package lrs.visitor;

import lrs.*;

/**
* Weaves ("Zips") together two distinct LRStructs that do not
* share any node to yield a single LRStruct.
* For example, when we zip (5, 3, 9), the host, together
* with (-7, 0, 8, -15), the input parameter, both lists
* become (5, -7, 3, 0, 9, 8, -15).
*/

public class Zipper implements IAlgo {
    public static final Zipper Singleton = new Zipper();
    private Zipper() {
    }

    /**
     * host is empty, so there is nothing to zip with.
     * Just let host become the input list.
     * @param host empty
     * @param input the other LRStruct.
     */
    public Object emptyCase(LRStruct host, Object input) {
        return host.execute(Becomes.Singleton, input);
    }

    /**
     * Recursively weave the input LRStruct with host's rest.
     * The result is both input and host'rest share the same
     * head node.  All we need to do next is assign host to
     * input so that input "becomes" the host and share the same
     * head node as host.
     * @param host not empty
     * @param input the other LRStruct.
     */
    public Object nonEmptyCase(final LRStruct host, Object input) {
        ((LRStruct)input).execute(this, host.getRest());
        return ((LRStruct)input).execute(Becomes.Singleton, host);
    }
}