function Queue() {
  this.head = new Item('head', null);
}
module.exports = Queue;

Queue.prototype.append = function append(kind, value) {
  var item = new Item(kind, value);
  this.head.prepend(item);
  return item;
};

Queue.prototype.isEmpty = function isEmpty() {
  return this.head.prev === this.head;
};

Queue.prototype.first = function first() {
  return this.head.next;
};

function Item(kind, value) {
  this.prev = this;
  this.next = this;
  this.kind = kind;
  this.value = value;
}

Item.prototype.prepend = function prepend(other) {
  other.prev = this.prev;
  other.next = this;
  other.prev.next = other;
  other.next.prev = other;
};

Item.prototype.dequeue = function dequeue() {
  var prev = this.prev;
  var next = this.next;

  prev.next = next;
  next.prev = prev;
  this.prev = this;
  this.next = this;

  return this.value;
};

Item.prototype.isEmpty = function isEmpty() {
  return this.prev === this;
};