scan0.c
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上传日期:2022-08-06
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- /* mpz_scan0 -- search for a 0 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 u>0 search since there might not be a 0
- bit before the end of the data. mpn_scan1 could be used for the inverted
- search under u<0, but usually the search won't go very far so it seems
- reasonable to inline that code. */
- mp_bitcnt_t
- mpz_scan0 (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;
- /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
- u<0. Notice this test picks up all cases of u==0 too. */
- if (starting_limb >= abs_size)
- return (size >= 0 ? starting_bit : ULONG_MAX);
- limb = *p;
- if (size >= 0)
- {
- /* 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 isn't all ones. If the end is reached then
- the zero bit immediately past the end is returned. */
- while (limb == GMP_NUMB_MAX)
- {
- p++;
- if (p == u_end)
- return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
- limb = *p;
- }
- /* Now seek low 1 bit. */
- limb = ~limb;
- }
- 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;
- }
- /* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
- If limb==0 here then this isn't the beginning of twos complement
- inversion, but that doesn't matter because limb==0 is a zero bit
- immediately (-1 is all ones for below). */
- limb--;
- inverted:
- /* Now seeking a 1 bit. */
- /* 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 the high limb is zero after masking, then no 1 bits past
- starting_bit. */
- p++;
- if (p == u_end)
- return ULONG_MAX;
- /* 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);
- }
- }
- }
- ASSERT (limb != 0);
- count_trailing_zeros (cnt, limb);
- return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
- }