Function random_dirent

zic.c:1549–1595

Generate a randomish name in the same directory as *NAME. If *NAMEALLOC, put the name into *NAMEALLOC which is assumed to be that returned by a previous call and is thus already almost set up and equal to *NAME; otherwise, allocate a new name and put its address into both *NAMEALLOC and *NAME. */

   that returned by a previous call and is thus already almost set up
   and equal to *NAME; otherwise, allocate a new name and put its
   address into both *NAMEALLOC and *NAME.  */
static void
random_dirent(char const **name, char **namealloc)
{
  char const *src = *name;
  char *dst = *namealloc;
  static char const prefix[] = ".zic";
  static char const alphabet[] =
    "abcdefghijklmnopqrstuvwxyz"
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    "0123456789";
  enum { prefixlen = sizeof prefix - 1, alphabetlen = sizeof alphabet - 1 };
  int suffixlen = 6;
  char const *lastslash = strrchr(src, '/');
  ptrdiff_t dirlen = lastslash ? lastslash + 1 - src : 0;
  int i;
  uint_fast64_t r;
  uint_fast64_t base = alphabetlen;

  /* BASE**6 */
  uint_fast64_t base__6 = base * base * base * base * base * base;

  /* The largest uintmax_t that is a multiple of BASE**6.  Any random
     uintmax_t value that is this value or greater, yields a biased
     remainder when divided by BASE**6.  UNFAIR_MIN equals the
     mathematical value of ((UINTMAX_MAX + 1) - (UINTMAX_MAX + 1) % BASE**6)
     computed without overflow.  */
  uint_fast64_t unfair_min = - ((UINTMAX_MAX % base__6 + 1) % base__6);

  if (!dst) {
    char *cp = dst = xmalloc(size_sum(dirlen, prefixlen + suffixlen + 1));
    cp = mempcpy(cp, src, dirlen);
    cp = mempcpy(cp, prefix, prefixlen);
    cp[suffixlen] = '\0';
    *name = *namealloc = dst;
  }

  for (;; check_for_signal()) {
    r = get_rand_u64();
    if (r < unfair_min)
      break;
  }

  for (i = 0; i < suffixlen; i++) {
    dst[dirlen + prefixlen + i] = alphabet[r % alphabetlen];
    r /= alphabetlen;
  }
}

/* For diagnostics the directory, and file name relative to that