Added Tarjan SCC computation algorithm and associated tests.

utils/misc/src/main/java/org/onlab/graph/TarjanGraphSearch.java

@@ -0,0 +1,195 @@
+package org.onlab.graph;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * Tarjan algorithm for searching a graph and producing results describing
+ * the graph SCC (strongly-connected components).
+ */
+public class TarjanGraphSearch<V extends Vertex, E extends Edge<V>>
+        implements GraphSearch<V, E> {
+
+    /**
+     * {@inheritDoc}
+     * <p/>
+     * This implementation produces results augmented with information on
+     * SCCs within the graph.
+     * <p/>
+     * To prevent traversal of an edge, the {@link EdgeWeight#weight} should
+     * return a negative value as an edge weight.
+     */
+    @Override
+    public SCCResult<V, E> search(Graph<V, E> graph, EdgeWeight<V, E> weight) {
+        SCCResult<V, E> result = new SCCResult<>(graph);
+        for (V vertex : graph.getVertexes()) {
+            VertexData data = result.data(vertex);
+            if (data == null) {
+                connect(graph, vertex, weight, result);
+            }
+        }
+        return result.build();
+    }
+
+    /**
+     * Scans the specified graph, using recursion, and produces SCC results.
+     *
+     * @param graph  graph to search
+     * @param vertex current vertex to scan and connect
+     * @param weight optional edge weight
+     * @param result graph search result
+     * @return augmentation vertexData for the current vertex
+     */
+    private VertexData<V> connect(Graph<V, E> graph, V vertex,
+                                  EdgeWeight<V, E> weight,
+                                  SCCResult<V, E> result) {
+        VertexData<V> data = result.addData(vertex);
+
+        // Scan through all egress edges of the current vertex.
+        for (E edge : graph.getEdgesFrom(vertex)) {
+            V nextVertex = edge.dst();
+
+            // If edge weight is negative, skip it.
+            if (weight != null && weight.weight(edge) < 0) {
+                continue;
+            }
+
+            // Attempt to get the augmentation vertexData for the next vertex.
+            VertexData<V> nextData = result.data(nextVertex);
+            if (nextData == null) {
+                // Next vertex has not been visited yet, so do this now.
+                nextData = connect(graph, nextVertex, weight, result);
+                data.lowLink = Math.min(data.lowLink, nextData.lowLink);
+
+            } else if (result.visited(nextData)) {
+                // Next vertex has been visited, which means it is in the
+                // same cluster as the current vertex.
+                data.lowLink = Math.min(data.lowLink, nextData.index);
+            }
+        }
+
+        if (data.lowLink == data.index) {
+            result.addCluster(data);
+        }
+        return data;
+    }
+
+    /**
+     * Graph search result augmented with SCC vertexData.
+     */
+    public static final class SCCResult<V extends Vertex, E extends Edge<V>>
+            implements Result {
+
+        private final Graph<V, E> graph;
+        private List<Set<V>> clusterVertexes = new ArrayList<>();
+        private List<Set<E>> clusterEdges = new ArrayList<>();
+
+        private int index = 0;
+        private final Map<V, VertexData<V>> vertexData = new HashMap<>();
+        private final List<VertexData<V>> visited = new ArrayList<>();
+
+        private SCCResult(Graph<V, E> graph) {
+            this.graph = graph;
+        }
+
+        /**
+         * Returns the number of SCC clusters in the graph.
+         *
+         * @return number of clusters
+         */
+        public int clusterCount() {
+            return clusterEdges.size();
+        }
+
+        /**
+         * Returns the list of strongly connected vertex clusters.
+         *
+         * @return list of strongly connected vertex sets
+         */
+        public List<Set<V>> clusterVertexes() {
+            return clusterVertexes;
+        }
+
+        /**
+         * Returns the list of edges linking strongly connected vertex clusters.
+         *
+         * @return list of strongly connected edge sets
+         */
+        public List<Set<E>> clusterEdges() {
+            return clusterEdges;
+        }
+
+        // Gets the augmentation vertexData for the specified vertex
+        private VertexData<V> data(V vertex) {
+            return vertexData.get(vertex);
+        }
+
+        // Adds augmentation vertexData for the specified vertex
+        private VertexData<V> addData(V vertex) {
+            VertexData<V> d = new VertexData<>(vertex, index);
+            vertexData.put(vertex, d);
+            visited.add(0, d);
+            index++;
+            return d;
+        }
+
+        // Indicates whether the given vertex has been visited
+        private boolean visited(VertexData data) {
+            return visited.contains(data);
+        }
+
+        // Adds a new cluster for the specified vertex
+        private void addCluster(VertexData data) {
+            Set<V> vertexes = findClusterVertices(data);
+            clusterVertexes.add(vertexes);
+            clusterEdges.add(findClusterEdges(vertexes));
+        }
+
+        private Set<V> findClusterVertices(VertexData data) {
+            VertexData<V> nextVertexData;
+            Set<V> vertexes = new HashSet<>();
+            do {
+                nextVertexData = visited.remove(0);
+                vertexes.add(nextVertexData.vertex);
+            } while (data != nextVertexData);
+            return Collections.unmodifiableSet(vertexes);
+        }
+
+        private Set<E> findClusterEdges(Set<V> vertexes) {
+            Set<E> edges = new HashSet<>();
+            for (V vertex : vertexes) {
+                for (E edge : graph.getEdgesFrom(vertex)) {
+                    if (vertexes.contains((edge.dst()))) {
+                        edges.add(edge);
+                    }
+                }
+            }
+            return Collections.unmodifiableSet(edges);
+        }
+
+        public SCCResult<V, E> build() {
+            clusterVertexes = Collections.unmodifiableList(clusterVertexes);
+            clusterEdges = Collections.unmodifiableList(clusterEdges);
+            return this;
+        }
+    }
+
+    // Augments the vertex to assist in determining SCC clusters.
+    private static final class VertexData<V extends Vertex> {
+        final V vertex;
+        int index;
+        int lowLink;
+
+        private VertexData(V vertex, int index) {
+            this.vertex = vertex;
+            this.index = index;
+            this.lowLink = index;
+        }
+    }
+
+}

utils/misc/src/test/java/org/onlab/graph/BellmanFordGraphSearchTest.java

@@ -31,17 +31,17 @@ public class BellmanFordGraphSearchTest extends BreadthFirstSearchTest {
 
     @Test
     public void searchGraphWithNegativeCycles() {
-        Set<TestVertex> vertexes = new HashSet<>(vertices());
+        Set<TestVertex> vertexes = new HashSet<>(vertexes());
         vertexes.add(Z);
 
         Set<TestEdge> edges = new HashSet<>(edges());
         edges.add(new TestEdge(G, Z, 1.0));
         edges.add(new TestEdge(Z, G, -2.0));
 
-        g = new AdjacencyListsGraph<>(vertexes, edges);
+        graph = new AdjacencyListsGraph<>(vertexes, edges);
 
         GraphPathSearch<TestVertex, TestEdge> search = graphSearch();
-        Set<Path<TestVertex, TestEdge>> paths = search.search(g, A, H, weight).paths();
+        Set<Path<TestVertex, TestEdge>> paths = search.search(graph, A, H, weight).paths();
         assertEquals("incorrect paths count", 1, paths.size());
 
         Path p = paths.iterator().next();
@@ -50,10 +50,10 @@ public class BellmanFordGraphSearchTest extends BreadthFirstSearchTest {
         assertEquals("incorrect path length", 5, p.edges().size());
         assertEquals("incorrect path cost", 5.0, p.cost(), 0.1);
 
-        paths = search.search(g, A, G, weight).paths();
+        paths = search.search(graph, A, G, weight).paths();
         assertEquals("incorrect paths count", 0, paths.size());
 
-        paths = search.search(g, A, null, weight).paths();
+        paths = search.search(graph, A, null, weight).paths();
         printPaths(paths);
         assertEquals("incorrect paths count", 6, paths.size());
     }

utils/misc/src/test/java/org/onlab/graph/BreadthFirstSearchTest.java

@@ -29,10 +29,10 @@ public class BreadthFirstSearchTest extends AbstractGraphPathSearchTest {
 
     // Executes the default test
     protected void executeDefaultTest(int pathCount, int pathLength, double pathCost) {
-        g = new AdjacencyListsGraph<>(vertices(), edges());
+        graph = new AdjacencyListsGraph<>(vertexes(), edges());
 
         GraphPathSearch<TestVertex, TestEdge> search = graphSearch();
-        Set<Path<TestVertex, TestEdge>> paths = search.search(g, A, H, weight).paths();
+        Set<Path<TestVertex, TestEdge>> paths = search.search(graph, A, H, weight).paths();
         assertEquals("incorrect paths count", 1, paths.size());
 
         Path p = paths.iterator().next();
@@ -41,7 +41,7 @@ public class BreadthFirstSearchTest extends AbstractGraphPathSearchTest {
         assertEquals("incorrect path length", pathLength, p.edges().size());
         assertEquals("incorrect path cost", pathCost, p.cost(), 0.1);
 
-        paths = search.search(g, A, null, weight).paths();
+        paths = search.search(graph, A, null, weight).paths();
         printPaths(paths);
         assertEquals("incorrect paths count", pathCount, paths.size());
     }

utils/misc/src/test/java/org/onlab/graph/DepthFirstSearchTest.java

@@ -33,11 +33,11 @@ public class DepthFirstSearchTest extends AbstractGraphPathSearchTest {
 
     protected void executeDefaultTest(int minLength, int maxLength,
                                       double minCost, double maxCost) {
-        g = new AdjacencyListsGraph<>(vertices(), edges());
+        graph = new AdjacencyListsGraph<>(vertexes(), edges());
         DepthFirstSearch<TestVertex, TestEdge> search = graphSearch();
 
         DepthFirstSearch<TestVertex, TestEdge>.SpanningTreeResult result =
-                search.search(g, A, H, weight);
+                search.search(graph, A, H, weight);
         Set<Path<TestVertex, TestEdge>> paths = result.paths();
         assertEquals("incorrect path count", 1, paths.size());
 
@@ -57,12 +57,12 @@ public class DepthFirstSearchTest extends AbstractGraphPathSearchTest {
     }
 
     public void executeBroadSearch() {
-        g = new AdjacencyListsGraph<>(vertices(), edges());
+        graph = new AdjacencyListsGraph<>(vertexes(), edges());
         DepthFirstSearch<TestVertex, TestEdge> search = graphSearch();
 
         // Perform narrow path search to a specific destination.
         DepthFirstSearch<TestVertex, TestEdge>.SpanningTreeResult result =
-                search.search(g, A, null, weight);
+                search.search(graph, A, null, weight);
         assertEquals("incorrect paths count", 7, result.paths().size());
 
         int[] types = new int[]{0, 0, 0, 0};

utils/misc/src/test/java/org/onlab/graph/DijkstraGraphSearchTest.java

@@ -32,22 +32,22 @@ public class DijkstraGraphSearchTest extends BreadthFirstSearchTest {
 
     @Test
     public void noPath() {
-        g = new AdjacencyListsGraph<>(of(A, B, C, D),
+        graph = new AdjacencyListsGraph<>(of(A, B, C, D),
-                                      of(new TestEdge(A, B, 1),
+                                          of(new TestEdge(A, B, 1),
-                                         new TestEdge(B, A, 1),
+                                             new TestEdge(B, A, 1),
-                                         new TestEdge(C, D, 1),
+                                             new TestEdge(C, D, 1),
-                                         new TestEdge(D, C, 1)));
+                                             new TestEdge(D, C, 1)));
         GraphPathSearch<TestVertex, TestEdge> gs = graphSearch();
-        Set<Path<TestVertex, TestEdge>> paths = gs.search(g, A, B, weight).paths();
+        Set<Path<TestVertex, TestEdge>> paths = gs.search(graph, A, B, weight).paths();
         printPaths(paths);
         assertEquals("incorrect paths count", 1, paths.size());
         assertEquals("incorrect path cost", 1.0, paths.iterator().next().cost(), 0.1);
 
-        paths = gs.search(g, A, D, weight).paths();
+        paths = gs.search(graph, A, D, weight).paths();
         printPaths(paths);
         assertEquals("incorrect paths count", 0, paths.size());
 
-        paths = gs.search(g, A, null, weight).paths();
+        paths = gs.search(graph, A, null, weight).paths();
         printPaths(paths);
         assertEquals("incorrect paths count", 1, paths.size());
         assertEquals("incorrect path cost", 1.0, paths.iterator().next().cost(), 0.1);
@@ -55,40 +55,40 @@ public class DijkstraGraphSearchTest extends BreadthFirstSearchTest {
 
     @Test
     public void simpleMultiplePath() {
-        g = new AdjacencyListsGraph<>(of(A, B, C, D),
+        graph = new AdjacencyListsGraph<>(of(A, B, C, D),
-                                      of(new TestEdge(A, B, 1),
+                                          of(new TestEdge(A, B, 1),
-                                         new TestEdge(A, C, 1),
+                                             new TestEdge(A, C, 1),
-                                         new TestEdge(B, D, 1),
+                                             new TestEdge(B, D, 1),
-                                         new TestEdge(C, D, 1)));
+                                             new TestEdge(C, D, 1)));
-        executeSearch(graphSearch(), g, A, D, weight, 2, 2.0);
+        executeSearch(graphSearch(), graph, A, D, weight, 2, 2.0);
     }
 
     @Test
     public void denseMultiplePath() {
-        g = new AdjacencyListsGraph<>(of(A, B, C, D, E, F, G),
+        graph = new AdjacencyListsGraph<>(of(A, B, C, D, E, F, G),
-                                      of(new TestEdge(A, B, 1),
+                                          of(new TestEdge(A, B, 1),
-                                         new TestEdge(A, C, 1),
+                                             new TestEdge(A, C, 1),
-                                         new TestEdge(B, D, 1),
+                                             new TestEdge(B, D, 1),
-                                         new TestEdge(C, D, 1),
+                                             new TestEdge(C, D, 1),
-                                         new TestEdge(D, E, 1),
+                                             new TestEdge(D, E, 1),
-                                         new TestEdge(D, F, 1),
+                                             new TestEdge(D, F, 1),
-                                         new TestEdge(E, G, 1),
+                                             new TestEdge(E, G, 1),
-                                         new TestEdge(F, G, 1),
+                                             new TestEdge(F, G, 1),
-                                         new TestEdge(A, G, 4)));
+                                             new TestEdge(A, G, 4)));
-        executeSearch(graphSearch(), g, A, G, weight, 5, 4.0);
+        executeSearch(graphSearch(), graph, A, G, weight, 5, 4.0);
     }
 
     @Test
     public void dualEdgeMultiplePath() {
-        g = new AdjacencyListsGraph<>(of(A, B, C, D, E, F, G, H),
+        graph = new AdjacencyListsGraph<>(of(A, B, C, D, E, F, G, H),
-                                      of(new TestEdge(A, B, 1), new TestEdge(A, C, 3),
+                                          of(new TestEdge(A, B, 1), new TestEdge(A, C, 3),
-                                         new TestEdge(B, D, 2), new TestEdge(B, C, 1),
+                                             new TestEdge(B, D, 2), new TestEdge(B, C, 1),
-                                         new TestEdge(B, E, 4), new TestEdge(C, E, 1),
+                                             new TestEdge(B, E, 4), new TestEdge(C, E, 1),
-                                         new TestEdge(D, H, 5), new TestEdge(D, E, 1),
+                                             new TestEdge(D, H, 5), new TestEdge(D, E, 1),
-                                         new TestEdge(E, F, 1), new TestEdge(F, D, 1),
+                                             new TestEdge(E, F, 1), new TestEdge(F, D, 1),
-                                         new TestEdge(F, G, 1), new TestEdge(F, H, 1),
+                                             new TestEdge(F, G, 1), new TestEdge(F, H, 1),
-                                         new TestEdge(A, E, 3), new TestEdge(B, D, 1)));
+                                             new TestEdge(A, E, 3), new TestEdge(B, D, 1)));
-        executeSearch(graphSearch(), g, A, E, weight, 3, 3.0);
+        executeSearch(graphSearch(), graph, A, E, weight, 3, 3.0);
     }
 
 }

utils/misc/src/test/java/org/onlab/graph/GraphTest.java

@@ -19,7 +19,7 @@ public class GraphTest {
     static final TestVertex H = new TestVertex("H");
     static final TestVertex Z = new TestVertex("Z");
 
-    protected Graph<TestVertex, TestEdge> g;
+    protected Graph<TestVertex, TestEdge> graph;
 
     protected EdgeWeight<TestVertex, TestEdge> weight =
             new EdgeWeight<TestVertex, TestEdge>() {
@@ -35,7 +35,7 @@ public class GraphTest {
         }
     }
 
-    protected Set<TestVertex> vertices() {
+    protected Set<TestVertex> vertexes() {
         return of(A, B, C, D, E, F, G, H);
     }
 

utils/misc/src/test/java/org/onlab/graph/TarjanGraphSearchTest.java

@@ -0,0 +1,110 @@
+package org.onlab.graph;
+
+import org.junit.Test;
+
+import static com.google.common.collect.ImmutableSet.of;
+import static org.junit.Assert.assertEquals;
+import static org.onlab.graph.TarjanGraphSearch.SCCResult;
+
+/**
+ * Tarjan graph search tests.
+ */
+public class TarjanGraphSearchTest extends GraphTest {
+
+    private void validate(SCCResult<TestVertex, TestEdge> result, int cc) {
+        System.out.println("Cluster count: " + result.clusterVertexes().size());
+        System.out.println("Clusters: " + result.clusterVertexes());
+        assertEquals("incorrect cluster count", cc, result.clusterCount());
+    }
+
+    private void validate(SCCResult<TestVertex, TestEdge> result,
+                          int i, int vc, int ec) {
+        assertEquals("incorrect cluster count", vc, result.clusterVertexes().get(i).size());
+        assertEquals("incorrect edge count", ec, result.clusterEdges().get(i).size());
+    }
+
+    @Test
+    public void basic() {
+        graph = new AdjacencyListsGraph<>(vertexes(), edges());
+        TarjanGraphSearch<TestVertex, TestEdge> gs = new TarjanGraphSearch<>();
+        SCCResult<TestVertex, TestEdge> result = gs.search(graph, null);
+        validate(result, 6);
+    }
+
+    @Test
+    public void singleCluster() {
+        graph = new AdjacencyListsGraph<>(vertexes(),
+                                          of(new TestEdge(A, B, 1),
+                                             new TestEdge(B, C, 1),
+                                             new TestEdge(C, D, 1),
+                                             new TestEdge(D, E, 1),
+                                             new TestEdge(E, F, 1),
+                                             new TestEdge(F, G, 1),
+                                             new TestEdge(G, H, 1),
+                                             new TestEdge(H, A, 1)));
+
+        TarjanGraphSearch<TestVertex, TestEdge> gs = new TarjanGraphSearch<>();
+        SCCResult<TestVertex, TestEdge> result = gs.search(graph, null);
+        validate(result, 1);
+        validate(result, 0, 8, 8);
+    }
+
+    @Test
+    public void twoUnconnectedCluster() {
+        graph = new AdjacencyListsGraph<>(vertexes(),
+                                          of(new TestEdge(A, B, 1),
+                                             new TestEdge(B, C, 1),
+                                             new TestEdge(C, D, 1),
+                                             new TestEdge(D, A, 1),
+                                             new TestEdge(E, F, 1),
+                                             new TestEdge(F, G, 1),
+                                             new TestEdge(G, H, 1),
+                                             new TestEdge(H, E, 1)));
+        TarjanGraphSearch<TestVertex, TestEdge> gs = new TarjanGraphSearch<>();
+        SCCResult<TestVertex, TestEdge> result = gs.search(graph, null);
+        validate(result, 2);
+        validate(result, 0, 4, 4);
+        validate(result, 1, 4, 4);
+    }
+
+    @Test
+    public void twoWeaklyConnectedClusters() {
+        graph = new AdjacencyListsGraph<>(vertexes(),
+                                          of(new TestEdge(A, B, 1),
+                                             new TestEdge(B, C, 1),
+                                             new TestEdge(C, D, 1),
+                                             new TestEdge(D, A, 1),
+                                             new TestEdge(E, F, 1),
+                                             new TestEdge(F, G, 1),
+                                             new TestEdge(G, H, 1),
+                                             new TestEdge(H, E, 1),
+                                             new TestEdge(B, E, 1)));
+        TarjanGraphSearch<TestVertex, TestEdge> gs = new TarjanGraphSearch<>();
+        SCCResult<TestVertex, TestEdge> result = gs.search(graph, null);
+        validate(result, 2);
+        validate(result, 0, 4, 4);
+        validate(result, 1, 4, 4);
+    }
+
+    @Test
+    public void twoClustersConnectedWithIgnoredEdges() {
+        graph = new AdjacencyListsGraph<>(vertexes(),
+                                          of(new TestEdge(A, B, 1),
+                                             new TestEdge(B, C, 1),
+                                             new TestEdge(C, D, 1),
+                                             new TestEdge(D, A, 1),
+                                             new TestEdge(E, F, 1),
+                                             new TestEdge(F, G, 1),
+                                             new TestEdge(G, H, 1),
+                                             new TestEdge(H, E, 1),
+                                             new TestEdge(B, E, -1),
+                                             new TestEdge(E, B, -1)));
+
+        TarjanGraphSearch<TestVertex, TestEdge> gs = new TarjanGraphSearch<>();
+        SCCResult<TestVertex, TestEdge> result = gs.search(graph, weight);
+        validate(result, 2);
+        validate(result, 0, 4, 4);
+        validate(result, 1, 4, 4);
+    }
+
+}