141 lines
4.4 KiB
C
141 lines
4.4 KiB
C
/*=============================================================================
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This file is part of FLINT.
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FLINT is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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FLINT is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with FLINT; if not, write to the Free Software
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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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=============================================================================*/
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/******************************************************************************
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Copyright (C) 2008, 2009 William Hart
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Copyright (C) 2010, 2011 Sebastian Pancratz
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******************************************************************************/
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#include <stdlib.h>
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#include <gmp.h>
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#include "flint.h"
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#include "fmpz.h"
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#include "fmpz_vec.h"
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#include "fmpz_mod_poly.h"
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#define FMPZ_MOD_POLY_DIVREM_DIVCONQUER_CUTOFF 16
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void
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_fmpz_mod_poly_divrem_divconquer_recursive(fmpz * Q, fmpz * BQ, fmpz * W,
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const fmpz * A, const fmpz * B, slong lenB,
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const fmpz_t invB, const fmpz_t p)
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{
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if (lenB <= FMPZ_MOD_POLY_DIVREM_DIVCONQUER_CUTOFF)
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{
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_fmpz_vec_zero(BQ, lenB - 1);
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_fmpz_vec_set(BQ + (lenB - 1), A + (lenB - 1), lenB);
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_fmpz_mod_poly_divrem_basecase(Q, BQ, BQ, 2 * lenB - 1, B, lenB, invB, p);
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_fmpz_mod_poly_neg(BQ, BQ, lenB - 1, p);
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_fmpz_vec_set(BQ + (lenB - 1), A + (lenB - 1), lenB);
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}
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else
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{
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const slong n2 = lenB / 2;
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const slong n1 = lenB - n2;
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fmpz * W1 = W;
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fmpz * W2 = W + lenB;
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const fmpz * p1 = A + 2 * n2;
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const fmpz * p2;
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const fmpz * d1 = B + n2;
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const fmpz * d2 = B;
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const fmpz * d3 = B + n1;
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const fmpz * d4 = B;
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fmpz * q1 = Q + n2;
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fmpz * q2 = Q;
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fmpz * dq1 = BQ + n2;
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fmpz * d1q1 = BQ + 2 * n2;
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fmpz *d2q1, *d3q2, *d4q2, *t;
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/*
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Set q1 to p1 div d1, a 2 n1 - 1 by n1 division so q1 ends up
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being of length n1; d1q1 = d1 q1 is of length 2 n1 - 1
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*/
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_fmpz_mod_poly_divrem_divconquer_recursive(q1, d1q1, W1,
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p1, d1, n1, invB, p);
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/*
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Compute d2q1 = d2 q1, of length lenB - 1
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*/
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d2q1 = W1;
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_fmpz_mod_poly_mul(d2q1, q1, n1, d2, n2, p);
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/*
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Compute dq1 = d1 q1 x^n2 + d2 q1, of length 2 n1 + n2 - 1
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*/
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_fmpz_vec_swap(dq1, d2q1, n2);
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_fmpz_mod_poly_add(dq1 + n2, dq1 + n2, n1 - 1, d2q1 + n2, n1 - 1, p);
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/*
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Compute t = A/x^n2 - dq1, which has length 2 n1 + n2 - 1, but we
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are not interested in the top n1 coeffs as they will be zero, so
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this has effective length n1 + n2 - 1
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For the following division, we want to set {p2, 2 n2 - 1} to the
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top 2 n2 - 1 coeffs of this
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Since the bottom n2 - 1 coeffs of p2 are irrelevant for the
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division, we in fact set {t, n2} to the relevant coeffs
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*/
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t = BQ;
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_fmpz_mod_poly_sub(t, A + n2 + (n1 - 1), n2, dq1 + (n1 - 1), n2, p);
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p2 = t - (n2 - 1);
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/*
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Compute q2 = t div d3, a 2 n2 - 1 by n2 division, so q2 will have
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length n2; let d3q2 = d3 q2, of length 2 n2 - 1
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*/
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d3q2 = W1;
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_fmpz_mod_poly_divrem_divconquer_recursive(q2, d3q2, W2,
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p2, d3, n2, invB, p);
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/*
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Compute d4q2 = d4 q2, of length n1 + n2 - 1 = lenB - 1
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*/
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d4q2 = W2;
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_fmpz_mod_poly_mul(d4q2, d4, n1, q2, n2, p);
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/*
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Compute dq2 = d3q2 x^n1 + d4q2, of length n1 + 2 n2 - 1
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*/
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_fmpz_vec_swap(BQ, d4q2, n2);
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_fmpz_mod_poly_add(BQ + n2, BQ + n2, n1 - 1, d4q2 + n2, n1 - 1, p);
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_fmpz_mod_poly_add(BQ + n1, BQ + n1, 2 * n2 - 1, d3q2, 2 * n2 - 1, p);
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/*
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Note Q = q1 x^n2 + q2, and BQ = dq1 x^n2 + dq2
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*/
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}
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}
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