package scheme2.visitor;

import scheme2.AListFW;
import scheme2.IListAlgo;

public class MoveInParallel implements IListAlgo
{
	AListFW _tailing; // _tailing "follows" the host while it traverses to its end.

	public MoveInParallel(AListFW list)
	{
		_tailing = list;
	}
   
	/**
	 * @param host 
	 * @param input not used
	 */
	public Object forEmpty(AListFW host, Object input)
	{
	    /*when the leading list gets to the end, we return the list that followed it*/
		return _tailing;
	}

	/**
	 * @param host 
	 * @param input 
	 */
	public Object forNonEmpty(AListFW host, Object input)
	{
	    /*tailing "follows" the host while it traverses to its end*/
		_tailing = _tailing.getRest();
		/*tailing is moved one spot ahead and now we call MoveInParallel for 
		  the rest of the list*/
		return host.getRest().execute (this, null);
	}
}