pqc/external/flint-2.4.3/ulong_extras/mulmod_preinv.c

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2014-05-18 22:03:37 +00:00
/*=============================================================================
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, 2010, 2012 William Hart
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "ulong_extras.h"
mp_limb_t
n_mulmod_preinv(mp_limb_t a, mp_limb_t b, mp_limb_t n,
mp_limb_t ninv, ulong norm)
{
mp_limb_t q0, q1, r, p_hi, p_lo;
/* multiply */
umul_ppmm(p_hi, p_lo, a, b);
/* renormalise product */
if (norm)
{
p_lo = (p_lo >> norm) + (p_hi << (FLINT_BITS - norm));
p_hi = (p_hi >> norm);
}
/* reduce mod n */
{
umul_ppmm(q1, q0, ninv, p_hi);
add_ssaaaa(q1, q0, q1, q0, p_hi, p_lo);
r = (p_lo - (q1 + 1) * n);
if (r >= q0)
r += n;
return (r < n ? r : r - n);
}
}