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/* mpz_scan1 -- search for a 1 bit.
Copyright 2000-2002, 2004, 2012 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 https://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) __GMP_NOTHROW
{
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 - 1;
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 ? ~(mp_bitcnt_t) 0 : starting_bit);
/* This is an important case, where sign is not relevant! */
if (starting_bit == 0)
goto short_cut;
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. */
if (p == u_end)
return ~(mp_bitcnt_t) 0;
/* Otherwise search further for a non-zero limb. The high limb is
non-zero, if nothing else. */
search_nonzero:
do
{
ASSERT (p != u_end);
p++;
short_cut:
limb = *p;
}
while (limb == 0);
}
}
else
{
/* If there's a non-zero limb before ours then we're in the ones
complement region. */
if (mpn_zero_p (u_ptr, starting_limb)) {
if (limb == 0)
/* Seeking for the first non-zero bit, it is the same for u and -u. */
goto search_nonzero;
/* Adjust so ~limb implied by searching for 0 bit becomes -limb. */
limb--;
}
/* 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)
{
if (p == u_end)
return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
p++;
limb = *p;
}
/* Now seeking low 1 bit. */
limb = ~limb;
}
ASSERT (limb != 0);
count_trailing_zeros (cnt, limb);
return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
}
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