'use strict';
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const Queue = require('yocto-queue');
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const pLimit = concurrency => {
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if (!((Number.isInteger(concurrency) || concurrency === Infinity) && concurrency > 0)) {
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throw new TypeError('Expected `concurrency` to be a number from 1 and up');
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}
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const queue = new Queue();
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let activeCount = 0;
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const next = () => {
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activeCount--;
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if (queue.size > 0) {
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queue.dequeue()();
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}
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};
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const run = async (fn, resolve, ...args) => {
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activeCount++;
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const result = (async () => fn(...args))();
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resolve(result);
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try {
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await result;
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} catch {}
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next();
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};
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const enqueue = (fn, resolve, ...args) => {
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queue.enqueue(run.bind(null, fn, resolve, ...args));
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(async () => {
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// This function needs to wait until the next microtask before comparing
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// `activeCount` to `concurrency`, because `activeCount` is updated asynchronously
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// when the run function is dequeued and called. The comparison in the if-statement
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// needs to happen asynchronously as well to get an up-to-date value for `activeCount`.
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await Promise.resolve();
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if (activeCount < concurrency && queue.size > 0) {
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queue.dequeue()();
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}
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})();
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};
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const generator = (fn, ...args) => new Promise(resolve => {
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enqueue(fn, resolve, ...args);
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});
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Object.defineProperties(generator, {
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activeCount: {
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get: () => activeCount
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},
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pendingCount: {
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get: () => queue.size
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},
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clearQueue: {
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value: () => {
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queue.clear();
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}
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}
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});
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return generator;
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};
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module.exports = pLimit;
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