NDCG.java

// Copyright (C) 2011 Zeno Gantner, Chris Newell
//
// This file is part of MyMediaLite.
//
// MyMediaLite is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// MyMediaLite is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
//  You should have received a copy of the GNU General Public License
//  along with MyMediaLite.  If not, see <http://www.gnu.org/licenses/>.

package org.mymedialite.eval.measures;

import java.util.Collection;
import java.util.HashSet;
import java.util.List;

/**
 * Normalized discounted cumulative gain (NDCG) of a list of ranked items.
 * See http://recsyswiki.com/wiki/Discounted_Cumulative_Gain
 * @version 2.03
 */
public class NDCG {

  // Prevent instantiation.
  private NDCG() {}

  /**
   * Compute the normalized discounted cumulative gain (NDCG) of a list of ranked items.
   * 
   * See http://recsyswiki.com/wiki/Discounted_Cumulative_Gain
   * 
   * @param ranked_items a list of ranked item IDs, the highest-ranking item first
   * @param correct_items a collection of positive/correct item IDs
   * @param ignore_items a collection of item IDs which should be ignored for the evaluation
   * @return the NDCG for the given data
   */
  public static double compute(
      List<Integer> ranked_items,
      Collection<Integer> correct_items,
      Collection<Integer> ignore_items) {
    
    if (ignore_items == null)
      ignore_items = new HashSet<Integer>();

    double dcg   = 0;
    double idcg  = computeIDCG(correct_items.size());
    int left_out = 0;

    for (int i = 0; i < ranked_items.size(); i++) {
      int item_id = ranked_items.get(i);
      if (ignore_items.contains(item_id)) {
        left_out++;
        continue;
      }

      if (!correct_items.contains(item_id))
        continue;

      // compute NDCG part
      int rank = i + 1 - left_out;
      dcg += Math.log(2) / Math.log(rank + 1);
            
    }

    return dcg / idcg;
  }

  /**
   * Computes the ideal DCG given the number of positive items..
   * 
   * See http://recsyswiki.com/wiki/Discounted_Cumulative_Gain
   * 
   * @return the ideal DCG
   * <param name='n'>the number of positive items
   */
  static double computeIDCG(int n)
  {
    double idcg = 0;
    for (int i = 0; i < n; i++)
      idcg += Math.log(2) / Math.log(i + 2);
    return idcg;
  }
}