package listFW.visitor;

import listFW.*;

/**
 * Reverses the host list by accumulating the
 * reverse as one traverses the list.
 * This is best done via a helper visitor.
 * The accumulated reverse list can be
 * viewed as a "stack": just stack the first
 * as one traverses down the list; by the time
 * the end is reached, the accumulated stack
 * is the reverse list.
 * Remember that the end of a list is marked
 * by the empty list.
 * An IListFactory is passed to the constructor.
 * @author D. X. Nguyen
 * @custom Copyright 2002 - All rights reserved
 */
public class Reverse implements IListAlgo {

    private IListFactory _fact;

    public Reverse (IListFactory f) {
        _fact = f;
    }

    /**
    * Returns the host since the reverse of
    * the empty list is the empty list.
    * @param host an IEmptyList
    * @param nu not used
    * @return host
    */
    public Object emptyCase(IEmptyList host, Object nu) {
        return host;
    }

    /**
    * Passes to the host's rest the list consisting
    * of the host's first as the accumulated
    * reverse of the list preceding the host's rest,
    * and asks for help to reverse the host.
    * @param host
    * @param nu nut used
    * @return INEList, the reverse of host.
    */
    public Object nonEmptyCase(INEList host, Object nu) {
        IList acc = _fact.makeNEList(host.getFirst(),
                                  _fact.makeEmptyList());
        return  host.getRest().execute(new HelpReverse(_fact),
                                        acc);
    }
}