Benchmark.java

import org.apache.mahout.math.DenseVector;
import org.apache.mahout.math.Vector;
import org.apache.mahout.math.function.IntObjectProcedure;
import org.apache.mahout.math.map.OpenIntObjectHashMap;

import java.util.Random;

public class Benchmark {

  static int NUM_FEATURES = 20;
  static Random RANDOM = new Random(0xdeadbeef);

  public static void main(String[] args) {

    int numItems = 2500000;
    OpenIntObjectHashMap<Vector> M = new OpenIntObjectHashMap<Vector>(numItems);

    for (int itemID = 0; itemID < numItems; itemID++) {
      M.put(itemID, randomVector(NUM_FEATURES));
    }

    int numUsers = 50;


    for (int n = 0; n < numUsers; n++) {

      final Vector userFeatures = randomVector(NUM_FEATURES);

      long start = System.currentTimeMillis();

      M.forEachPair(new IntObjectProcedure<Vector>() {
        @Override
        public boolean apply(int itemID, Vector itemFeatures) {
          userFeatures.dot(itemFeatures);
          return true;
        }
      });

      long pause = System.currentTimeMillis();

      M.forEachPair(new IntObjectProcedure<Vector>() {
        @Override
        public boolean apply(int itemID, Vector itemFeatures) {
          dot(userFeatures, itemFeatures);
          return true;
        }
      });
      long end = System.currentTimeMillis();

      System.out.println("dot() " + (pause - start) + "ms, direct " + (end - pause) + "ms");
    }


  }

  static double dot(Vector one, Vector two) {
    double sum = 0;
      for (int n = 0; n < NUM_FEATURES; n++) {
        sum += one.getQuick(n) * two.getQuick(n);
      }
    return sum;
  }

  static Vector randomVector(int size) {
    double[] values = new double[size];
    for (int n = 0; n < size; n++) {
      values[n] = RANDOM.nextDouble();
    }
    return new DenseVector(values, true);
  }

}