140 lines
4.4 KiB
C
140 lines
4.4 KiB
C
/* mpz_scan1 -- search for a 1 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 2.1 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; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#include "gmp.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 inverted u<0 search since there might not
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be a 0 bit before the end of the data. mpn_scan1 could be used under u>0
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(except when in the high limb), but usually the search won't go very far
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so it seems reasonable to inline that code. */
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unsigned long
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mpz_scan1 (mpz_srcptr u, unsigned long 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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unsigned long 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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/* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
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Notice this test picks up any u==0 too. */
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if (starting_limb >= abs_size)
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return (size >= 0 ? ULONG_MAX : starting_bit);
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limb = *p;
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if (size >= 0)
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{
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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 it's the high limb which is zero after masking, then there's
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no 1 bits after 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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/* Otherwise search further for a non-zero limb. The high limb is
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non-zero, 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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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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if (limb == 0)
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{
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/* Skip zero limbs, to find the start of twos complement. The
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high limb is non-zero, if nothing else. This search is
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necessary so the -limb is applied at the right spot. */
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do
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{
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p++;
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ASSERT (p < u_end);
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limb = *p;
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}
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while (limb == 0);
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/* Apply twos complement, and look for a 1 bit in that. Since
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limb!=0 here, also have (-limb)!=0 so there's certainly a 1
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bit. */
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limb = -limb;
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goto got_limb;
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}
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/* Adjust so ~limb implied by searching for 0 bit becomes -limb. */
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limb--;
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inverted:
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/* Now seeking a 0 bit. */
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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 is not all ones. If the end is reached
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then the zero immediately past the end is the result. */
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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 abs_size * GMP_NUMB_BITS;
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limb = *p;
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}
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/* Now seeking low 1 bit. */
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limb = ~limb;
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}
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got_limb:
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ASSERT (limb != 0);
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count_trailing_zeros (cnt, limb);
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return (p - u_ptr) * GMP_NUMB_BITS + cnt;
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}
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