2018-3-2 面试题（面试结束时的版本）

concurrency.js

// 实现一个方法 parallel(tasks, concurrency)，让 tasks 并发执行（并控制并发数为 concurrency)
// 其中 tasks 为一个数组，每一个元素都是一个方法返回一个 promise
// 当所有 tasks 执行完成时，resolve 一个数组保存所有的结果
// 当任意一个 task 执行失败时，reject 这个错误

const assert = require('assert')

function task (input) {
  return () => new Promise(resolve => {
    setTimeout(() => resolve(input), 1000)
  })
}

parallel([
  task(1),
  task(2),
  task(3),
  task(4),
  task(5),
], 2)
.then(res => assert.deepEqual(res, [1, 2, 3, 4, 5]));

function parallel (tasks, concurrency) {
  return new Promise((resolve, reject) => {
    const result = tasks.slice()
    let promiseChain = Promise.resolve()
    let running = 0
    let index = 0

    function runTask (i) {
      running++

      const taskPromise = tasks[i]()
      promiseChain = promiseChain.then(() => taskPromise)

      return taskPromise.then(taskResult => {
        console.log(taskResult)
        result[i] = taskResult
        running--
      })
    }

    function scheduleNextTask () {
      if (running < concurrency) {
        runTask(index++).then(() => {
          if (index === tasks.length) {
            resolve(promiseChain.then(() => result))
          } else {
            scheduleNextTask()
          }
        })
      }
    }

    while (running < concurrency && index < tasks.length) {
      scheduleNextTask()
    }
  })
}