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| const links = ['link1', 'link2', 'link1', 'link3', 'link1', 'link2', 'link2', 'link7']; | |
| const expected = [1, 2, 1, 3, 1, 2, 2, 7]; | |
| const db = { link1: '1', link2: '2', link3: '3', link7: '7' }; | |
| function fetch(url) { | |
| console.log(`fetching: ${url}`) | |
| const response = db[url]; | |
| if (response === undefined) { | |
| throw new Error('No such url'); | |
| } | |
| return new Promise(resolve => { | |
| const timeout = Math.random() * 1000 + 1000 + 20; | |
| setTimeout(() => resolve(response), timeout); | |
| }); | |
| } | |
| function callOnce(fn) { | |
| this.called = false; | |
| return (...args) => { | |
| if (this.called) { | |
| throw new Error('Callback called second time'); | |
| } | |
| this.called = true; | |
| return fn(...args); | |
| }; | |
| } | |
| function test(results) { | |
| if (!Array.isArray(results) || results.toString() !== expected.toString()) { | |
| console.error(`Expected: ${expected.toString()}`); | |
| console.error(`Received: ${results.toString()}`); | |
| return | |
| } | |
| console.log('Test passed'); | |
| } | |
| console.clear(); | |
| parallelLimit(links, 3, callOnce(test)); | |
| function parallelLimit(urls, streams, cb) { | |
| if (urls.length === 0) { | |
| cb([]); | |
| } | |
| const copyLinks = [...urls]; | |
| let currentStreams = 0; | |
| let callCount = 0; | |
| const res = []; | |
| function request(count) { | |
| if (copyLinks.length === 0) { | |
| return; | |
| } | |
| if (currentStreams < streams) { | |
| const currentLink = copyLinks.splice(0, 1); | |
| currentStreams++; | |
| callCount++; | |
| fetch(currentLink).then(result => { | |
| currentStreams--; | |
| res[count] = result; | |
| }).catch(err => { | |
| res[count] = err; | |
| }).finally(() => { | |
| request(callCount); | |
| if (urls.length === res.length) { | |
| cb(res); | |
| } | |
| }) | |
| } | |
| } | |
| for (let i = 0; i < streams; i++) { | |
| request(callCount); | |
| } | |
| } |
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