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

138 lines
3.7 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) 2010 Sebastian Pancratz
Copyright (C) 2011 Fredrik Johansson
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "nmod_vec.h"
#include "nmod_poly.h"
#include "ulong_extras.h"
/* pointer to (x/Q)^i */
#define Ri(ii) (R + (n-1)*((ii)-1))
void
_nmod_poly_revert_series_lagrange_fast(mp_ptr Qinv,
mp_srcptr Q, slong n, nmod_t mod)
{
slong i, j, k, m;
mp_ptr R, S, T, tmp;
if (n >= 1) Qinv[0] = UWORD(0);
if (n >= 2) Qinv[1] = n_invmod(Q[1], mod.n);
if (n <= 2)
return;
m = n_sqrt(n);
R = _nmod_vec_init((n - 1) * m);
S = _nmod_vec_init(n - 1);
T = _nmod_vec_init(n - 1);
_nmod_poly_inv_series(Ri(1), Q + 1, n - 1, mod);
for (i = 2; i <= m; i++)
_nmod_poly_mullow(Ri(i), Ri(i-1), n - 1, Ri(1), n - 1, n - 1, mod);
for (i = 2; i < m; i++)
Qinv[i] = nmod_div(Ri(i)[i-1], i, mod);
_nmod_vec_set(S, Ri(m), n - 1);
for (i = m; i < n; i += m)
{
Qinv[i] = nmod_div(S[i-1], i, mod);
for (j = 1; j < m && i + j < n; j++)
{
mp_limb_t s;
int nlimbs = _nmod_vec_dot_bound_limbs(i + j, mod);
NMOD_VEC_DOT(s, k, i + j, S[k], Ri(j)[i+j-1-k], mod, nlimbs);
Qinv[i+j] = nmod_div(s, i+j, mod);
}
if (i + 1 < n)
{
_nmod_poly_mullow(T, S, n - 1, Ri(m), n - 1, n - 1, mod);
tmp = S; S = T; T = tmp;
}
}
_nmod_vec_clear(R);
_nmod_vec_clear(S);
_nmod_vec_clear(T);
}
void
nmod_poly_revert_series_lagrange_fast(nmod_poly_t Qinv,
const nmod_poly_t Q, slong n)
{
mp_ptr Qinv_coeffs, Q_coeffs;
nmod_poly_t t1;
slong Qlen;
Qlen = Q->length;
if (Qlen < 2 || Q->coeffs[0] != 0 || Q->coeffs[1] == 0)
{
flint_printf("Exception (nmod_poly_revert_series_lagrange_fast). Input must \n"
"have zero constant and an invertible coefficient of x^1.\n");
abort();
}
if (Qlen < n)
{
Q_coeffs = _nmod_vec_init(n);
flint_mpn_copyi(Q_coeffs, Q->coeffs, Qlen);
flint_mpn_zero(Q_coeffs + Qlen, n - Qlen);
}
else
Q_coeffs = Q->coeffs;
if (Q == Qinv && Qlen >= n)
{
nmod_poly_init2(t1, Q->mod.n, n);
Qinv_coeffs = t1->coeffs;
}
else
{
nmod_poly_fit_length(Qinv, n);
Qinv_coeffs = Qinv->coeffs;
}
_nmod_poly_revert_series_lagrange_fast(Qinv_coeffs, Q_coeffs, n, Q->mod);
if (Q == Qinv && Qlen >= n)
{
nmod_poly_swap(Qinv, t1);
nmod_poly_clear(t1);
}
Qinv->length = n;
if (Qlen < n)
_nmod_vec_clear(Q_coeffs);
_nmod_poly_normalise(Qinv);
}