/*
    * 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;
    }
  }