package OOscheme.visitor;

import OOscheme.*;

/**
 * Extracts the even-indexed elements of a list.
 * @author DXN
 */
public class EvenIndexed implements IListAlgo {

    public static final EvenIndexed Singleton = new EvenIndexed();
    private EvenIndexed() {
    }

    /**
     * The list consisting of the even-indexed elements of an empty list is
     * an empty list.
     * @param host an empty list.
     * @param nu not used
     * @return IEmptyList, the host itself.
     */
    public Object emptyCase(IEmptyList host, Object nu) {
        return host;
    }

    /**
     * The even-indexed elements of a non-empty list consist of its first
     * followed by the odd-indexed elements of its rest.
     * @param host a non-empty list
     * @param nu not used
     * @return IList
     */
    public Object nonEmptyCase(INEList host, Object nu) {
        return new NEList(host.getFirst(),
                          (IList)host.getRest().execute(OddIndexed.Singleton, null));
    }
}

package OOscheme.visitor;

import OOscheme.*;

/**
 * Extracts the odd-indexed elements of a list.
 * @author DXN
 */
public class OddIndexed implements IListAlgo {

    public static final OddIndexed Singleton = new OddIndexed();
    private OddIndexed() {
    }

    /**
     * The list consisting of the odd-indexed elements of an empty list is
     * an empty list.
     * @param host an empty list.
     * @param nu not used
     * @return IEmptyList, the host itself.
     */
    public Object emptyCase(IEmptyList host, Object nu) {
        return host;
    }

    /**
     * The odd-indexed elements of a non-empty list are the even-indexed
     * elements of its rest.
     * @param host a non-empty list
     * @param nu not used
     * @return IList
     */
    public Object nonEmptyCase(INEList host, Object nu) {
        return host.getRest().execute(EvenIndexed.Singleton, null);
    }
}