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;
}