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/* mpz_congruent_2exp_p -- test congruence of mpz mod 2^n.
Copyright 2001, 2002 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"
int
mpz_congruent_2exp_p (mpz_srcptr a, mpz_srcptr c, unsigned long d)
{
unsigned long i, dlimbs, dbits;
mp_ptr ap, cp;
mp_limb_t dmask, alimb, climb, sum;
mp_size_t asize_signed, csize_signed, asize, csize;
if (ABSIZ(a) < ABSIZ(c))
MPZ_SRCPTR_SWAP (a, c);
dlimbs = d / GMP_NUMB_BITS;
dbits = d % GMP_NUMB_BITS;
dmask = (CNST_LIMB(1) << dbits) - 1;
ap = PTR(a);
cp = PTR(c);
asize_signed = SIZ(a);
asize = ABS(asize_signed);
csize_signed = SIZ(c);
csize = ABS(csize_signed);
if (csize_signed == 0)
goto a_zeros;
if ((asize_signed ^ csize_signed) >= 0)
{
/* same signs, direct comparison */
/* a==c for limbs in common */
if (mpn_cmp (ap, cp, MIN (csize, dlimbs)) != 0)
return 0;
/* if that's all of dlimbs, then a==c for remaining bits */
if (csize > dlimbs)
return ((ap[dlimbs]-cp[dlimbs]) & dmask) == 0;
a_zeros:
/* a remains, need all zero bits */
/* if d covers all of a and c, then must be exactly equal */
if (asize <= dlimbs)
return asize == csize;
/* whole limbs zero */
for (i = csize; i < dlimbs; i++)
if (ap[i] != 0)
return 0;
/* partial limb zero */
return (ap[dlimbs] & dmask) == 0;
}
else
{
/* different signs, negated comparison */
/* common low zero limbs, stopping at first non-zeros, which must
match twos complement */
i = 0;
for (;;)
{
ASSERT (i < csize); /* always have a non-zero limb on c */
alimb = ap[i];
climb = cp[i];
sum = (alimb + climb) & GMP_NUMB_MASK;
if (i >= dlimbs)
return (sum & dmask) == 0;
i++;
/* require both zero, or first non-zeros as twos-complements */
if (sum != 0)
return 0;
if (alimb != 0)
break;
}
/* further limbs matching as ones-complement */
for (;;)
{
if (i >= csize)
break;
alimb = ap[i];
climb = cp[i];
sum = (alimb + climb + 1) & GMP_NUMB_MASK;
if (i >= dlimbs)
return (sum & dmask) == 0;
if (sum != 0)
return 0;
i++;
}
/* no more c, so require all 1 bits in a */
if (asize < dlimbs)
return 0; /* not enough a */
/* whole limbs */
for ( ; i < dlimbs; i++)
if (ap[i] != GMP_NUMB_MAX)
return 0;
/* if only whole limbs, no further fetches from a */
if (dbits == 0)
return 1;
/* need enough a */
if (asize == dlimbs)
return 0;
return ((ap[dlimbs]+1) & dmask) == 0;
}
}
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