package queues;

import lrs.*;
import java.util.Enumeration;

/**
 * A queue implemented using LRStruct.
 *
 * Version 1.
 *
 * @author Alan L. Cox
 * @since 03/15/01
 */
public class LRSQueue implements IQueue
{
    private LRStruct _objects = new LRStruct();
    
    /**
     * Adds an to the LRSQueue.
     * @param object the object to enqueue
     * This enqueue is O(n) since it needs to traverse to the tail of the list
     * to add the element
     */
    public void enqueue(Object object)
    {
	_objects.insertEnd(object);
    }

    /**
     * Removes the first object from the LRSQueue.
     * @return the dequeued object
     * This dequeue is O(1) since it removes from the front
     */
    public Object dequeue()
    {     
       return _objects.removeFront();
    }

    /**
     * Enumerates the queue from oldest to newest object.
     * @return an Enumeration of the queue.
     * This enumeration is O(1) since it contains single statements.
     */
    public Enumeration enumeration()
    {
	/**
	 * We can't use an anonymous inner class to implement this
	 * Enumeration because of the cosntructor.
	 */
	class LRSEnumeration implements Enumeration
	{
	    private LRStruct  _next;
	    
	    LRSEnumeration(LRStruct lrs)
	    {
		_next = lrs;
	    }	      

	    public boolean hasMoreElements()
	    {
		return Boolean.TRUE == _next.execute(new IAlgo()
		    {
			public Object emptyCase(LRStruct host, Object input)
			{
			    return Boolean.FALSE;
			}

			public Object nonEmptyCase(LRStruct host, Object input)
			{
			    return Boolean.TRUE;
			}
		    }, null);
	    }

	    public Object nextElement()
	    {
		return _next.execute(new IAlgo()
		    {
			public Object emptyCase(LRStruct host, Object input)
			{
			    return null;
			}
			
			public Object nonEmptyCase(LRStruct host, Object input)
			{
			    Object object = _next.getFirst();
			    
			    _next = _next.getRest();
			    
			    return object;
			}
		    }, null);
	    }
	}
	return new LRSEnumeration(_objects);
    }
}