/**
 * This file is part of Haketilo.
 *
 * Function: Facilitate use of IndexedDB within Haketilo.
 *
 * Copyright (C) 2021 Wojtek Kosior <koszko@koszko.org>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * As additional permission under GNU GPL version 3 section 7, you
 * may distribute forms of that code without the copy of the GNU
 * GPL normally required by section 4, provided you include this
 * license notice and, in case of non-source distribution, a URL
 * through which recipients can access the Corresponding Source.
 * If you modify file(s) with this exception, you may extend this
 * exception to your version of the file(s), but you are not
 * obligated to do so. If you do not wish to do so, delete this
 * exception statement from your version.
 *
 * As a special exception to the GPL, any HTML file which merely
 * makes function calls to this code, and for that purpose
 * includes it by reference shall be deemed a separate work for
 * copyright law purposes. If you modify this code, you may extend
 * this exception to your version of the code, but you are not
 * obligated to do so. If you do not wish to do so, delete this
 * exception statement from your version.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 * I, Wojtek Kosior, thereby promise not to sue for violation of this file's
 * license. Although I request that you do not make use this code in a
 * proprietary program, I am not going to enforce this in court.
 */

/*
 * IMPORTS_START
 * IMPORT initial_data
 * IMPORT entities
 * IMPORTS_END
 */

/* Update when changes are made to database schema. Must have 3 elements */
const db_version = [1, 0, 0];

const nr_reductor = ([i, s], num) => [i - 1, s + num * 1024 ** i];
const version_nr = ver => Array.reduce(ver.slice(0, 3), nr_reductor, [2, 0])[1];

const stores = 	[
    ["files",     {keyPath: "hash_key"}],
    ["file_uses", {keyPath: "hash_key"}],
    ["resources", {keyPath: "identifier"}],
    ["mappings",  {keyPath: "identifier"}]
];

let db = null;

/* Generate a Promise that resolves when an IndexedDB request succeeds. */
async function wait_request(idb_request)
{
    let resolve, reject;
    const waiter = new Promise((...cbs) => [resolve, reject] = cbs);
    [idb_request.onsuccess, idb_request.onerror] = [resolve, reject];
    return waiter;
}

/* asynchronous wrapper for IDBObjectStore's get() method. */
async function idb_get(transaction, store_name, key)
{
    const req = transaction.objectStore(store_name).get(key);
    return (await wait_request(req)).target.result;
}

/* asynchronous wrapper for IDBObjectStore's put() method. */
async function idb_put(transaction, store_name, object)
{
    return wait_request(transaction.objectStore(store_name).put(object));
}

/* asynchronous wrapper for IDBObjectStore's delete() method. */
async function idb_del(transaction, store_name, key)
{
    return wait_request(transaction.objectStore(store_name).delete(key));
}

/* Open haketilo database, asynchronously return an IDBDatabase object. */
async function get_db(data=initial_data)
{
    if (db)
	return db;

    let resolve, reject;
    const waiter = new Promise((...cbs) => [resolve, reject] = cbs);

    const request = indexedDB.open("haketilo", version_nr(db_version));
    request.onsuccess       = resolve;
    request.onerror         = ev => reject("db error: " + ev.target.errorCode);
    request.onupgradeneeded = resolve;

    const event = await waiter;
    const opened_db = event.target.result;

    if (event instanceof IDBVersionChangeEvent) {
	/*
	 * When we move to a new database schema, we will add upgrade logic
	 * here.
	 */
	if (event.oldVersion > 0)
	    throw "bad db version: " + event.oldVersion;

	let store;
	for (const [store_name, key_mode] of stores)
	    store = opened_db.createObjectStore(store_name, key_mode);

	const context = make_context(store.transaction, data.files);

	await _save_items(data.resources, data.mappings, context);
    }

    db = opened_db;

    return db;
}

/* Helper function used by start_items_transaction() and get_db(). */
function make_context(transaction, files)
{
    files = files || {};
    const context = {transaction, files, file_uses: {}};

    let resolve, reject;
    context.result = new Promise((...cbs) => [resolve, reject] = cbs);

    context.transaction.oncomplete = resolve;
    context.transaction.onerror = reject;

    return context;
}

/*
 * item_store_names should be an array with either string "mappings", string
 * "resources" or both. files should be a dict with values being contents of
 * files that are to be possibly saved in this transaction and keys of the form
 * `sha256-<file's-sha256-sum>`.
 *
 * Returned is a context object wrapping the transaction and handling the
 * counting of file references in IndexedDB.
 */
async function start_items_transaction(item_store_names, files)
{
    const db = await haketilodb.get();
    const scope = [...item_store_names, "files", "file_uses"];
    return make_context(db.transaction(scope, "readwrite"), files);
}

async function incr_file_uses(context, file_ref, by=1)
{
    const hash_key = file_ref.hash_key;
    let uses = context.file_uses[hash_key];
    if (uses === undefined) {
	uses = await idb_get(context.transaction, "file_uses", hash_key);
	if (uses)
	    [uses.new, uses.initial] = [false, uses.uses];
	else
	    uses = {hash_key, uses: 0, new: true, initial: 0};

	context.file_uses[hash_key] = uses;
    }

    uses.uses = uses.uses + by;
}

const decr_file_uses = (ctx, file_ref) => incr_file_uses(ctx, file_ref, -1);

async function finalize_items_transaction(context)
{
    for (const uses of Object.values(context.file_uses)) {
	if (uses.uses < 0)
	    console.error("internal error: uses < 0 for file " + uses.hash_key);

	const is_new       = uses.new;
	const initial_uses = uses.initial;
	const hash_key     = uses.hash_key;

	delete uses.new;
	delete uses.initial;

	if (uses.uses < 1) {
	    if (!is_new) {
		idb_del(context.transaction, "file_uses", hash_key);
		idb_del(context.transaction, "files",     hash_key);
	    }

	    continue;
	}

	if (uses.uses === initial_uses)
	    continue;

	idb_put(context.transaction, "file_uses", uses);

	if (initial_uses > 0)
	    continue;

	const file = context.files[hash_key];
	if (file === undefined) {
	    context.transaction.abort();
	    throw "file not present: " + hash_key;
	}

	idb_put(context.transaction, "files", {hash_key, contents: file});
    }

    return context.result;
}

async function with_items_transaction(cb, item_store_names, files={})
{
    const context = await start_items_transaction(item_store_names, files);
    await cb(context);
    await finalize_items_transaction(context);
}

/*
 * How a sample data argument to the function below might look like:
 *
 * data = {
 *     resources: {
 *         "resource1": {
 *             "1": {
 *                 // some stuff
 *             },
 *             "1.1": {
 *                 // some stuff
 *             }
 *         },
 *         "resource2": {
 *             "0.4.3": {
 *                 // some stuff
 *             }
 *         },
 *     },
 *     mappings: {
 *         "mapping1": {
 *             "2": {
 *                 // some stuff
 *             }
 *         },
 *         "mapping2": {
 *             "0.1": {
 *                 // some stuff
 *             }
 *         },
 *     },
 *     files: {
 *         "sha256-f9444510dc7403e41049deb133f6892aa6a63c05591b2b59e4ee5b234d7bbd99": "console.log(\"hello\");\n",
 *         "sha256-b857cd521cc82fff30f0d316deba38b980d66db29a5388eb6004579cf743c6fd": "console.log(\"bye\");"
 *     }
 * }
 */
async function save_items(transaction, data)
{
    const items_store_names = ["resources", "mappings"];
    const context = start_items_transaction(items_store_names, data.files);

    return _save_items(data.resources, data.mappings, context);
}

async function _save_items(resources, mappings, context)
{
    resources = Object.values(resources || {}).map(entities.get_newest);
    mappings  = Object.values(mappings  || {}).map(entities.get_newest);

    for (const item of resources.concat(mappings))
	await save_item(item, context);

    await finalize_items_transaction(context);
}

/*
 * Save given definition of a resource/mapping to IndexedDB. If the definition
 * (passed as `item`) references files that are not already present in
 * IndexedDB, those files should be present as values of the `files_sha256`
 * object with keys being of the form `sha256-<file's-sha256-sum>`.
 *
 * context should be one returned from start_items_transaction() and should be
 * later passed to finalize_items_transaction() so that files depended on are
 * added to IndexedDB and files that are no longer depended on after this
 * operation are removed from IndexedDB.
 */
async function save_item(item, context)
{
    const store_name = {resource: "resources", mapping: "mappings"}[item.type];

    for (const file_ref of entities.get_files(item))
	await incr_file_uses(context, file_ref);

    await _remove_item(store_name, item.identifier, context, false);
    await idb_put(context.transaction, store_name, item);
}

/* Helper function used by remove_item() and save_item(). */
async function _remove_item(store_name, identifier, context)
{
    const item = await idb_get(context.transaction, store_name, identifier);
    if (item !== undefined) {
	for (const file_ref of entities.get_files(item))
	    await decr_file_uses(context, file_ref);
    }
}

/*
 * Remove definition of a resource/mapping from IndexedDB.
 *
 * context should be one returned from start_items_transaction() and should be
 * later passed to finalize_items_transaction() so that files depended on are
 * added to IndexedDB and files that are no longer depended on after this
 * operation are removed from IndexedDB.
 */
async function remove_item(store_name, identifier, context)
{
    await _remove_item(store_name, identifier, context);
    await idb_del(context.transaction, store_name, identifier);
}

const remove_resource =
      (identifier, ctx) => remove_item("resources", identifier, ctx);
const remove_mapping =
      (identifier, ctx) => remove_item("mappings",  identifier, ctx);

const haketilodb = {
    get: get_db,
    save_items,
    save_item,
    remove_resource,
    remove_mapping,
    start_items_transaction,
    finalize_items_transaction
};

/*
 * EXPORTS_START
 * EXPORT haketilodb
 * EXPORT idb_get
 * EXPORT idb_put
 * EXPORT idb_del
 * EXPORTS_END
 */