misc/xcrcgen.c

   1 /*
   2  * Produce a "generalized CRC" table.  Assumes a platform with
   3  * /dev/urandom -- otherwise reimplement get_random_byte().
   4  */
   5
   6 #include <errno.h>
   7 #include <fcntl.h>
   8 #include <inttypes.h>
   9 #include <stdio.h>
  10 #include <stdlib.h>
  11 #include <unistd.h>
  12
  13 static uint8_t get_random_byte(void)
  14 {
  15     static int fd = -1;
  16     uint8_t buf;
  17     int rv;
  18
  19     if (fd < 0)
  20         fd = open("/dev/urandom", O_RDONLY);
  21
  22     do {
  23         errno = 0;
  24         rv = read(fd, &buf, 1);
  25         if (rv < 1 && errno != EAGAIN)
  26             abort();
  27     } while (rv < 1);
  28
  29     return buf;
  30 }
  31
  32 static void random_permute(uint8_t *buf)
  33 {
  34     int i, j, k;
  35     int m;
  36
  37     for (i = 0; i < 256; i++)
  38         buf[i] = i;
  39
  40     m = 255;
  41     for (i = 255; i > 0; i--) {
  42         if (i <= (m >> 1))
  43             m >>= 1;
  44         do {
  45             j = get_random_byte() & m;
  46         } while (j > i);
  47         k = buf[i];
  48         buf[i] = buf[j];
  49         buf[j] = k;
  50     }
  51 }
  52
  53 static void xcrc_table(uint64_t *buf)
  54 {
  55     uint8_t perm[256];
  56     int i, j;
  57
  58     memset(buf, 0, 8*256);      /* Make static checkers happy */
  59
  60     for (i = 0; i < 8; i++) {
  61         random_permute(perm);
  62         for (j = 0; j < 256; j++)
  63             buf[j] = (buf[j] << 8) | perm[j];
  64     }
  65 }
  66
  67 int main(void)
  68 {
  69     int i;
  70     uint64_t buf[256];
  71
  72     xcrc_table(buf);
  73
  74     for (i = 0; i < 256; i++) {
  75         printf("%016"PRIx64"\n", buf[i]);
  76     }
  77
  78     return 0;
  79 }