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/*******************************************************************************
 *
 * 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.util;

import hudson.Util;

import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.Stack;

/**
 * Traverses a directed graph and if it contains any cycle, throw an exception.
 *
 * @author Kohsuke Kawaguchi
 */
public abstract class CyclicGraphDetector {

    private final Set visited = new HashSet();
    private final Set visiting = new HashSet();
    private final Stack path = new Stack();

    public void run(Iterable allNodes) throws CycleDetectedException {
        for (N n : allNodes) {
            visit(n);
        }
    }

    /**
     * List up edges from the given node (by listing nodes that those edges
     * point to.)
     *
     * @return Never null.
     */
    protected abstract Iterable getEdges(N n);

    private void visit(N p) throws CycleDetectedException {
        if (!visited.add(p)) {
            return;
        }

        visiting.add(p);
        path.push(p);
        for (N q : getEdges(p)) {
            if (q == null) {
                continue;   // ignore unresolved references
            }
            if (visiting.contains(q)) {
                detectedCycle(q);
            }
            visit(q);
        }
        visiting.remove(p);
        path.pop();
    }

    private void detectedCycle(N q) throws CycleDetectedException {
        int i = path.indexOf(q);
        path.push(q);
        throw new CycleDetectedException(path.subList(i, path.size()));
    }

    public static final class CycleDetectedException extends Exception {
        //TODO: review and check whether we can do it private

        public final List cycle;

        public List getCycle() {
            return cycle;
        }

        public CycleDetectedException(List cycle) {
            super("Cycle detected: " + Util.join(cycle, " -> "));
            this.cycle = cycle;
        }
    }
}