added code to check cycles in a directed graph

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@25681 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/util/CyclicGraphDetector.java

+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<N> {
+    private final Set<N> visited = new HashSet<N>();
+    private final Set<N> visiting = new HashSet<N>();
+    private final Stack<N> path = new Stack<N>();
+
+    public void run(Iterable<? extends N> 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<? extends N> 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 {
+        public final List cycle;
+
+        public CycleDetectedException(List cycle) {
+            super("Cycle detected: "+Util.join(cycle," -> "));
+            this.cycle = cycle;
+        }
+    }
+}

core/src/test/java/hudson/util/CyclicGraphDetectorTest.java

+package hudson.util;
+
+import hudson.util.CyclicGraphDetector.CycleDetectedException;
+import junit.framework.TestCase;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.LinkedHashSet;
+import java.util.Set;
+
+/**
+ * @author Kohsuke Kawaguchi
+ */
+public class CyclicGraphDetectorTest extends TestCase {
+    private class Edge {
+        String src,dst;
+
+        private Edge(String src, String dst) {
+            this.src = src;
+            this.dst = dst;
+        }
+    }
+
+    private class Graph extends ArrayList<Edge> {
+        Graph e(String src, String dst) {
+            add(new Edge(src,dst));
+            return this;
+        }
+
+        Set<String> nodes() {
+            Set<String> nodes = new LinkedHashSet<String>();
+            for (Edge e : this) {
+                nodes.add(e.src);
+                nodes.add(e.dst);
+            }
+            return nodes;
+        }
+
+        Set<String> edges(String from) {
+            Set<String> edges = new LinkedHashSet<String>();
+            for (Edge e : this) {
+                if (e.src.equals(from))
+                    edges.add(e.dst);
+            }
+            return edges;
+        }
+
+        /**
+         * Performs a cycle check. 
+         */
+        void check() throws Exception {
+            new CyclicGraphDetector<String>() {
+                protected Set<String> getEdges(String s) {
+                    return edges(s);
+                }
+            }.run(nodes());
+        }
+
+        void mustContainCycle(String... members) throws Exception {
+            try {
+                check();
+                fail("Cycle expected");
+            } catch (CycleDetectedException e) {
+                String msg = "Expected cycle of " + Arrays.asList(members) + " but found " + e.cycle;
+                for (String s : members)
+                    assertTrue(msg, e.cycle.contains(s));
+            }
+        }
+    }
+
+    public void testCycle() throws Exception {
+        new Graph().e("A","B").e("B","C").e("C","A").mustContainCycle("A","B","C");
+    }
+
+    public void testCycle2() throws Exception {
+        new Graph().e("A","B").e("B","C").e("C","C").mustContainCycle("C");
+    }
+
+    public void testCycle3() throws Exception {
+        new Graph().e("A","B").e("B","C").e("C","D").e("B","E").e("E","D").e("E","A").mustContainCycle("A","B","E");
+    }
+
+}