package lrs.visitor;
import lrs.*;

/**
 * Merges the non-empty input list into the host list.  In the end, both lists share the same head node.
 * Serves as a helper visitor for Merger.  Uses Becomes.
 * @author Dung X. Nguyen - Copyright 1999 - All rights reserved.
 * @dependency Becomes uses
 */
public class MergeHelper implements IAlgo
{
	/** @SBGen Singleton Variable   */
	public final static MergeHelper Singleton = new MergeHelper ();

	/** @SBGen Constructor   */
	private MergeHelper()
	{
	}
	/**
	 * @param host 
	 * @param input a non-empty LRStruct. 
	 * @return 
	 */
	public Object nullCase(LRStruct host, Object input)
	{
		return host.execute (Becomes.Singleton, input);
	}

	/**
	 * @param host a non-empty LRStruct
	 * @param input a non-empty LRStruct.
	 * @return 
	 */
	public Object nonNullCase(LRStruct host, Object input)
	{
      LRStruct inList = (LRStruct)input;
      int hostFirst = ((Integer)host.getFirst ()).intValue ();
      int inputFirst = ((Integer)inList.getFirst ()).intValue ();
      if (hostFirst <= inputFirst)
      {
         host.getRest ().execute(this, inList);
         return inList.execute(Becomes.Singleton, host);
      }
      else
      {
         inList.getRest ().execute (this, host);
         return host.execute(Becomes.Singleton, inList);
      }
	}
}