/*
    * 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();
    }
  }