/*******************************************************************************
 *
 * Copyright (c) 2004-2010 Oracle Corporation.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *
 *
 *******************************************************************************/ 

package hudson.scm;

import hudson.model.Action;

/**
 * Immutable object that represents revisions of the files in the repository,
 * used to represent the result of
 * {@linkplain SCM#poll(hudson.model.AbstractProject, hudson.Launcher, hudson.FilePath, hudson.model.TaskListener, SCMRevisionState) a SCM polling}.
 *
 * <p> This object is used so that the successive polling can compare the tip of
 * the repository now vs what it was when it was last polled. (Before 1.345,
 * Hudson was only able to compare the tip of the repository vs the state of the
 * workspace, which resulted in a problem like HUDSON-2180.
 *
 * <p> {@link SCMRevisionState} is persisted as an action to
 * {@link hudson.model.AbstractBuild}.
 *
 * @author Kohsuke Kawaguchi
 * @since 1.345
 */
public abstract class SCMRevisionState implements Action {

    public String getIconFileName() {
        return null;
    }

    public String getDisplayName() {
        return null;
    }

    public String getUrlName() {
        return null;
    }

    /*
     I can't really make this comparable because comparing two revision states often requires
     non-trivial computation and conversations with the repository (mainly to figure out
     which changes are insignificant and which are not.)

     So instead, here we opt to a design where we tell SCM upfront about what we are comparing
     against (baseline), and have it give us the new state and degree of change in PollingResult.
     */
    public static SCMRevisionState NONE = new None();

    private static final class None extends SCMRevisionState {
    }
}