scan0.c
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上传日期:2022-08-06
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- /* 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 "gmp.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;
- }