Initial revision

git-svn-id: https://svn.apache.org/repos/asf/jakarta/commons/sandbox/graph/trunk@144575 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/commons/graph/GraphAlgorithms.java

@@ -0,0 +1,149 @@
+package org.apache.commons.graph;
+
+/* ====================================================================
+ * The Apache Software License, Version 1.1
+ *
+ * Copyright (c) 2001 The Apache Software Foundation.  All rights
+ * reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * 3. The end-user documentation included with the redistribution,
+ *    if any, must include the following acknowledgment:
+ *       "This product includes software developed by the
+ *        Apache Software Foundation (http://www.apache.org/)."
+ *    Alternately, this acknowledgment may appear in the software itself,
+ *    if and wherever such third-party acknowledgments normally appear.
+ *
+ * 4. The names "Apache" and "Apache Software Foundation" and
+ *    "Apache BCEL" must not be used to endorse or promote products
+ *    derived from this software without prior written permission. For
+ *    written permission, please contact apache@apache.org.
+ *
+ * 5. Products derived from this software may not be called "Apache",
+ *    "Apache BCEL", nor may "Apache" appear in their name, without
+ *    prior written permission of the Apache Software Foundation.
+ *
+ * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED.  IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
+ * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ */
+
+import java.util.*;
+
+/**
+ * Collection of graph algorithms such as transposeGraph.
+ *
+ * @version $Id: GraphAlgorithms.java,v 1.1 2001/11/12 16:05:18 jvanzyl Exp $
+ * @author  <A HREF="http://www.inf.fu-berlin.de/~dahm">M. Dahm</A>
+ */
+public abstract class GraphAlgorithms implements Constants {
+  /**
+   * Compute the inverse of a graph, i.e. with all edges reverted
+   */
+  public static final Graph transposeGraph(Graph g) {
+    if(!g.isDirected())
+      throw new GraphError("Can only transpose directed graphs");
+
+    Graph graph = new Graph(true);
+
+    for(Iterator i=g.getVertices().iterator(); i.hasNext();)
+      graph.addVertex((Vertex)i.next());
+
+    for(Iterator i=g.getEdges().iterator(); i.hasNext();) {
+      Edge f = (Edge)i.next();
+
+      graph.addEdge(g.getTarget(f), g.getSource(f), f);
+    }
+
+    return graph;
+  }
+
+  /**
+   * Test whether the SCC g is a loop.
+   *
+   * @param super_g super graph containing the subset of vertices
+   * @param g subset of super_g (g must be SCC of super_g)
+   */
+  public static final boolean isLoop(Graph super_g, Graph loop) { 
+    return findLoopEntry(super_g, loop) != null;
+  }
+
+  /**
+   * Test whether the SCC g is a loop.
+   *
+   * @param super_g super graph containing the subset of vertices
+   * @param g subset of super_g (g must be SCC of super_g)
+   * @return root node of loop, i.e. the entry point, or null if this is no loop
+   */
+  public static final Vertex findLoopEntry(Graph super_g, Graph loop) {
+    switch(loop.getNoVertices()) {
+    case 0:
+      return null; // certainly not a loop
+
+    case 1: // singleton must point to itself, i.e. be a self-loop
+      Vertex   v     = loop.getVertexArray()[0];
+      Vertex[] edges = loop.getVertexArray(v);
+
+      for(int i=0; i < edges.length; i++)
+	if(edges[i] == v)
+	  return v;
+
+      return null;
+    }
+
+    /* Make sure that the root node is the only entry point to the
+     * SCC, i.e.  there is no way from a node v \in (super_g \ g) into
+     * a node in g except via the root node.
+     *
+     * So we have to find the root node first ...
+     */
+    Vertex   root     = null;
+    Vertex[] vertices = super_g.getVertexArray();
+
+    for(int i=0; i < vertices.length; i++) {
+      Vertex v = vertices[i];
+
+      if(loop.contains(v)) // v is in the SCC, no check needed
+	continue;
+      
+      Vertex[] edges = super_g.getVertexArray(v); // All nodes reachable from v
+      for(int j=0; j < edges.length; j++) {
+	Vertex u = edges[j];
+
+	if(loop.contains(u))
+	  if(root == null)
+	    root = u;
+	  else
+	    return null; // Can't be a loop then, i.e. has more than one root node
+      }
+    }
+  
+    return root; // Has loop a root node at all !?
+  }
+}