package lrs.visitor;
import lrs.*;

public class EveryOtherHelper implements IAlgo
{
	public final static EveryOtherHelper Singleton = new EveryOtherHelper ();

	private EveryOtherHelper()
	{
	}

	/**
	 * @param host 
	 * @param parent
	 * @return 
	 */
	public Object nullCase(LRStruct host, Object parent)
	{
		return new LRStruct ();
	}

	/**
    * Removes every other elements from host.
	 * @param host a non-empty LRStruct.
	 * @param parent a non-empty LRStruct - the parent of host.
	 * @return the list consisting of elements that were left out
	 */
	public Object nonNullCase(LRStruct host, Object parent)
	{
      LRStruct parentList = (LRStruct)parent;
      LRStruct hostTail = host.getRest ();
      parentList.setRest (hostTail);
      host.setRest ((LRStruct)hostTail.execute (EveryOther.Singleton, null));
      return host;

	}
}