package horstmann.ch03_queue;
/**
    A first-in, first-out bounded collection of messages.
 */
public class MessageQueue
{
  /**
       Constructs an empty message queue.
       @param capacity the maximum capacity of the queue
       <p><b>Precondition:</b> capacity > 0
   */
  public MessageQueue(int capacity)
  {
    elements = new Message[capacity];
    count = 0;
    head = 0;
    tail = 0;
  }

  /**
       Remove message at head.
       @return the message that has been removed from the queue
       <p><b>Precondition:</b> size() > 0
   */
  public Message remove()
  {
    Message r = elements[head];
    head = (head + 1) % elements.length;
    count--;
    return r;
  }

  /**
       Append a message at tail.
       @param aMessage the message to be appended
       <p><b>Precondition:</b> !isFull();
   */
  public void add(Message aMessage)
  {
    elements[tail] = aMessage;
    tail = (tail + 1) % elements.length;
    count++;
  }

  /**
       Get the total number of messages in the queue.
       @return the total number of messages in the queue
   */
  public int size()
  {
    return count;
  }

  /**
       Checks whether this queue is full
       @return true if the queue is full
   */
  public boolean isFull()
  {
    return count == elements.length;
  }

  /**
       Get message at head.
       @return the message that is at the head of the queue
       <p><b>Precondition:</b> size() > 0
   */
  public Message peek()
  {
    return elements[head];
  }

  private Message[] elements;
  private int head;
  private int tail;
  private int count;
}