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/*=============================================================================
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) 2012 Fredrik Johansson
******************************************************************************/
#include <gmp.h>
#include "flint.h"
#include "fmpz.h"
#include "fmpz_poly.h"
int
_fmpz_poly_sqrt_classical(fmpz * res, const fmpz * poly, slong len)
{
slong i, m;
int result;
/* the degree must be even */
if (len % 2 == 0)
return 0;
/* valuation must be even, and then can be reduced to 0 */
while (fmpz_is_zero(poly))
{
if (!fmpz_is_zero(poly + 1))
return 0;
fmpz_zero(res);
poly += 2;
len -= 2;
res++;
}
/* check whether a square root exists modulo 2 */
for (i = 1; i < len; i += 2)
if (!fmpz_is_even(poly + i))
return 0;
/* check endpoints */
if (!fmpz_is_square(poly) || (len > 1 && !fmpz_is_square(poly + len - 1)))
return 0;
/* square root of leading coefficient */
m = (len + 1) / 2;
fmpz_sqrt(res + m - 1, poly + len - 1);
result = 1;
/* do slong divison style 'square root with remainder' from top to bottom */
if (len > 1)
{
fmpz_t t, u;
fmpz * r;
fmpz_init(t);
fmpz_init(u);
r = _fmpz_vec_init(len);
_fmpz_vec_set(r, poly, len);
fmpz_mul_ui(u, res + m - 1, 2);
for (i = 1; i < m; i++)
{
fmpz_fdiv_qr(res + m - i - 1, t, r + len - i - 1, u);
if (!fmpz_is_zero(t))
{
result = 0;
break;
}
fmpz_mul_si(t, res + m - i - 1, -2);
_fmpz_vec_scalar_addmul_fmpz(r + len - 2*i, res + m - i, i - 1, t);
fmpz_submul(r + len - 2*i - 1, res + m - i - 1, res + m - i - 1);
}
for (i = m; i < len && result; i++)
if (!fmpz_is_zero(r + len - 1 - i))
result = 0;
_fmpz_vec_clear(r, len);
fmpz_clear(t);
fmpz_clear(u);
}
return result;
}
int
fmpz_poly_sqrt_classical(fmpz_poly_t b, const fmpz_poly_t a)
{
slong blen, len = a->length;
int result;
if (len % 2 == 0)
{
fmpz_poly_zero(b);
return len == 0;
}
if (b == a)
{
fmpz_poly_t tmp;
fmpz_poly_init(tmp);
result = fmpz_poly_sqrt_classical(tmp, a);
fmpz_poly_swap(b, tmp);
fmpz_poly_clear(tmp);
return result;
}
blen = len / 2 + 1;
fmpz_poly_fit_length(b, blen);
_fmpz_poly_set_length(b, blen);
result = _fmpz_poly_sqrt_classical(b->coeffs, a->coeffs, len);
if (!result)
_fmpz_poly_set_length(b, 0);
return result;
}
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