samoa-api/src/main/java/org/apache/samoa/moa/classifiers/core/splitcriteria/GiniSplitCriterion.java

package org.apache.samoa.moa.classifiers.core.splitcriteria;

/*
 * #%L
 * SAMOA
 * %%
 * Copyright (C) 2014 - 2015 Apache Software Foundation
 * %%
 * Licensed 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.
 * #L%
 */

import org.apache.samoa.moa.core.ObjectRepository;
import org.apache.samoa.moa.core.Utils;
import org.apache.samoa.moa.options.AbstractOptionHandler;
import org.apache.samoa.moa.tasks.TaskMonitor;

/**
 * Class for computing splitting criteria using Gini with respect to distributions of class values. The split criterion
 * is used as a parameter on decision trees and decision stumps.
 * 
 * @author Richard Kirkby (rkirkby@cs.waikato.ac.nz)
 * @version $Revision: 7 $
 */
public class GiniSplitCriterion extends AbstractOptionHandler implements
    SplitCriterion {

  private static final long serialVersionUID = 1L;

  @Override
  public double getMeritOfSplit(double[] preSplitDist, double[][] postSplitDists) {
    double totalWeight = 0.0;
    double[] distWeights = new double[postSplitDists.length];
    for (int i = 0; i < postSplitDists.length; i++) {
      distWeights[i] = Utils.sum(postSplitDists[i]);
      totalWeight += distWeights[i];
    }
    double gini = 0.0;
    for (int i = 0; i < postSplitDists.length; i++) {
      gini += (distWeights[i] / totalWeight)
          * computeGini(postSplitDists[i], distWeights[i]);
    }
    return 1.0 - gini;
  }

  @Override
  public double getRangeOfMerit(double[] preSplitDist) {
    return 1.0;
  }

  public static double computeGini(double[] dist, double distSumOfWeights) {
    double gini = 1.0;
    for (double aDist : dist) {
      double relFreq = aDist / distSumOfWeights;
      gini -= relFreq * relFreq;
    }
    return gini;
  }

  public static double computeGini(double[] dist) {
    return computeGini(dist, Utils.sum(dist));
  }

  @Override
  public void getDescription(StringBuilder sb, int indent) {
    // TODO Auto-generated method stub
  }

  @Override
  protected void prepareForUseImpl(TaskMonitor monitor,
      ObjectRepository repository) {
    // TODO Auto-generated method stub
  }
}