package schemeFactory.visitor;

import schemeFactory.*;

/**
 * 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.  The
 * end of a list is marked by the empty list.
 * @author D. X. Nguyen
 */
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
    * @param input not used
    * @return AList
    */
    public Object emptyCase(AList host, Object input) {
        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 input not used
    * @return AList
    */
    public Object nonEmptyCase(AList host, Object input) {
        AList accReverse = _fact.makeNEList(host.getFirst(), _fact.makeEmptyList());
        return  host.getRest().execute(new IListAlgo() {
            /**
            * Returns the accumulated reverse because this is the end of the list.
            * @param h not used
            * @param acc the accumulated reverse of the list preceding h.
            * @return AList
            */
            public Object emptyCase(AList h, Object acc) {
                return acc;
            }

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

        }, accReverse);
    }
}