package lrs.visitor;
import lrs.*;

/**
 * Removes the odd-indexed elements from the host LRStruct and retuns
 * an LRStruct containing these elements.
 * @author Dung X. Nguyen - Copyright 2005 - All rights reserved.
 */
public class Unzip implements IAlgo {
    public final static Unzip Singleton = new Unzip();
    private Unzip() {
    }

    /**
     * @param host is empty.
     * @param nu not used
     * @return an empty LRStruct.
     */
    public Object emptyCase(LRStruct host, Object... nu) {
        return new LRStruct();
    }

    /**
     * @param host is not empty
     * @param nu not used
     * @return LRStruct containing the odd indexed element of the orignal host.
     */
    public Object nonEmptyCase(LRStruct host, Object... nu) {
        // Hold on to the tail before setting the host's rest to a different list:
        LRStruct tail = host.getRest();
        // Get the odd indexed elements of the tail:
        LRStruct other = (LRStruct)tail.execute(this);
        // The odd indexed elements of the tail are the even-indexed elements
        // of the original host:
        host.setRest(other);
        return tail;
    }
}