pqc/external/flint-2.4.3/ulong_extras/factorial_mod2_preinv.c
2014-05-24 23:16:06 +02:00

83 lines
2.4 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) 2011 Fredrik Johansson
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "ulong_extras.h"
static const mp_limb_t small_factorials[] =
{
UWORD(1), UWORD(1), UWORD(2), UWORD(6), UWORD(24), UWORD(120), UWORD(720), UWORD(5040), UWORD(40320), UWORD(362880),
UWORD(3628800), UWORD(39916800), UWORD(479001600),
#if FLINT64
UWORD(6227020800), UWORD(87178291200),
UWORD(1307674368000), UWORD(20922789888000), UWORD(355687428096000), UWORD(6402373705728000),
UWORD(121645100408832000), UWORD(2432902008176640000),
#endif
};
#if FLINT64
#define MAX_SMALL_FACTORIAL 20
#else
#define MAX_SMALL_FACTORIAL 12
#endif
mp_limb_t n_factorial_mod2_preinv(ulong n, mp_limb_t p, mp_limb_t pinv)
{
mp_limb_t prod, hi, lo;
if (n <= MAX_SMALL_FACTORIAL)
return n_mod2_preinv(small_factorials[n], p, pinv);
if (n >= p)
return UWORD(0);
if (n >= UWORD(1000000))
return n_factorial_fast_mod2_preinv(n, p, pinv);
prod = small_factorials[MAX_SMALL_FACTORIAL];
lo = n;
n--;
/* TODO: speedup for n in the range of sqrt(UWORD_MAX) */
while (n > MAX_SMALL_FACTORIAL)
{
umul_ppmm(hi, lo, lo, n);
if (hi)
{
lo = n_ll_mod_preinv(hi, lo, p, pinv);
prod = n_mulmod2_preinv(prod, lo, p, pinv);
lo = UWORD(1);
}
n--;
}
return n_mulmod2_preinv(prod, lo, p, pinv);
}