package listFW.visitor;

import listFW.*;

/**
 * Reverses the host given the reverse of the list
 * preceding the host.
 * Passes the accumulated reverse to the constructor.
 * @author D. X. Nguyen
 * @custom Copyright 2002 - All rights reserved
 */
class HelpReverse implements IListAlgo {
    private IListFactory _fact;

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

    /**
    * Returns the accumulated reverse because
    * this is the end of the list.
    * @param host not used
    * @param acc an INEList, the accumulated reverse
    * of the list preceding host.
    * @return acc
    */
    public Object emptyCase(IEmptyList host, Object acc) {
        return acc;
    }

    /**
    * Accumulates the reverse by cons-ing the
    * host's first with the accumulated
    * reverse so far, and recur on the host's rest.
    * @param host the rest of the list to be reversed.
    * @param acc an INEList, the accumulated reverse
    * of the list preceding host.
    * @return IList
    */
    public Object nonEmptyCase(INEList host, Object acc) {
        return host.getRest().execute (this,
                _fact.makeNEList(host.getFirst(), (IList)acc));
    }
}