//
//  A Queue provided for the Spampede assignment
//  It should be better commented!
//


class Queue implements QueueInterface
{
  
  /*
   * an inner class, QCell, for the cells containing the Queue's data
   */  
  class QCell
  {
    private Object data;
    private QCell next;

    private QCell(Object data)
    {
      this.data = data;
      this.next = next;
    }
  }
    
  // the two data members of each Queue object
  private QCell front;  // the front of the queue (for dequeueing)
  private QCell back;   // the back of the queue (for enqueueing)

  /*
   * a constructor of an empty Queue
   */ 
  public Queue()
  {
    front = null;
    back = null;
  }

  /*
   * isEmpty returns true if the Queue is empty; false otherwise
   */
  public boolean isEmpty()
  {
    return (front == null && back == null);
  }

  /*
   * enqueue adds an element (a QCell containing data) onto the back of this Queue
   */
  public void enqueue(Object data)
  {
    QCell new_back = new QCell(data);
    if (this.isEmpty()) {
      front = back = new_back;
    } else {
      back.next = new_back;
      back = new_back;
    }
    return;
  }

  /*
   * dequeue removes an element from the front of this Queue and returns its data
   */
  public Object dequeue()
  {
    if (isEmpty()) {
      System.out.println("You can't dequeue from an empty queue.");
      return null;
    }
    Object o = front.data;
    if (front == back) {
      front = back = null;
    } else {
      front = front.next;
    }
    return o;
  }

  /*
   * peek returns the data at the front of this Queue
   */
  public Object peek()
  {
    if (isEmpty())
    {
      System.out.println("You can't peek an empty Queue!");
      return null;
    }
    Object data = front.data;
    return data;
  }
  
  /*
   * peekNext returns the data in the second-from-front of this Queue
   */
  public Object peekNext()
  {
    if (isEmpty() || front.next == null)
    {
      System.out.println("You can't peekNext in a Queue with < 2 elements!");
      return null;
    }
    Object data = front.next.data;
    return data;
  }

  /*
   * toString returns the String representation of this Queue
   */
  public String toString()
  {
    String result = "<FRONT> ";
    QCell current = this.front;
    while (current != null)
    {
      result += "" + current.data + " ";
      current = current.next;
    } 
    result += "<BACK>";
    return result;
  }

  /*
   * main tests our Queue class
   */
  public static void main(String[] args)
  {
    Queue ball = new Queue();
    System.out.println("ball is " + ball);
    ball.enqueue("Will");
    System.out.println("ball is " + ball);
    ball.enqueue("this");
    System.out.println("ball is " + ball);
    ball.enqueue("work?");
    System.out.println("ball is " + ball);
    ball.dequeue();
    System.out.println("ball is " + ball);
    ball.dequeue();
    System.out.println("ball is " + ball);
    ball.dequeue();
    System.out.println("ball is " + ball);
    ball.dequeue();
    System.out.println("ball is " + ball);
    ball.enqueue("Will");
    System.out.println("ball is " + ball);
    ball.enqueue("this");
    System.out.println("ball is " + ball);
    ball.enqueue("work?");
    System.out.println("ball is " + ball);
  }
}