#pragma warning(disable : 28110)

#include "ShanonEntropy.h"

constexpr DOUBLE M_LOG2E = 1.4426950408889634;

constexpr ULONG MAX_BYTE_SIZE = 256;

_Kernel_float_used_ DOUBLE shannonEntropy(PUCHAR buffer, size_t size) {
  if (IS_DEBUG_IRP)
    DbgPrint("!!! snFilter: Calc entropy started\n");
  DOUBLE entropy = 0.0;
  ULONG bucketByteVals[MAX_BYTE_SIZE] = {};
  for (ULONG i = 0; i < size; i++) {
    bucketByteVals[buffer[i]]++;
  }

  __try {
    for (ULONG i = 0; i < MAX_BYTE_SIZE; i++) {
      if (bucketByteVals[i] != 0) {
        DOUBLE
        val = (DOUBLE)bucketByteVals[i] / (DOUBLE)size;
        entropy += (-1) * val * log(val) * M_LOG2E;
      }
    }
  } __finally {
  }

  return entropy;
}