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

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;
        }
    }

}