/**
 * Allow promises to be queued up and executed in order.
 */
export class PromiseQueue {
  private queue: any[] = [];
  private locked: boolean = false;

  /** Add new promise to queue */
  add(p: () => Promise<any>, delay?: number) {
    this._add(p, false, delay);
  }

  /** Add new promise to the from of the queue */
  addToBeginning(p: () => Promise<any>, delay?: number) {
    this._add(p, true, delay);
  }

  /** Clear the queue and reset state. */
  clearQueue() {
    this.queue = [];
  }

  private delay = (ms: number) => () => new Promise(resolve => setTimeout(resolve, ms));

  _add = (p: () => Promise<any>, beginning: boolean = false, delay?: number) => {
    const newPromise = delay ? [p, this.delay(delay)] : [p];
    beginning ? this.queue.unshift(newPromise) : this.queue.push(newPromise);
    if (!this.locked) {
      this.next();
    }
  };

  private next() {
    this.locked = true;
    const p = this.queue.shift();
    const finished = () => {
      this.queue.length > 0 ? this.next() : (this.locked = false);
    };
    Promise.all(p.map((v: any) => v()))
      .then(finished)
      .catch(finished);
  }
}