/*

  * 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 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;

 import org.apache.hadoop.io.NullWritable;

 /**

  * The PageRank algorithm, with uniform transition probabilities on the edges

  * http://en.wikipedia.org/wiki/PageRank

  */

 public class PageRankComputation extends RandomWalkComputation<NullWritable> {

   @Override

   protected double transitionProbability(

       Vertex<LongWritable, DoubleWritable, NullWritable> vertex,

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

     // Uniform transition probability

     return stateProbability / vertex.getNumEdges();

   }

   @Override

   protected double recompute(

       Vertex<LongWritable, DoubleWritable, NullWritable> vertex,

       Iterable<DoubleWritable> partialRanks, double teleportationProbability) {

     // Rank contribution from incident neighbors

     double rankFromNeighbors = MathUtils.sum(partialRanks);

     // Rank contribution from dangling vertices

     double danglingContribution =

         getDanglingProbability() / getTotalNumVertices();

     // Recompute rank

     return (1d - teleportationProbability) *

         (rankFromNeighbors + danglingContribution) +

         teleportationProbability / getTotalNumVertices();

   }

 }