120 lines
3.7 KiB
C
120 lines
3.7 KiB
C
/* mpz_scan0 -- search for a 0 bit.
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Copyright 2000, 2001, 2002, 2004 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 3 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
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#include "mpir.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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/* mpn_scan0 can't be used for the u>0 search since there might not be a 0
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bit before the end of the data. mpn_scan1 could be used for the inverted
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search under u<0, but usually the search won't go very far so it seems
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reasonable to inline that code. */
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mp_bitcnt_t
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mpz_scan0 (mpz_srcptr u, mp_bitcnt_t starting_bit)
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{
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mp_srcptr u_ptr = PTR(u);
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mp_size_t size = SIZ(u);
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mp_size_t abs_size = ABS(size);
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mp_srcptr u_end = u_ptr + abs_size;
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mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS;
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mp_srcptr p = u_ptr + starting_limb;
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mp_limb_t limb;
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int cnt;
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/* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
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u<0. Notice this test picks up all cases of u==0 too. */
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if (starting_limb >= abs_size)
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return (size >= 0 ? starting_bit : ULONG_MAX);
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limb = *p;
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if (size >= 0)
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{
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/* Mask to 1 all bits before starting_bit, thus ignoring them. */
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limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
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/* Search for a limb which isn't all ones. If the end is reached then
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the zero bit immediately past the end is returned. */
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while (limb == GMP_NUMB_MAX)
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{
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p++;
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if (p == u_end)
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return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
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limb = *p;
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}
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/* Now seek low 1 bit. */
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limb = ~limb;
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}
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else
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{
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mp_srcptr q;
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/* If there's a non-zero limb before ours then we're in the ones
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complement region. Search from *(p-1) downwards since that might
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give better cache locality, and since a non-zero in the middle of a
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number is perhaps a touch more likely than at the end. */
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q = p;
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while (q != u_ptr)
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{
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q--;
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if (*q != 0)
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goto inverted;
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}
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/* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
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If limb==0 here then this isn't the beginning of twos complement
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inversion, but that doesn't matter because limb==0 is a zero bit
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immediately (-1 is all ones for below). */
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limb--;
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inverted:
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/* Now seeking a 1 bit. */
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/* Mask to 0 all bits before starting_bit, thus ignoring them. */
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limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
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if (limb == 0)
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{
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/* If the high limb is zero after masking, then no 1 bits past
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starting_bit. */
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p++;
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if (p == u_end)
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return ULONG_MAX;
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/* Search further for a non-zero limb. The high limb is non-zero,
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if nothing else. */
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for (;;)
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{
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limb = *p;
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if (limb != 0)
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break;
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p++;
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ASSERT (p < u_end);
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}
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}
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}
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ASSERT (limb != 0);
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count_trailing_zeros (cnt, limb);
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return (mp_bitcnt_t)((p - u_ptr) * GMP_NUMB_BITS + cnt);
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}
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