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/**
 * This file is part of Haketilo.
 *
 * Function: Wrap APIs that depend on some asynchronous initialization into
 *     promises.
 *
 * Copyright (C) 2021 Wojtek Kosior
 *
 * 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.
 */

/*
 * This module provides an easy way to wrap an async function into a promise
 * so that it only gets executed once.
 */

async function assign_result(state, result_producer)
{
    state.result = await result_producer();
    state.ready = true;
    for (let cb of state.waiting)
	setTimeout(cb, 0, state.result);
    state.waiting = undefined;
}

async function get_result(state)
{
    if (state.ready)
	return state.result;

    return new Promise((resolve, reject) => state.waiting.push(resolve));
}

function make_once(result_producer)
{
    let state = {waiting : [], ready : false, result : undefined};
    assign_result(state, result_producer);
    return () => get_result(state);
}

/*
 * EXPORTS_START
 * EXPORT make_once
 * EXPORTS_END
 */