source: trip-planner-front/node_modules/p-limit/index.js@ eed0bf8

'use strict';
const Queue = require('yocto-queue');

const pLimit = concurrency => {
        if (!((Number.isInteger(concurrency) || concurrency === Infinity) && concurrency > 0)) {
                throw new TypeError('Expected `concurrency` to be a number from 1 and up');
        }

        const queue = new Queue();
        let activeCount = 0;

        const next = () => {
                activeCount--;

                if (queue.size > 0) {
                        queue.dequeue()();
                }
        };

        const run = async (fn, resolve, ...args) => {
                activeCount++;

                const result = (async () => fn(...args))();

                resolve(result);

                try {
                        await result;
                } catch {}

                next();
        };

        const enqueue = (fn, resolve, ...args) => {
                queue.enqueue(run.bind(null, fn, resolve, ...args));

                (async () => {
                        // This function needs to wait until the next microtask before comparing
                        // `activeCount` to `concurrency`, because `activeCount` is updated asynchronously
                        // when the run function is dequeued and called. The comparison in the if-statement
                        // needs to happen asynchronously as well to get an up-to-date value for `activeCount`.
                        await Promise.resolve();

                        if (activeCount < concurrency && queue.size > 0) {
                                queue.dequeue()();
                        }
                })();
        };

        const generator = (fn, ...args) => new Promise(resolve => {
                enqueue(fn, resolve, ...args);
        });

        Object.defineProperties(generator, {
                activeCount: {
                        get: () => activeCount
                },
                pendingCount: {
                        get: () => queue.size
                },
                clearQueue: {
                        value: () => {
                                queue.clear();
                        }
                }
        });

        return generator;
};

module.exports = pLimit;