/**
 * 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: "sha256"}],
    ["file_uses", {keyPath: "sha256"}],
    ["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(initialization_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);

	await new Promise(resolve => store.transaction.oncomplete = resolve);

	save_items(db, initialization_data);
    }

    db = opened_db;

    return db;
}

/*
 * 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(db, data)
{
    const files = data.files;
    const resources =
	  Object.values(data.resources || []).map(entities.get_newest);
    const mappings =
	  Object.values(data.mappings || []).map(entities.get_newest);

    resources.concat(mappings).forEach(i => save_item(i, data.files, db));
}

/* helper function of save_item() */
async function get_file_uses(transaction, file_uses_sha256, file_ref)
{
    let uses = file_uses_sha256[file_ref.sha256];
    if (uses === undefined) {
	uses = await idb_get(transaction, "file_uses", file_ref.sha256);
	if (uses)
	    [uses.new, uses.initial] = [false, uses.uses];
	else
	    uses = {sha256: file_ref.sha256, uses: 0, new: true, initial: 0};

	file_uses_sha256[file_ref.sha256] = uses;
    }

    return uses;
}

/*
 * 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 their sha256 sums.
 */
async function save_item(item, files_sha256, db)
{
    const store_name = {resource: "resources", mapping: "mappings"}[item.type];
    const transaction =
	  db.transaction([store_name, "files", "file_uses"], "readwrite");

    let resolve, reject;
    const result = new Promise((...cbs) => [resolve, reject] = cbs);
    transaction.oncomplete = resolve;
    transaction.onerror = reject;

    const uses_sha256 = {};
    for (const file_ref of entities.get_files(item))
	(await get_file_uses(transaction, uses_sha256, file_ref)).uses++;

    const old_item = await idb_get(transaction, store_name, item.identifier);
    if (old_item !== undefined) {
	for (const file_ref of entities.get_files(old_item))
	    (await get_file_uses(transaction, uses_sha256, file_ref)).uses--;
    }

    for (const uses of Object.values(uses_sha256)) {
	if (uses.uses < 0)
	    console.error("internal error: uses < 0 for file " + uses.sha256);

	const [is_new, initial_uses] = [uses.new, uses.initial];
	delete uses.new;
	delete uses.initial;

	if (uses.uses < 1) {
	    if (!is_new) {
		idb_del(transaction, "file_uses", uses.sha256);
		idb_del(transaction, "files",     uses.sha256);
	    }

	    continue;
	}

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

	const file = files_sha256[uses.sha256];
	if (file === undefined)
	    throw "file not present: " + uses.sha256;

	idb_put(transaction, "files", {sha256: uses.sha256, contents: file});
	idb_put(transaction, "file_uses", uses);
    }

    idb_put(transaction, store_name, item);

    return result;
}

const haketilodb = {
    get:       get_db,
    save_item: save_item
};

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