/* Queue */
/**
 * insertion  - O(1)
 * removal    - O(1)
 * searching  - O(n)
 * access     - O(n)
 */

function Queue() { 
  let size = 0; 
  let first = null;
  let last = null;

  const Node = function(element){
    this.element = element; 
    this.next = null;
  };

  this.enqueue = function (element) {
    const node = new Node(element);
    
    if (first === null) {
      first = node;
      last = node;
    } else {
      last.next = node;
      last = node;
    }

    size++;
    return this;
  }

  this.dequeue = function () {
    if (first === null) {
      return undefined;
    }

    const currentNode = first;
    first = currentNode.next;
    size--;

    if (size === 0) {
      first = null;
      last = null;
    }

    return currentNode.element;
  }

  this.isEmpty = function () {
    return size === 0;
  };

  this.toArray = function () {
    let array = [];
    let currentNode = first;

    while (currentNode !== null) {
      array.push(currentNode.element);
      currentNode = currentNode.next;
    }

    return array;
  }

  this.size = function (){
    return size;
  };

  this.first = function (){
    return first.element;
  };

  this.last = function () {
    return last.element;
  };

  this.firstNode = function (){
    return first;
  };

  this.lastNode = function () {
    return last;
  };
}