pqc/external/flint-2.4.3/ulong_extras/lll_mod_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 William Hart
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "ulong_extras.h"
/*
New method of Moller and Granlund
see paper Improved Division by Invariant Integers (Torbjorn Granlund
and Niels Moller) (preprint): (pp. 4)
http://www.lysator.liu.se/~nisse/archive/draft-division-paper.pdf
*/
mp_limb_t
n_lll_mod_preinv(mp_limb_t a_hi, mp_limb_t a_mi, mp_limb_t a_lo,
mp_limb_t n, mp_limb_t ninv)
{
mp_limb_t q0, q1, r, norm;
count_leading_zeros(norm, n);
n <<= norm;
/*
we assume a_hi is already reduced
first reduce a_hi, a_mi mod n
*/
{
const mp_limb_t u1 = (a_hi << norm) + r_shift(a_mi, FLINT_BITS - norm);
const mp_limb_t u0 = (a_mi << norm);
umul_ppmm(q1, q0, ninv, u1);
add_ssaaaa(q1, q0, q1, q0, u1, u0);
r = (u0 - (q1 + 1) * n);
if (r >= q0)
r += n;
if (r < n)
a_mi = (r >> norm);
else
a_mi = ((r - n) >> norm);
}
/*
now a_mid is reduced mod n
so reduce a_mi, a_lo mod n
*/
{
const mp_limb_t u1 = (a_mi << norm) + r_shift(a_lo, FLINT_BITS - norm);
const mp_limb_t u0 = (a_lo << norm);
umul_ppmm(q1, q0, ninv, u1);
add_ssaaaa(q1, q0, q1, q0, u1, u0);
r = (u0 - (q1 + 1) * n);
if (r >= q0)
r += n;
if (r < n)
return (r >> norm);
else
return ((r - n) >> norm);
}
}