dimaslanjaka / universal-framework

/**

 * Creates a new queue. A queue is a first-in-first-out (FIFO) data structure -

 * items are added to the end of the queue and removed from the front.

 * @example

 * // create a new queue

 * var queue = new Queue();

 * // enqueue an item

 * queue.enqueue('item');

 * // dequeue an item

 * var item = queue.dequeue();

 * // get the item at the front of the queue

 * var item = queue.peek();

 * // determine the number of items in the queue

 * var length = queue.getLength();

 * // determine whether the queue is empty

 * var isEmpty = queue.isEmpty();

 */

function Queue() {

  // initialise the queue and offset

  var queue = [];

  var offset = 0;

  // Returns the length of the queue.

  this.getLength = function () {

    return queue.length - offset;

  };

  // Returns true if the queue is empty, and false otherwise.

  this.isEmpty = function () {

    return queue.length == 0;

  };

  /* Enqueues the specified item. The parameter is:

   *

   * item - the item to enqueue

   */

  this.enqueue = function (item) {

    queue.push(item);

  };

  /* Dequeues an item and returns it. If the queue is empty, the value

   * 'undefined' is returned.

   */

  this.dequeue = function () {

    // if the queue is empty, return immediately

    if (queue.length == 0) return undefined;

    // store the item at the front of the queue

    var item = queue[offset];

    // increment the offset and remove the free space if necessary

    if (++offset * 2 >= queue.length) {

      queue = queue.slice(offset);

      offset = 0;

    }

    // return the dequeued item

    return item;

  };

  /* Returns the item at the front of the queue (without dequeuing it). If the

   * queue is empty then undefined is returned.

   */

  this.peek = function () {

    return queue.length > 0 ? queue[offset] : undefined;