/*
 * Copyright (C) 2009  Lars Wirzenius
 *
 * This program 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.
 *
 * 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, see <http://www.gnu.org/licenses/>.
 */


#define _LARGEFILE64_SOURCE

#include <errno.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>


/* Choose a random integer in [0..n-1]. */
static long choose(long n)
{
    if (n == 0)
        return 0;
    
    long buckets = (RAND_MAX + 1L) / n;
    long max = buckets * n;
    long r;
    
    do {
        r = random();
    } while (r >= max);
    return r / buckets;
}


static int corrupt(const char *filename)
{
    int fd;
    struct stat st;
    
    fd = open(filename, O_RDWR | O_LARGEFILE);
    if (fd == -1) {
        fprintf(stderr, "Can't open %s: %s\n", filename, strerror(errno));
        return -1;
    }

    if (fstat(fd, &st) == -1) {
        fprintf(stderr, "Can't find length of %s: %s", filename, 
                strerror(errno));
        return -1;
    }

    long offset = choose(st.st_size);
    long bit = choose(CHAR_BIT);
    printf("%s: offset=%ld bit=%ld\n", filename, offset, bit);
    
    if (lseek(fd, offset, SEEK_SET) == (off_t) -1) {
        fprintf(stderr, "Can't seek in %s: %s", filename, strerror(errno));
        return -1;
    }
    
    unsigned char byte;
    if (read(fd, &byte, 1) == -1) {
        fprintf(stderr, "Can't read %s: %s", filename, strerror(errno));
        return -1;
    }
    
    /* This is where we toggle the bit. */
    unsigned mask = 1 << bit; // All zeroes, except for the interesting bit.
    unsigned interesting = byte & mask; // Only the interesting bit can be 1.
    unsigned toggled = interesting ^ mask; // Interesting bit has been toggled.
    unsigned others = byte & (~mask); // Interesting bit is 0.
    byte = others | toggled;
    
    /* Now write it back. */
    if (lseek(fd, offset, SEEK_SET) == (off_t) -1) {
        fprintf(stderr, "Can't seek in %s: %s", filename, strerror(errno));
        return -1;
    }
    if (write(fd, &byte, 1) != 1) {
        fprintf(stderr, "Can't write %s: %s", filename, strerror(errno));
        return -1;
    }

    if (close(fd) == -1) {
        fprintf(stderr, "Can't close %s: %s", filename, strerror(errno));
        return -1;
    }
    
    return 0;
}


int main(int argc, char **argv)
{
    int exit_code;
    
    srandom(time(NULL) + getpid());
    exit_code = 0;
    for (int i = 1; i < argc; ++i)
        if (corrupt(argv[i]) == -1)
            exit_code = 1;
    return exit_code;
}