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/* mpz_congruent_p -- test congruence of two mpz's.
Copyright 2001, 2002, 2005 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 either:
* 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.
or
* the GNU General Public License as published by the Free Software
Foundation; either version 2 of the License, or (at your option) any
later version.
or both in parallel, as here.
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 General Public License
for more details.
You should have received copies of the GNU General Public License and the
GNU Lesser General Public License along with the GNU MP Library. If not,
see https://www.gnu.org/licenses/. */
#include "gmp-impl.h"
#include "longlong.h"
/* For big divisors this code is only very slightly better than the user
doing a combination of mpz_sub and mpz_tdiv_r, but it's quite convenient,
and perhaps in the future can be improved, in similar ways to
mpn_divisible_p perhaps.
The csize==1 / dsize==1 special case makes mpz_congruent_p as good as
mpz_congruent_ui_p on relevant operands, though such a combination
probably doesn't occur often.
Alternatives:
If c<d then it'd work to just form a%d and compare a and c (either as
a==c or a+c==d depending on the signs), but the saving from avoiding the
abs(a-c) calculation would be small compared to the division.
Similarly if both a<d and c<d then it would work to just compare a and c
(a==c or a+c==d), but this isn't considered a particularly important case
and so isn't done for the moment.
Low zero limbs on d could be stripped and the corresponding limbs of a
and c tested and skipped, but doing so would introduce a borrow when a
and c differ in sign and have non-zero skipped limbs. It doesn't seem
worth the complications to do this, since low zero limbs on d should
occur only rarely. */
int
mpz_congruent_p (mpz_srcptr a, mpz_srcptr c, mpz_srcptr d)
{
mp_size_t asize, csize, dsize, sign;
mp_srcptr ap, cp, dp;
mp_ptr xp;
mp_limb_t alow, clow, dlow, dmask, r;
int result;
TMP_DECL;
dsize = SIZ(d);
if (UNLIKELY (dsize == 0))
return (mpz_cmp (a, c) == 0);
dsize = ABS(dsize);
dp = PTR(d);
if (ABSIZ(a) < ABSIZ(c))
MPZ_SRCPTR_SWAP (a, c);
asize = SIZ(a);
csize = SIZ(c);
sign = (asize ^ csize);
asize = ABS(asize);
ap = PTR(a);
if (csize == 0)
return mpn_divisible_p (ap, asize, dp, dsize);
csize = ABS(csize);
cp = PTR(c);
alow = ap[0];
clow = cp[0];
dlow = dp[0];
/* Check a==c mod low zero bits of dlow. This might catch a few cases of
a!=c quickly, and it helps the csize==1 special cases below. */
dmask = LOW_ZEROS_MASK (dlow) & GMP_NUMB_MASK;
alow = (sign >= 0 ? alow : -alow);
if (((alow-clow) & dmask) != 0)
return 0;
if (csize == 1)
{
if (dsize == 1)
{
cong_1:
if (sign < 0)
NEG_MOD (clow, clow, dlow);
if (ABOVE_THRESHOLD (asize, BMOD_1_TO_MOD_1_THRESHOLD))
{
r = mpn_mod_1 (ap, asize, dlow);
if (clow < dlow)
return r == clow;
else
return r == (clow % dlow);
}
if ((dlow & 1) == 0)
{
/* Strip low zero bits to get odd d required by modexact. If
d==e*2^n then a==c mod d if and only if both a==c mod e and
a==c mod 2^n, the latter having been done above. */
unsigned twos;
count_trailing_zeros (twos, dlow);
dlow >>= twos;
}
r = mpn_modexact_1c_odd (ap, asize, dlow, clow);
return r == 0 || r == dlow;
}
/* dlow==0 is avoided since we don't want to bother handling extra low
zero bits if dsecond is even (would involve borrow if a,c differ in
sign and alow,clow!=0). */
if (dsize == 2 && dlow != 0)
{
mp_limb_t dsecond = dp[1];
if (dsecond <= dmask)
{
unsigned twos;
count_trailing_zeros (twos, dlow);
dlow = (dlow >> twos) | (dsecond << (GMP_NUMB_BITS-twos));
ASSERT_LIMB (dlow);
/* dlow will be odd here, so the test for it even under cong_1
is unnecessary, but the rest of that code is wanted. */
goto cong_1;
}
}
}
TMP_MARK;
xp = TMP_ALLOC_LIMBS (asize+1);
/* calculate abs(a-c) */
if (sign >= 0)
{
/* same signs, subtract */
if (asize > csize || mpn_cmp (ap, cp, asize) >= 0)
ASSERT_NOCARRY (mpn_sub (xp, ap, asize, cp, csize));
else
ASSERT_NOCARRY (mpn_sub_n (xp, cp, ap, asize));
MPN_NORMALIZE (xp, asize);
}
else
{
/* different signs, add */
mp_limb_t carry;
carry = mpn_add (xp, ap, asize, cp, csize);
xp[asize] = carry;
asize += (carry != 0);
}
result = mpn_divisible_p (xp, asize, dp, dsize);
TMP_FREE;
return result;
}
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