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/*
 * (c) 2005 David B. Bracewell
 *
 * 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 com.davidbracewell.atlas.scoring;

import com.davidbracewell.atlas.Graph;
import com.davidbracewell.collection.Counter;
import com.davidbracewell.collection.Counters;
import com.google.common.base.Preconditions;

/**
 * @author David B. Bracewell
 */
public class PageRank extends AbstractVertexScorer {

  private static final long serialVersionUID = -7329282877612595474L;
  private final double dampingFactor;
  private final int maxIterations;
  private final double tolerance;

  public PageRank() {
    this(100, 0.85, 0.001);
  }

  public PageRank(int maxIterations, double dampingFactor, double tolerance) {
    this.maxIterations = maxIterations;
    this.dampingFactor = dampingFactor;
    this.tolerance = tolerance;
  }

  @Override
  public Counter score(Graph g) {
    Preconditions.checkNotNull(g, "The graph must not be null.");
    Counter pageRank = Counters.newHashMapCounter();
    for (V vertex : g.vertices()) {
      pageRank.increment(vertex, 1d / g.numberOfVertices());
    }

    double lastValue = 0d;
    for (int itr = 0; itr < maxIterations; itr++) {
      double thisValue = 0d;

      Counter thisRound = Counters.newHashMapCounter();
      for (V vertex : g.vertices()) {

        double pr = 0;

        for (V v2 : g.getPredecessors(vertex)) {
          pr += (pageRank.get(v2) / g.outDegree(v2));
        }

        pr = (1 - dampingFactor) + dampingFactor * pr;
        thisValue += pr;
        thisRound.set(vertex, pr);
      }

      if (Math.abs(thisValue - lastValue) < tolerance) {
        break;
      }
      lastValue = thisValue;
      pageRank = thisRound;
    }
    return pageRank;
  }

}//END OF PageRank