package lrs.visitor;

import lrs.*;
/**
* Helper to reverse a list.
* @since 10/05/01
* @author DXN
*/
class ReverseHelp implements IAlgo {
    public static final ReverseHelp Singleton = new ReverseHelp();

    private ReverseHelp() {
    }

    /**
     * Does nothing because the host is empty.
     * From top to host is the reverse of the original list.
     * @param host the remaining tail of the list to be reversed.
     * @param top from top to host is the reversed list so far.
     * @return null.
     */
    public Object forEmpty(LRStruct host, Object top) {
        return null;
    }

    /**
     * Removes the host's first and insert it to the front of the top, and
     * recurs.
     * @param host the remaining tail of the list to be reversed.
     * @param top from top to host is the reversed list so far.
     * @return null.
     */
    public Object forNonEmpty(LRStruct host, Object top) {
        Object hostFirst = host.removeFront(); // host has "advanced".
        ((LRStruct)top).insertFront(hostFirst);
        return host.execute(this, top);
    }
}