yandex-sync-queue.js

class ListNode {
  constructor (val) {
    this.val = val;
    this.next = this.prev = null;
  }
}

class Queue {
  constructor () {
    this._head = null;
    this._tail = null;
    this._cur = null;
  }

  push (fn) {
    const oldTail = this._tail;
    const newTail = new ListNode( fn );
    this._tail = newTail;
    if (oldTail) {
      oldTail.prev = newTail;
    }
    newTail.next = oldTail;
    if (!this._head) {
      this._head = newTail;
    }

    if (!this._cur) {
      this.next();
    }
  }

  next () {
    if (!this._head) {
      return ( this._cur = null );
    }

    const oldHead = this._head;
    const newHead = oldHead.prev;
    if (newHead) {
      newHead.next = null;
    } else {
      this._tail = null;
    }
    this._head = newHead;
    oldHead.prev = null;

    oldHead.val();
    this._cur = oldHead;
  }
}