entropy.h

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#pragma once
 
#include <cmath>
#include <map>
namespace pesieve {
 
    namespace stats {
 
        template <typename T> size_t generateHistogram(IN T buffer[], IN size_t bufferSize, OUT std::map<T, size_t> &counts)
        {
            if (!buffer || !bufferSize) return 0;
 
            for (size_t i = 0; i < bufferSize; ++i) {
                const T val = buffer[i];
                counts[val]++;
            }
            return counts.size();
        }
 
        // Shannon's Entropy calculation based on: https://stackoverflow.com/questions/20965960/shannon-entropy
        template <typename T>
        double calcShannonEntropy(std::map<T, size_t>& histogram, size_t totalSize)
        {
            if (!totalSize) return 0;
            double entropy = 0;
            for (auto it = histogram.begin(); it != histogram.end(); ++it) {
                double p_x = (double)it->second / totalSize;
                if (p_x > 0) entropy -= p_x * log(p_x) / log((double)2);
            }
            return entropy;
        }
 
        template <typename T> static double ShannonEntropy(T buffer[], size_t bufferSize)
        {
            std::map<T, size_t> counts;
            if (!generateHistogram<T>(buffer, bufferSize, counts)) {
                return 0;
            }
            return calcShannonEntropy<T>(counts, bufferSize);
        }
 
    }; // namespace stats
 
}; //namespace pesieve