/*:: type Task = {
  task: () => Promise;
  _completion: Promise
};*/

/**
 * Copyright (c) 2015-present, Parse, LLC.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @flow
 */
class TaskQueue {
  /*:: queue: Array<Task>;*/
  constructor() {
    this.queue = [];
  }

  enqueue(task
  /*: () => Promise*/
  )
  /*: Promise*/
  {
    let res;
    let rej;
    const taskComplete = new Promise((resolve, reject) => {
      res = resolve;
      rej = reject;
    });
    taskComplete.resolve = res;
    taskComplete.reject = rej;
    this.queue.push({
      task: task,
      _completion: taskComplete
    });

    if (this.queue.length === 1) {
      task().then(() => {
        this._dequeue();

        taskComplete.resolve();
      }, error => {
        this._dequeue();

        taskComplete.reject(error);
      });
    }

    return taskComplete;
  }

  _dequeue() {
    this.queue.shift();

    if (this.queue.length) {
      const next = this.queue[0];
      next.task().then(() => {
        this._dequeue();

        next._completion.resolve();
      }, error => {
        this._dequeue();

        next._completion.reject(error);
      });
    }
  }

}

module.exports = TaskQueue;