package lrs.visitor;

import lrs.*;

/**
 * Represents an abstract algorithm on an LRStruct.  Acts as a visitor to a
 * LRStruct host.
 * @author Dung X. Nguyen  Copyright 2005 - All rights reserved.
 * @since 03/29/05
 */
public class SwapEnds implements IAlgo {
    
    public static final SwapEnds Singleton = new SwapEnds();
    private SwapEnds(){}
    
    public Object emptyCase(LRStruct host, Object... nu){
        return null; // there is nothing to swap.
    }
    
    
    public Object nonEmptyCase(final LRStruct host, Object... nu){
        LRStruct taillist = host.getRest();
        
        return taillist.execute(new IAlgo(){
            
            public Object emptyCase(LRStruct child, Object... nu){
                return null;  // host is a single element list.
            }
            
            public Object nonEmptyCase(LRStruct child, Object... nu){
                return child.getRest().execute(new IAlgo(){
                    
                    public Object emptyCase(LRStruct c, Object... p){
                        Integer lastElement = (Integer)((LRStruct)p[0]).getFirst();
                        Integer firstElement = (Integer)host.getFirst();                        
                        host.setFirst(lastElement);
                        ((LRStruct) p[0]).setFirst(firstElement);
                        return null;
                    }
                    
                    public Object nonEmptyCase(LRStruct c, Object... nu){
                        return c.getRest().execute(this, c);
                    }
                }, child);
            }
        });
    }
}