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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) 2008, 2009, 2011 William Hart
Copyright (C) 2010 Sebastian Pancratz
******************************************************************************/
#include <stdlib.h>
#include <gmp.h>
#include "flint.h"
#include "nmod_vec.h"
#include "nmod_poly.h"
void
_nmod_poly_div_divconquer_recursive(mp_ptr Q, mp_ptr W, mp_ptr V,
mp_srcptr A, mp_srcptr B, slong lenB, nmod_t mod)
{
if (lenB <= NMOD_DIV_DIVCONQUER_CUTOFF)
{
_nmod_poly_div_basecase(Q, V, A, 2 * lenB - 1, B, lenB, mod);
}
else
{
const slong n2 = lenB / 2;
const slong n1 = lenB - n2;
mp_ptr W1 = W;
mp_ptr W2 = W + n2;
mp_srcptr p1 = A + 2 * n2;
mp_srcptr p2;
mp_srcptr d1 = B + n2;
mp_srcptr d2 = B;
mp_srcptr d3 = B + n1;
mp_ptr q1 = Q + n2;
mp_ptr q2 = Q;
mp_ptr d1q1 = q2 + n2 - (n1 - 1);
mp_ptr d2q1, t;
/*
Set q1 to p1 div d1, a 2 n1 - 1 by n1 division so q1 ends up
being of length n1; low(d1q1) = d1 q1 is of length n1 - 1
*/
_nmod_poly_divrem_divconquer_recursive(q1, d1q1, W1, V, p1, d1, n1, mod);
/*
Compute bottom n1 + n2 - 1 coeffs of d2q1 = d2 q1
*/
d2q1 = W1;
_nmod_poly_mullow(d2q1, q1, n1, d2, n2, n1 + n2 - 1, mod);
/*
Compute dq1 = d1 q1 x^n2 + d2 q1, of length n1 + n2 - 1
Split it into a segment of length n1 - 1 at which is ignored
and a piece of length n2 at BQ.
*/
if (n2 > n1 - 1)
W1[0] = d2q1[n1 - 1];
_nmod_vec_add(W1 + n2 - (n1 - 1), d1q1, d2q1 + n2, n1 - 1, mod);
/*
Compute t = A/x^n2 - dq1, which has length 2 n1 + n2 - 1, but we
are not interested in the top n1 coeffs as they will be zero, so
this has effective length n1 + n2 - 1
For the following division, we want to set {p2, 2 n2 - 1} to the
top 2 n2 - 1 coeffs of this
Since the bottom n2 - 1 coeffs of p2 are irrelevant for the
division, we in fact set {t, n2} to the relevant coeffs
*/
t = W1;
_nmod_vec_sub(t, A + n2 + (n1 - 1), t, n2, mod);
p2 = t - (n2 - 1);
/*
Compute q2 = t div d3, a 2 n2 - 1 by n2 division, so q2 will have
length n2;
*/
_nmod_poly_div_divconquer_recursive(q2, W2, V, p2, d3, n2, mod);
/*
Note Q = q1 x^n2 + q2
*/
}
}
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