storedkey and/or val can
 * be NULL.
 */
int ht_get(hashtable_t *ht, const void *key, void **storedkey, void **val);

/*
 * Works like ht_get() but is thread-safe with regard to other calls to
 * ht_get_threadsafe() on the same hashtable. Note that the hash and compare
 * functions supplied to the hashtable also have to be thread-safe (that
 * requirement is of course met for those used by ht_string_init()).
 */
int ht_get_threadsafe(hashtable_t *ht, const void *key,
		      void **storedkey, void **val);

/* Works like the above but also removes the pair from ht if found. */
int ht_rem(hashtable_t *ht, const void *key, void **storedkey, void **val);

/*
 * De-initializes the hashtable freeing all its structures. The programmer is
 * responsible for freeing keys and values if they were allocated from the heap
 * (see ht_map_destroy() below).
 */
void ht_destroy(hashtable_t *ht);

/* Calls ht_finish_resizing(), then maps through ht. */
void ht_map(hashtable_t *ht, void *arg,
	    void (*mapfunc)(const void *key, void *val, void *arg));

/*
 * It might be tempting to use ht_map() to free() all keys and values stored in
 * ht and then call ht_destroy(). If you think about it, ht_map() would leave
 * hashtable in a broken state - with keys being deallocated. Depending on the
 * implementation, ht_destroy() could cope with that, but we'd rather not
 * guarrantee anything, so here's another function just for that - mapping
 * through entries and destroying the hashtable immediately after, explicitly
 * allowing the mapping function to deallocate keys.
 */
void ht_map_destroy(hashtable_t *ht, void *arg,
		    void (*mapfunc)(void *key, void *val, void *arg));

/*
 * If hashtable is in the process of being rehashed, this function
 * processes it to the end. Otherwise - it does nothing.
 */
void ht_finish_resizing(hashtable_t *ht);

/* Included, since strings are commonly used as keys. */
size_t ht_string_hash(const char *key);

/*
 * May fail with HT_NO_MEM. Initializes ht for use with string keys
 * (using ht_string_hash() and strcmp()).
 */
int ht_string_init(hashtable_t *ht);

#endif /* HASHTABLE_H */