package lrs.visitor;
import lrs.*;

/**
 * @author Dung X. Nguyen - Copyright 2005 - All rights reserved.
 */
public class RunningSum implements IAlgo {
    public final static RunningSum Singleton = new RunningSum();
    private RunningSum() {
    }
    
    /**
     */
    public Object emptyCase(LRStruct host, Object... nu) {
        return null;
    }
    
    /**
     */
    public Object nonEmptyCase(LRStruct host, Object... nu) {
        return host.getRest().execute(new IAlgo() {
            public Object emptyCase(LRStruct child, Object... nu){
                return null;  // host is a single element list.
            }
            
            public Object nonEmptyCase(LRStruct child, Object... acc){
                Integer sum = (Integer)acc[0] + (Integer)child.getFirst();
                child.setFirst(sum);
                return child.getRest().execute(this, sum);
            }
        }, host.getFirst());
    }
}