package lrs.visitor;
import lrs.*;

public class MergeSortHelper implements IAlgo
{
  public static final MergeSortHelper Singleton = new MergeSortHelper ();
  private MergeSortHelper()
  {
  }

	/**
	 * @param host 
	 * @param input LRStruct parent of host
	 * @return null
	 */
	public Object nullCase(LRStruct host, Object input)
	{
		return null;  // there is nothing to sort
	}

	/**
	 * @param host 
	 * @param input LRStruct parent of host
	 * @return null.
	 */
	public Object nonNullCase(LRStruct host, Object input)
	{
    LRStruct hostParent = (LRStruct)input;
    LRStruct sublist = (LRStruct)hostParent.execute (EveryOther.Singleton, null);
    hostParent.execute (MergeSorter.Singleton, null);
		sublist.execute (MergeSorter.Singleton, null);
    hostParent.execute (Merger.Singleton, sublist);
    return null;
	}
}