mpir/mpn/generic/scan0.c

/* mpn_scan0 -- Scan from a given bit position for the next clear bit.

Copyright 1994, 1996, 2001, 2002, 2004 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */

#include "mpir.h"
#include "gmp-impl.h"
#include "longlong.h"

/* Argument constraints:
   1. U must sooner or later have a limb with a clear bit.
 */

mp_bitcnt_t
mpn_scan0 (mp_srcptr up, mp_bitcnt_t starting_bit)
{
  mp_size_t starting_word;
  mp_limb_t alimb;
  int cnt;
  mp_srcptr p;

  /* Start at the word implied by STARTING_BIT.  */
  starting_word = starting_bit / GMP_NUMB_BITS;
  p = up + starting_word;
  alimb = *p++ ^ GMP_NUMB_MASK;

  /* Mask off any bits before STARTING_BIT in the first limb.  */
  alimb &= - (mp_limb_t) 1 << (starting_bit % GMP_NUMB_BITS);

  while (alimb == 0)
    alimb = *p++ ^ GMP_NUMB_MASK;

  count_trailing_zeros (cnt, alimb);
  return (p - up - 1) * GMP_NUMB_BITS + cnt;
}