gistfile1.js

import Namespace from '../core/Namespace'

const self = undefined
const internal = Namespace()

class Task {
  constructor(semaphore, callback) {
    const promises = [
      new Promise((resolve, reject) => {
        this.resolve = resolve
        this.reject = reject
      }),
      new Promise(resolve => {
        this.let = resolve
      }).then(() => {
        callback(this.resolve, this.reject)
      }),
    ]
    this.promise = Promise.all(promises)
      .then(values => {
        semaphore.signal()
        return values[0]
      }, reason => {
        semaphore.signal()
        return Promise.reject(reason)
      })
  }
}

export default class Semaphore {
  constructor(capacity) {
    const self = internal(this)
    self.capacity = capacity
    self.available = capacity
    self.queue = []
  }

  wait(callback) {
    const self = internal(this)
    const task = new Task(this, callback)
    if (self.available === 0) {
      self.queue.push(task)
    } else {
      --self.available
      task.let()
    }
    return task.promise
  }

  signal() {
    const self = internal(this)
    if (self.queue.length === 0) {
      ++self.available
    } else {
      self.queue.shift().let()
    }
  }

  // Properties

  get capacity() {
    const self = internal(this)
    return self.capacity
  }

  get available() {
    const self = internal(this)
    return self.available
  }
}