package lrs.visitor;

import lrs.*;
/**
* Reverses the host using anonymous class as a helper visitor.  Creates the
* helper object once.  Avoids having to name the helper.
* @author DXN
*/
public class ReverseAnonymous implements IAlgo {
    public static final ReverseAnonymous Singleton = new ReverseAnonymous ();

    /**
     * Helper visitor as an anonymous class.  This is not an inner class.
     */
    private static final IAlgo reverseHelp= new IAlgo () {
        /**
         * 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);
        }
    };

    private ReverseAnonymous () {
    }

    /**
     * Does nothing: the empty host is the reverse of itself.
     * @return null
     */
    public Object forEmpty(LRStruct host, Object input) {
        return null;
    }

    /**
     * Asks for help to recursively move down the host, removes one element at
     * a time and inserting it to the front of the host.  In the end, the host
     * is reversed.
     * @return null
     */
    public Object forNonEmpty(LRStruct host, Object input) {
        return host.getRest().execute(reverseHelp, host);
    }
}