root/tools/entropy/entropy.c

/* entropy.c
 *
 * Copyright (C) 2008 Tillmann Werner <tillmann.werner@gmx.de>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License Version 2 as
 * published by the Free Software Foundation.  You may not use, modify or
 * distribute this program under any other version of the GNU General
 * Public License.
 *
 * This program 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 this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 *
 * Description: Command line utility to calculate file entropy
 */


#include <fcntl.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>


typedef struct bstring {
        ssize_t         len;
        u_char          *data;
} bstring;


inline double calc_ent(bstring bstr) {
        ssize_t         i;
        u_int32_t       bytefreqs[256];
        double          e;
        double          log2;

        e       = 0;
        log2    = log(2);       // factor for changing the log base

        // calculate byte frequencies
        memset(bytefreqs, 0, 256 * sizeof(u_int32_t));
        for (i=0; i<bstr.len; i++) bytefreqs[bstr.data[i]]++;

        // calculate entropy
        for (i=0; i<=255; i++)
                if (bytefreqs[i])
                        e -= (double)bytefreqs[i]/(double)bstr.len * log((double)bytefreqs[i]/(double)bstr.len) / log2;

        return(e);
}

inline bstring bstr_map(const char* filename) {
        u_char buffer[BUFSIZ];
        int fd, bytes_read, i;
        bstring bstr;

        if (filename == NULL) {
                fprintf(stderr, "Error - Unable to map file into memory: No filename given.\n");
                exit(EXIT_FAILURE);
        }
        
        if ((fd = open(filename, O_RDONLY)) == -1) {
                fprintf(stderr, "Unable to open file %s: %m.\n", filename);
                exit(EXIT_FAILURE);
        }

        memset(&bstr, 0, sizeof(bstring));
        while ((bytes_read = read(fd, buffer, BUFSIZ)) > 0) {
                if ((bstr.data = (void *) realloc(bstr.data, (bstr.len + bytes_read) * sizeof(u_char))) == NULL) {
                        fprintf(stderr, "Unable to allocate memory: %m.\n");
                        exit(EXIT_FAILURE);
                }
                memset(bstr.data+bstr.len, 0, bytes_read * sizeof(u_char));
                for (i=0; i<bytes_read; i++) bstr.data[bstr.len+i] = buffer[i];
                        
                bstr.len += bytes_read;
        }
        if (bytes_read < 0) {
                fprintf(stderr, "Unable to read from %s: %m.\n", filename);
                exit(EXIT_FAILURE);
        }
        close(fd);

        return(bstr);
}


inline void bstr_unmap(bstring bstr) {
        free(bstr.data);
        memset(&bstr, 0, sizeof(bstring));

        return;
}

int main(int argc, char **argv) {
        int     i;
        bstring bstr;

        if (argc < 2) {
                printf ("Usage: %s file1 [file2 file3 ...]\n", argv[0]);
                exit(EXIT_FAILURE);
        }

        for (i=1; i<argc; i++) {
                bstr = bstr_map(argv[i]);
                printf ("%f  %s\n", calc_ent(bstr), argv[i]);
                bstr_unmap(bstr);
        }

        return(EXIT_SUCCESS);
}