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

104 lines
2.8 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, 2010 William Hart
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "ulong_extras.h"
/*
mp_limb_t
n_ll_mod_preinv(mp_limb_t a_hi, mp_limb_t a_lo, mp_limb_t n, mp_limb_t ninv)
{
mp_limb_t q, r, norm;
if (a_hi > n) a_hi = n_mod2_preinv(a_hi, n, ninv);
count_leading_zeros(norm, n);
udiv_qrnnd_preinv(q, r, (a_hi<<norm) + r_shift(a_lo, FLINT_BITS-norm), a_lo<<norm, n<<norm, ninv);
return (r>>norm);
}
*/
/*
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_ll_mod_preinv(mp_limb_t a_hi, 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);
/* reduce u1 first i.e. u1 = n_mod2_preinv(u1, n, ninv) */
if (a_hi >= n)
{
const mp_limb_t u1 = r_shift(a_hi, FLINT_BITS - norm);
const mp_limb_t u0 = (a_hi << norm);
n <<= 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_hi = (r >> norm);
else
a_hi = ((r - n) >> norm);
}
else
n <<= norm;
/* now reduce the rest of the way */
{
const mp_limb_t u1 = (a_hi << 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);
}
}