package listFW.visitor;
import listFW.*;
import listFW.factory.*;
import listFW.visitor.*;

/**
 * Returns the even-indexed elements of the host.
 * @author DXN
 */
public class Even implements IListAlgo {
    private IListFactory _fact;
    private IListAlgo _oddHelp;
    
    public Even(IListFactory f) {
        _fact = f;
        _oddHelp = new OddHelp(_fact, this);
    }
          
    public Object emptyCase(IEmptyList host, Object nu) {
        return host;
    }

    /**
     */
    public Object nonEmptyCase(INEList host, Object input) {
        IList evenRest = (IList)host.getRest().execute(_oddHelp, null);
        return _fact.makeNEList(host.getFirst(), evenRest);
    }
}

/**
 * Helper returning the odd-indexed elements of the host.
 * @author DXN
 */
class OddHelp implements IListAlgo {
    private IListFactory _fact;
    private IListAlgo _even;
    
    public OddHelp(IListFactory f, IListAlgo e) {
        _fact = f;
        _even = e;
    }
    
    public Object emptyCase(IEmptyList host, Object nu) {
        return host;
    }

    /**
     */
    public Object nonEmptyCase(INEList host, Object input) {
        return host.getRest().execute(_even, null);
    }
}