/*

  * Licensed to the Apache Software Foundation (ASF) under one

  * or more contributor license agreements.  See the NOTICE file

  * distributed with this work for additional information

  * regarding copyright ownership.  The ASF licenses this file

  * to you under the Apache License, Version 2.0 (the

  * "License"); you may not use this file except in compliance

  * with the License.  You may obtain a copy of the License at

  *

  *     http://www.apache.org/licenses/LICENSE-2.0

  *

  * Unless required by applicable law or agreed to in writing, software

  * distributed under the License is distributed on an "AS IS" BASIS,

  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

  * See the License for the specific language governing permissions and

  * limitations under the License.

  */

 package org.apache.giraph.examples;

 import com.google.common.base.Preconditions;

 import org.apache.giraph.edge.Edge;

 import org.apache.giraph.graph.Vertex;

 import org.apache.giraph.utils.MathUtils;

 import org.apache.hadoop.io.DoubleWritable;

 import org.apache.hadoop.io.LongWritable;

 /**

  * Executes "RandomWalkWithRestart", a random walk on the graph which is biased

  * towards a source vertex. The resulting probabilities of staying at a given

  * vertex can be interpreted as a measure of proximity to the source vertex.

  */

 public class RandomWalkWithRestartComputation

     extends RandomWalkComputation<DoubleWritable> {

   /** Configuration parameter for the source vertex */

   static final String SOURCE_VERTEX = RandomWalkWithRestartComputation.class

       .getName() + ".sourceVertex";

   /**

    * Checks whether the currently executed vertex is the source vertex

    * @param vertex Vertex

    * @return is the currently executed vertex the source vertex?

    */

   private boolean isSourceVertex(Vertex<LongWritable, ?, ?> vertex) {

     return ((RandomWalkWorkerContext) getWorkerContext()).isSource(

         vertex.getId().get());

   }

   /**

    * Returns the number of source vertices.

    * @return The number of source vertices.

    */

   private int numSourceVertices() {

     return ((RandomWalkWorkerContext) getWorkerContext()).numSources();

   }

   @Override

   protected double transitionProbability(

       Vertex<LongWritable, DoubleWritable, DoubleWritable>

           vertex,

       double stateProbability, Edge<LongWritable, DoubleWritable> edge) {

     return stateProbability * edge.getValue().get();

   }

   @Override

   protected double recompute(

       Vertex<LongWritable, DoubleWritable, DoubleWritable> vertex,

       Iterable<DoubleWritable> transitionProbabilities,

       double teleportationProbability) {

     int numSourceVertices = numSourceVertices();

     Preconditions.checkState(numSourceVertices > 0, "No source vertex found");

     double stateProbability = MathUtils.sum(transitionProbabilities);

     // Add the contribution of dangling nodes (weakly preferential

     // implementation: dangling nodes redistribute uniformly)

     stateProbability += getDanglingProbability() / getTotalNumVertices();

     // The random walk might teleport back to one of the source vertexes

     stateProbability *= 1 - teleportationProbability;

     if (isSourceVertex(vertex)) {

       stateProbability += teleportationProbability / numSourceVertices;

     }

     return stateProbability;

   }

 }