pqc/external/flint-2.4.3/nmod_poly/resultant_euclidean.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) 2007, 2008 William Hart
Copyright (C) 2011 Sebastian Pancratz
******************************************************************************/
#include <stdlib.h>
#include <gmp.h>
#include "flint.h"
#include "mpn_extras.h"
#include "nmod_vec.h"
#include "nmod_poly.h"
mp_limb_t
_nmod_poly_resultant_euclidean(mp_srcptr poly1, slong len1,
mp_srcptr poly2, slong len2, nmod_t mod)
{
if (poly1 == poly2)
{
return 0;
}
else if (len2 == 1)
{
if (len1 == 1)
{
return 1;
}
else if (len1 == 2)
{
return poly2[0];
}
else
{
return n_powmod2_ui_preinv(poly2[0], len1 - 1, mod.n, mod.ninv);
}
}
else /* len1 >= len2 >= 2 */
{
mp_limb_t res = 1;
mp_ptr u, v, r, t, w;
slong l0, l1, l2;
mp_limb_t lc;
w = _nmod_vec_init(3 * len1);
u = w;
v = w + len1;
r = v + len1;
_nmod_vec_set(u, poly1, len1);
_nmod_vec_set(v, poly2, len2);
l1 = len1;
l2 = len2;
do
{
l0 = l1;
l1 = l2;
lc = v[l1 - 1];
_nmod_poly_rem(r, u, l0, v, l1, mod);
l2 = l1 - 1;
MPN_NORM(r, l2);
{
t = u;
u = v;
v = r;
r = t;
}
if (l2 >= 1)
{
lc = n_powmod2_preinv(lc, l0 - l2, mod.n, mod.ninv);
res = n_mulmod2_preinv(res, lc, mod.n, mod.ninv);
if (((l0 | l1) & 1) == 0)
{
res = nmod_neg(res, mod);
}
}
else
{
if (l1 == 1)
{
lc = n_powmod2_preinv(lc, l0 - 1, mod.n, mod.ninv);
res = n_mulmod2_preinv(res, lc, mod.n, mod.ninv);
}
else
{
res = 0;
}
}
}
while (l2 > 0);
_nmod_vec_clear(w);
return res;
}
}
mp_limb_t
nmod_poly_resultant_euclidean(const nmod_poly_t f, const nmod_poly_t g)
{
const slong len1 = f->length;
const slong len2 = g->length;
mp_limb_t r;
if (len1 == 0 || len2 == 0)
{
r = 0;
}
else
{
if (len1 >= len2)
{
r = _nmod_poly_resultant_euclidean(f->coeffs, len1,
g->coeffs, len2, f->mod);
}
else
{
r = _nmod_poly_resultant_euclidean(g->coeffs, len2,
f->coeffs, len1, f->mod);
if (((len1 | len2) & WORD(1)) == WORD(0))
r = nmod_neg(r, f->mod);
}
}
return r;
}