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

122 lines
3.3 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, 2011 Sebastian Pancratz
Copyright (C) 2013 William Hart
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "fmpz.h"
#include "fmpz_poly.h"
void
_fmpz_poly_preinvert(fmpz * Binv, const fmpz * B, slong len)
{
if (len == 1) /* B is +-1 */
{
fmpz_set(Binv, B);
}
else
{
const slong alloc = len + FLINT_MAX(len, 3 * FMPZ_POLY_INV_NEWTON_CUTOFF);
slong *a, i, m, n = len;
fmpz *T, *W;
T = _fmpz_vec_init(alloc);
W = T + len;
for (i = 1; (WORD(1) << i) < n; i++) ;
a = (slong *) flint_malloc(i * sizeof(slong));
a[i = 0] = n;
while (n >= FMPZ_POLY_INV_NEWTON_CUTOFF)
a[++i] = (n = (n + 1) / 2);
if (len != n)
_fmpz_poly_reverse(T, B, len, len); /* only reverse if it ... */
/* Base case */
{
fmpz *Brev = W + 2 * FMPZ_POLY_INV_NEWTON_CUTOFF;
if (len != n)
_fmpz_poly_reverse(Brev, T, n, n); /* ... won't be undone */
else Brev = (fmpz *) B;
_fmpz_vec_zero(W, 2*n - 2);
fmpz_one(W + (2*n - 2));
_fmpz_poly_div_basecase(Binv, W, W, 2*n - 1, Brev, n);
_fmpz_poly_reverse(Binv, Binv, n, n);
}
for (i--; i >= 0; i--)
{
m = n;
n = a[i];
_fmpz_poly_mullow(W, T, n, Binv, m, n);
_fmpz_poly_mullow(Binv + m, Binv, m, W + m, n - m, n - m);
_fmpz_vec_neg(Binv + m, Binv + m, n - m);
}
_fmpz_vec_clear(T, alloc);
flint_free(a);
}
}
void
fmpz_poly_preinvert(fmpz_poly_t B_inv, const fmpz_poly_t B)
{
slong n = B->length;
fmpz_poly_t temp;
fmpz * Binv_coeffs;
if (n == 0)
{
flint_printf("Exception (fmpz_poly_preinvert). Division by zero.\n");
abort();
}
if (B == B_inv)
{
fmpz_poly_init2(temp, n);
Binv_coeffs = temp->coeffs;
} else
{
fmpz_poly_fit_length(B_inv, n);
Binv_coeffs = B_inv->coeffs;
}
_fmpz_poly_preinvert(Binv_coeffs, B->coeffs, n);
if (B == B_inv)
{
_fmpz_poly_set_length(temp, n);
fmpz_poly_swap(B_inv, temp);
fmpz_poly_clear(temp);
} else
_fmpz_poly_set_length(B_inv, n);
/* no need to normalise */
}