package listFW.visitor;

import listFW.*;

/**
 * Helps computes the number of elements in a list
 * by accumulating the length.
 * @author D. X. Nguyen
 * @since Copyright 2002 - DXN All rights reserved
 */
public class GetLenHelp implements IListAlgo {

    public static final GetLenHelp Singleton = new GetLenHelp();

    private GetLenHelp() {
    }

    /**
     * Returns the accumulated length acc because the
     * empty list marks the end
     * of the enclosing list.
     * @param host an empty list.
     * @param acc the accumulated length of the preceding list.
     * @return Integer
     */
    public Object emptyCase(IEmptyList host, Object acc) {
        return acc;
    }


    /**
     * Passes the accumulated length (1) to the rest and
     * asks for help to
     * compute the length.
     * @param host a non-empty list.
     * @param acc the accumulated of the preceding list.
     * @return Integer
     */
    public Object nonEmptyCase(INEList host, Object acc) {
        int accLen = 1 + ((Integer)acc).intValue();
        return host.getRest().execute(this, new Integer(accLen));
    }
}