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 <string.h>
  12 #include <unistd.h>
  13
  14 static uint8_t get_random_byte(void)
  15 {
  16     static int fd = -1;
  17     uint8_t buf;
  18     int rv;
  19
  20     if (fd < 0)
  21         fd = open("/dev/urandom", O_RDONLY);
  22
  23     do {
  24         errno = 0;
  25         rv = read(fd, &buf, 1);
  26         if (rv < 1 && errno != EAGAIN)
  27             abort();
  28     } while (rv < 1);
  29
  30     return buf;
  31 }
  32
  33 static void random_permute(uint8_t *buf)
  34 {
  35     int i, j, k;
  36     int m;
  37
  38     for (i = 0; i < 256; i++)
  39         buf[i] = i;
  40
  41     m = 255;
  42     for (i = 255; i > 0; i--) {
  43         if (i <= (m >> 1))
  44             m >>= 1;
  45         do {
  46             j = get_random_byte() & m;
  47         } while (j > i);
  48         k = buf[i];
  49         buf[i] = buf[j];
  50         buf[j] = k;
  51     }
  52 }
  53
  54 static void xcrc_table(uint64_t *buf)
  55 {
  56     uint8_t perm[256];
  57     int i, j;
  58
  59     memset(buf, 0, 8*256);      /* Make static checkers happy */
  60
  61     for (i = 0; i < 8; i++) {
  62         random_permute(perm);
  63         for (j = 0; j < 256; j++)
  64             buf[j] = (buf[j] << 8) | perm[j];
  65     }
  66 }
  67
  68 int main(void)
  69 {
  70     int i;
  71     uint64_t buf[256];
  72
  73     xcrc_table(buf);
  74
  75     for (i = 0; i < 256; i++) {
  76         printf("%016"PRIx64"\n", buf[i]);
  77     }
  78
  79     return 0;
  80 }