package stacks;

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

/**
 * A stack implemented using LRStruct.
 *
 * A stack manages its objects in a Last In, First Out (LIFO) order. 
 *
 * @author Alan L. Cox
 * @since 03/15/01
 */
public class LRSStack implements IStack
{
    private LRStruct _objects = new LRStruct();
    
    /**
     * Adds a new object to the LRSStack.
     * @param data is new object added to the stack.
     * This push method is O(1) since it only adds to the front of the list.
     */
    public void push(Object data)
    {
	_objects.insertFront(data);
    }

    /**
     * Removes and returns the newest object from the stack.
     *
     *@return the popped object
     * This pop method is O(1) since it only removes from the front of the list
     */
    public Object pop()
    {
	return _objects.removeFront();
    }

    /**
     * Enumerates the stack from newest to oldest object.
     * @return an Enumeration of the stack.
     * This enumeration method 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);
    }
}