package lrs.visitor;
import lrs.*;

/**
 * Uses reverse accumulation to remove duplicates from the host list.
 * @author Dung X. Nguyen - Copyright 2005 - All rights reserved.
 */
public class RemDupFwd implements IAlgo {
    public final static RemDupFwd Singleton = new RemDupFwd();
    private RemDupFwd() {
    }
    
    /**
     */
    public Object emptyCase(LRStruct host, Object... nu) {
        return host;
    }
    
    /**
     */
    public Object nonEmptyCase(final LRStruct host, Object... nu) {
        final LRStruct seenSoFar = new LRStruct();
        seenSoFar.insertFront(host.getFirst());
        return host.getRest().execute(new IAlgo() {
            public Object emptyCase(LRStruct h, Object... nu) {
                return host;
            }
            
            /**
             */
            public Object nonEmptyCase(LRStruct h, Object... nu) {
                // if h.first has been seen, remove it from h and recurs:
                if ((Boolean)seenSoFar.execute(Contain.Singleton, h.getFirst())) {
                    h.removeFront();
                    return h.execute(this);
                }
                // else add h.first to the seenSoFar list and recurs on h.rest:
                else {
                    seenSoFar.insertFront(h.getFirst());
                    return h.getRest().execute(this);
                }
            }
        });
    }
}