Function seq_fast

src/seq.c:461–538

Print all whole numbers from A to B, inclusive -- to stdout, each followed by a newline. Then exit. */

/* Print all whole numbers from A to B, inclusive -- to stdout, each
   followed by a newline.  Then exit.  */
static void
seq_fast (char const *a, char const *b, uintmax_t step)
{
  /* Skip past any redundant leading '0's.  Without this, our naive cmp
     function would declare 000 to be larger than 99.  */
  a = trim_leading_zeros (a);
  b = trim_leading_zeros (b);

  idx_t p_len = strlen (a);
  idx_t b_len = strlen (b);
  bool inf = b_len == 3 && memeq (b, "inf", 4);

  /* Allow for at least 31 digits without realloc.
     1 more than p_len is needed for the inf case.  */
  enum { INITIAL_ALLOC_DIGITS = 31 };
  idx_t inc_size = MAX (MAX (p_len + 1, b_len), INITIAL_ALLOC_DIGITS);
  /* Ensure we only increase by at most 1 digit at buffer boundaries.  */
  static_assert (SEQ_FAST_STEP_LIMIT_DIGITS < INITIAL_ALLOC_DIGITS - 1);

  /* Copy A (sans NUL) to end of new buffer.  */
  char *p0 = xmalloc (inc_size);
  char *endp = p0 + inc_size;
  char *p = memcpy (endp - p_len, a, p_len);

  /* Reduce number of write calls which is seen to
     give a speed-up of more than 2x over naive stdio code
     when printing the first 10^9 integers.  */
  char buf[BUFSIZ];
  char *buf_end = buf + sizeof buf;
  char *bufp = buf;

  while (inf || cmp (p, endp - p, b, b_len) <= 0)
    {
      /* Append number, flushing output buffer while the number's
         digits do not fit with room for a separator or terminator.  */
      char *pp = p;
      while (buf_end - bufp <= endp - pp)
        {
          memcpy (bufp, pp, buf_end - bufp);
          pp += buf_end - bufp;
          if (full_write (STDOUT_FILENO, buf, sizeof buf) != sizeof buf)
            write_error ();
          bufp = buf;
        }

      /* The rest of the number, followed by a separator or terminator,
         will fit.  Tentatively append a separator.  */
      bufp = mempcpy (bufp, pp, endp - pp);
      *bufp++ = *separator;

      /* Grow number buffer if needed for the inf case.  */
      if (p == p0)
        {
          char *new_p0 = xpalloc (NULL, &inc_size, 1, -1, 1);
          idx_t saved_p_len = endp - p;
          endp = new_p0 + inc_size;
          p = memcpy (endp - saved_p_len, p0, saved_p_len);
          free (p0);