Entropy.java

package storm.applications.util.math;

import java.util.List;
import java.util.Map;

public class Entropy {
    public static double calculateEntropyHistogram(int totalCount, Map<Double, List<String>> histogram) {
        double entropy = 0.0;

        for(Map.Entry<Double, List<String>> entry : histogram.entrySet()) {
            double prob = (double)entry.getValue().size() / totalCount;
            entropy -= prob * log2(prob);
        }

        return entropy;
    }

    public static double calculateEntropyNormalDistribution(double sigma) {
        if(sigma == 0) {
                sigma = 0.000001;
        }
        return 0.5 * Math.log(2 * Math.PI * Math.E * Math.pow(sigma, 2));
    }


    private static double log2(double value) {
        return Math.log(value) / Math.log(2);
    }
}