scan1.c
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上传日期:2022-08-06
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- /* mpz_scan1 -- search for a 1 bit.
- Copyright 2000, 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"
- /* mpn_scan0 can't be used for the inverted u<0 search since there might not
- be a 0 bit before the end of the data. mpn_scan1 could be used under u>0
- (except when in the high limb), but usually the search won't go very far
- so it seems reasonable to inline that code. */
- mp_bitcnt_t
- mpz_scan1 (mpz_srcptr u, mp_bitcnt_t starting_bit)
- {
- mp_srcptr u_ptr = PTR(u);
- mp_size_t size = SIZ(u);
- mp_size_t abs_size = ABS(size);
- mp_srcptr u_end = u_ptr + abs_size;
- mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS;
- mp_srcptr p = u_ptr + starting_limb;
- mp_limb_t limb;
- int cnt;
- /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
- Notice this test picks up any u==0 too. */
- if (starting_limb >= abs_size)
- return (size >= 0 ? ULONG_MAX : starting_bit);
- limb = *p;
- if (size >= 0)
- {
- /* Mask to 0 all bits before starting_bit, thus ignoring them. */
- limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
- if (limb == 0)
- {
- /* If it's the high limb which is zero after masking, then there's
- no 1 bits after starting_bit. */
- p++;
- if (p == u_end)
- return ULONG_MAX;
- /* Otherwise search further for a non-zero limb. The high limb is
- non-zero, if nothing else. */
- for (;;)
- {
- limb = *p;
- if (limb != 0)
- break;
- p++;
- ASSERT (p < u_end);
- }
- }
- }
- else
- {
- mp_srcptr q;
- /* If there's a non-zero limb before ours then we're in the ones
- complement region. Search from *(p-1) downwards since that might
- give better cache locality, and since a non-zero in the middle of a
- number is perhaps a touch more likely than at the end. */
- q = p;
- while (q != u_ptr)
- {
- q--;
- if (*q != 0)
- goto inverted;
- }
- if (limb == 0)
- {
- /* Skip zero limbs, to find the start of twos complement. The
- high limb is non-zero, if nothing else. This search is
- necessary so the -limb is applied at the right spot. */
- do
- {
- p++;
- ASSERT (p < u_end);
- limb = *p;
- }
- while (limb == 0);
- /* Apply twos complement, and look for a 1 bit in that. Since
- limb!=0 here, also have (-limb)!=0 so there's certainly a 1
- bit. */
- limb = -limb;
- goto got_limb;
- }
- /* Adjust so ~limb implied by searching for 0 bit becomes -limb. */
- limb--;
- inverted:
- /* Now seeking a 0 bit. */
- /* Mask to 1 all bits before starting_bit, thus ignoring them. */
- limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
- /* Search for a limb which is not all ones. If the end is reached
- then the zero immediately past the end is the result. */
- while (limb == GMP_NUMB_MAX)
- {
- p++;
- if (p == u_end)
- return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
- limb = *p;
- }
- /* Now seeking low 1 bit. */
- limb = ~limb;
- }
- got_limb:
- ASSERT (limb != 0);
- count_trailing_zeros (cnt, limb);
- return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
- }