utils.h

#ifndef UTILS_H
#define UTILS_H 1

#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define MAXLEN 961 // maximum string length

static inline size_t makehist(unsigned char const * const buf, ssize_t * const hist, size_t len) {
  ssize_t wherechar[256];
  size_t i, histlen;
  histlen = 0;
  for (i = 0; i < 256; i++)
    wherechar[i] = -1;
  for (i = 0; i < len; i++) {
    if (wherechar[buf[i]] == -1) {
      wherechar[(int)buf[i]] = histlen;
      histlen++;
    }
    hist[wherechar[(int)buf[i]]]++;
  }
  return histlen;
}

static inline float entropy(u_int8_t const * const buf, size_t len) {
  float entropy = 0.0f;
  u_int32_t byte_counters[256];
  size_t i;

  memset(byte_counters, 0, sizeof(byte_counters));

  for(i = 0; i < len; ++i) {
    byte_counters[buf[i]]++;
  }

  for(i = 0; i < sizeof(byte_counters) / sizeof(byte_counters[0]); ++i) {
    if(byte_counters[i] == 0) {
      continue;
    }

    float const p = (float)byte_counters[i] / len;
    entropy += p * log2f(1 / p);
  }

  return entropy;
}
#endif