source: imaps-frontend/node_modules/webpack/lib/util/processAsyncTree.js

/*
        MIT License http://www.opensource.org/licenses/mit-license.php
        Author Tobias Koppers @sokra
*/

"use strict";

/**
 * @template T
 * @template {Error} E
 * @param {Iterable<T>} items initial items
 * @param {number} concurrency number of items running in parallel
 * @param {function(T, function(T): void, function(E=): void): void} processor worker which pushes more items
 * @param {function(E=): void} callback all items processed
 * @returns {void}
 */
const processAsyncTree = (items, concurrency, processor, callback) => {
        const queue = Array.from(items);
        if (queue.length === 0) return callback();
        let processing = 0;
        let finished = false;
        let processScheduled = true;

        /**
         * @param {T} item item
         */
        const push = item => {
                queue.push(item);
                if (!processScheduled && processing < concurrency) {
                        processScheduled = true;
                        process.nextTick(processQueue);
                }
        };

        /**
         * @param {E | null | undefined} err error
         */
        const processorCallback = err => {
                processing--;
                if (err && !finished) {
                        finished = true;
                        callback(err);
                        return;
                }
                if (!processScheduled) {
                        processScheduled = true;
                        process.nextTick(processQueue);
                }
        };

        const processQueue = () => {
                if (finished) return;
                while (processing < concurrency && queue.length > 0) {
                        processing++;
                        const item = /** @type {T} */ (queue.pop());
                        processor(item, push, processorCallback);
                }
                processScheduled = false;
                if (queue.length === 0 && processing === 0 && !finished) {
                        finished = true;
                        callback();
                }
        };

        processQueue();
};

module.exports = processAsyncTree;