Reduce.java

package hip.ch7.pagerank.mr;

import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;

public class Reduce
    extends Reducer<Text, Text, Text, Text> {

  public static final double CONVERGENCE_SCALING_FACTOR = 1000.0;
  public static final double DAMPING_FACTOR = 0.85;
  public static String CONF_NUM_NODES_GRAPH = "pagerank.numnodes";
  private int numberOfNodesInGraph;

  public static enum Counter {
    CONV_DELTAS
  }

  @Override
  protected void setup(Context context)
      throws IOException, InterruptedException {
    numberOfNodesInGraph = context.getConfiguration().getInt(
        CONF_NUM_NODES_GRAPH, 0);
  }

  private Text outValue = new Text();

  public void reduce(Text key, Iterable<Text> values,
                     Context context)
      throws IOException, InterruptedException {

    System.out.println("input -> K[" + key + "]");

    double summedPageRanks = 0;
    Node originalNode = new Node();

    for (Text textValue : values) {
      System.out.println("  input -> V[" + textValue + "]");

      Node node = Node.fromMR(textValue.toString());

      if (node.containsAdjacentNodes()) {
        // the original node
        //
        originalNode = node;
      } else {
        summedPageRanks += node.getPageRank();
      }
    }

    double dampingFactor =
        ((1.0 - DAMPING_FACTOR) / (double) numberOfNodesInGraph);

    double newPageRank =
        dampingFactor + (DAMPING_FACTOR * summedPageRanks);

    double delta = originalNode.getPageRank() - newPageRank;

    originalNode.setPageRank(newPageRank);

    outValue.set(originalNode.toString());

    System.out.println(
        "  output -> K[" + key + "],V[" + outValue + "]");
    context.write(key, outValue);


    int scaledDelta =
        Math.abs((int) (delta * CONVERGENCE_SCALING_FACTOR));

    System.out.println("Delta = " + scaledDelta);

    context.getCounter(Counter.CONV_DELTAS).increment(scaledDelta);
  }
}