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/* Chrysalide - Outil d'analyse de fichiers binaires
* entropy.c - calcul de l'entropie d'un contenu binaire
*
* Copyright (C) 2024 Cyrille Bagard
*
* This file is part of Chrysalide.
*
* Chrysalide is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 3 of the License, or
* (at your option) any later version.
*
* Chrysalide is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with Chrysalide. If not, see <http://www.gnu.org/licenses/>.
*/
#include "entropy.h"
#include <math.h>
#include <string.h>
/******************************************************************************
* *
* Paramètres : data = séquence d'octets à traiter. *
* len = quantité de ces octets. *
* *
* Description : Détermine l'entropie d'un contenu binaire. *
* *
* Retour : Valeur d'entropie du contenu fourni. *
* *
* Remarques : - *
* *
******************************************************************************/
double compute_entropy(const bin_t *data, size_t len)
{
double result; /* Valeur calculée à renvoyer */
unsigned long counters[256]; /* Décompte des valeurs */
const bin_t *d_max; /* Borne de fin de parcours #1 */
const bin_t *d_iter; /* Boucle de parcours #1 */
double log_2;
unsigned long *c_max; /* Borne de fin de parcours #2 */
unsigned long *c_iter; /* Boucle de parcours #2 */
double freq; /* Fréquence liée à une valeur */
result = 0.0;
memset(counters, 0, sizeof(counters));
d_max = data + len;
for (d_iter = data; d_iter < d_max; d_iter++)
counters[*d_iter]++;
log_2 = log(256.0);
c_max = counters + 256;
for (c_iter = counters; c_iter < c_max; c_iter++)
{
if (*c_iter == 0lu)
continue;
freq = ((double)*c_iter) / ((double)len);
result -= freq * (log(freq) / log_2);
}
return result;
}
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