package lrs.visitor;
import lrs.*;

/**
* Assigns the input list to the host.
* host thus "becomes" the input list.
* In the end, both host and the input list share the same head node.
*/
public class Becomes implements IAlgo
{
	public final static Becomes Singleton = new Becomes ();

	private Becomes()
	{
	}

	/**
	 * @param host 
	 * @param input a LRStruct;
	 * @return null
	 */
	public Object nullCase(LRStruct host, Object input)
	{
      return assign (host, (LRStruct)input);
	}

	/**
	 * @param host 
	 * @param input a LRStruct;
	 * @return null
	 */
	public Object nonNullCase(LRStruct host, Object input)
	{
      return assign (host, (LRStruct)input);
	}

   private Object assign (LRStruct lhs, LRStruct rhs)
   {
      lhs.insertFront (null);
      lhs.setRest (rhs);
      lhs.removeFront();
		return null;
   }
}