98 lines
3.1 KiB
C
98 lines
3.1 KiB
C
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/*=============================================================================
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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) 2009 William Hart
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******************************************************************************/
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#include <gmp.h>
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#include "flint.h"
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#include "ulong_extras.h"
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#include "fmpz.h"
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void
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fmpz_submul_ui(fmpz_t f, const fmpz_t g, ulong x)
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{
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fmpz c1 = *g, r;
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if ((x == 0) || (c1 == 0))
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return; /* product is zero */
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r = *f;
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if (r == 0)
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{
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fmpz_mul_ui(f, g, x); /* we are subtracting product from 0 */
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fmpz_neg(f, f);
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return;
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}
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if (!COEFF_IS_MPZ(c1)) /* c1 is small */
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{
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mp_limb_t prod[2];
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ulong uc1 = FLINT_ABS(c1);
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__mpz_struct * mpz_ptr;
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mpz_t temp;
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umul_ppmm(prod[1], prod[0], uc1, x); /* compute product */
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if (prod[1] == 0) /* product fits in one limb */
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{
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if (c1 < WORD(0))
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fmpz_add_ui(f, f, prod[0]);
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else
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fmpz_sub_ui(f, f, prod[0]);
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return;
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}
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else if ((prod[1] == 1) && (!COEFF_IS_MPZ(r)) && ((r ^ c1) >= WORD(0)))
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{
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/*
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only chance at cancellation is if product is one bit past
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a limb and f is small and same sign as this product
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*/
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ulong ur = FLINT_ABS(r);
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if (ur > prod[0]) /* cancellation will occur */
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{
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fmpz_set_ui(f, prod[0] - ur);
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if (r > WORD(0))
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fmpz_neg(f, f);
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return;
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}
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}
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/*
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in all remaining cases res is either big already,
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or will be big in the end
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*/
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mpz_ptr = _fmpz_promote_val(f);
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/* set up a temporary, cheap mpz_t to contain prod */
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temp->_mp_d = prod;
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temp->_mp_size = (c1 < WORD(0) ? -2 : 2);
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mpz_sub(mpz_ptr, mpz_ptr, temp);
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_fmpz_demote_val(f); /* cancellation may have occurred */
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}
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else /* c1 is large */
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{
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__mpz_struct *mpz_ptr = _fmpz_promote_val(f);
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flint_mpz_submul_ui(mpz_ptr, COEFF_TO_PTR(c1), x);
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_fmpz_demote_val(f); /* cancellation may have occurred */
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}
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}
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