pqc/external/flint-2.4.3/ulong_extras/mod2_preinv.c

73 lines
2.1 KiB
C
Raw Normal View History

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"
/*
mp_limb_t n_mod2_preinv(mp_limb_t a, mp_limb_t n, mp_limb_t ninv)
{
unsigned int norm;
mp_limb_t q, r;
count_leading_zeros(norm, n);
udiv_qrnnd_preinv(q, r, r_shift(a, FLINT_BITS-norm), a<<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_mod2_preinv(mp_limb_t a, mp_limb_t n, mp_limb_t ninv)
{
unsigned int norm;
mp_limb_t q1, q0, r;
count_leading_zeros(norm, n);
n <<= norm;
{
const mp_limb_t u1 = r_shift(a, FLINT_BITS-norm);
const mp_limb_t u0 = (a << 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;
return (r < n) ? (r >> norm) : ((r - n) >> norm);
}
}