pqc/external/flint-2.4.3/fmpz/submul_ui.c
2014-05-24 23:16:06 +02:00

98 lines
3.1 KiB
C

/*=============================================================================
This file is part of FLINT.
FLINT is free software; you can redistribute it and/or modify
it under the terms of 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.
FLINT 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 a copy of the GNU General Public License
along with FLINT; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
=============================================================================*/
/******************************************************************************
Copyright (C) 2009 William Hart
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "ulong_extras.h"
#include "fmpz.h"
void
fmpz_submul_ui(fmpz_t f, const fmpz_t g, ulong x)
{
fmpz c1 = *g, r;
if ((x == 0) || (c1 == 0))
return; /* product is zero */
r = *f;
if (r == 0)
{
fmpz_mul_ui(f, g, x); /* we are subtracting product from 0 */
fmpz_neg(f, f);
return;
}
if (!COEFF_IS_MPZ(c1)) /* c1 is small */
{
mp_limb_t prod[2];
ulong uc1 = FLINT_ABS(c1);
__mpz_struct * mpz_ptr;
mpz_t temp;
umul_ppmm(prod[1], prod[0], uc1, x); /* compute product */
if (prod[1] == 0) /* product fits in one limb */
{
if (c1 < WORD(0))
fmpz_add_ui(f, f, prod[0]);
else
fmpz_sub_ui(f, f, prod[0]);
return;
}
else if ((prod[1] == 1) && (!COEFF_IS_MPZ(r)) && ((r ^ c1) >= WORD(0)))
{
/*
only chance at cancellation is if product is one bit past
a limb and f is small and same sign as this product
*/
ulong ur = FLINT_ABS(r);
if (ur > prod[0]) /* cancellation will occur */
{
fmpz_set_ui(f, prod[0] - ur);
if (r > WORD(0))
fmpz_neg(f, f);
return;
}
}
/*
in all remaining cases res is either big already,
or will be big in the end
*/
mpz_ptr = _fmpz_promote_val(f);
/* set up a temporary, cheap mpz_t to contain prod */
temp->_mp_d = prod;
temp->_mp_size = (c1 < WORD(0) ? -2 : 2);
mpz_sub(mpz_ptr, mpz_ptr, temp);
_fmpz_demote_val(f); /* cancellation may have occurred */
}
else /* c1 is large */
{
__mpz_struct *mpz_ptr = _fmpz_promote_val(f);
flint_mpz_submul_ui(mpz_ptr, COEFF_TO_PTR(c1), x);
_fmpz_demote_val(f); /* cancellation may have occurred */
}
}