/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2020 Peter Holm <pho@FreeBSD.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Flip one or more bits in a file.
 */

#include <sys/types.h>
#include <sys/stat.h>

#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

extern char *optarg;
extern int optind;

static long
random_long(long mi, long ma)
{
        return (arc4random()  % (ma - mi + 1) + mi);
}

static void
flip(void *ap, size_t len)
{
        unsigned char *cp;
        int byte;
        unsigned char bit, buf, mask, old __unused;

        cp = (unsigned char *)ap;
        byte = random_long(0, len - 1);
        bit = random_long(0,7);
        mask = ~(1 << bit);
        buf = cp[byte];
        old = cp[byte];
        buf = (buf & mask) | (~buf & ~mask);
        cp[byte] = buf;
#if defined(DEBUG)
        printf("Change %2x to %2x at %d by flipping bit %d\n",
            old, buf, byte, bit);
#endif
}

static void
trash(char *c)
{
        if (arc4random() % 2 == 1)
                *c = 0;
        else
                arc4random_buf(c, sizeof(c));
}

int
main(int argc, char *argv[])
{
        struct stat st;
        off_t pos;
        size_t size;
        int c, fd, i, times;

        times = 1;
        size = 0;
        while ((c = getopt(argc, argv, "n:s:")) != -1) {
                switch (c) {
                        case 'n':
                                times = atoi(optarg);
                                break;
                        case 's':
                                size = atol(optarg);
                                break;
                        case '?':
                        default:
                                fprintf(stderr,
                                    "Usage: %s [ -n <num> <file>]\n",
                                    argv[0]);
                                exit(1);
                }
        }
        argc -= optind;
        argv += optind;

        if (argc != 1) {
                fprintf(stderr, "Missing file name\n");
                exit(1);
        }

        if ((fd = open(argv[0], O_RDWR)) == -1)
                err(1, "open(%s)", argv[0]);

        if (size == 0) {
                if (fstat(fd, &st) == -1)
                        err(1, "stat %s", argv[0]);
                if ((st.st_mode & S_IFREG) == 0)
                        errx(1, "%s must be a regular file\n", argv[0]);
                size = st.st_size;
        }

        for (i = 0; i < times; i++) {
                char ch;

                pos = arc4random() % size;
                if (lseek(fd, pos, SEEK_SET) == -1)
                        err(1, "lseek()");
                if (read(fd, &ch, 1) != 1)
                        err(1, "read()");
                if (arc4random() % 100 < 98)
                        flip(&ch, 1);
                else
                        trash(&ch);
                if (lseek(fd, pos, SEEK_SET) == -1)
                        err(1, "lseek()");
                if (write(fd, &ch, 1) != 1)
                        err(1, "write()");
        }

        return (0);
}