128 lines
3.6 KiB
C
128 lines
3.6 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 <stdio.h>
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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 "ulong_extras.h"
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#include "fmpz.h"
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ulong
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z_gcdinv(ulong * inv, slong a, ulong b)
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{
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ulong g, ua = FLINT_ABS(a);
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if (ua >= b)
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ua %= b;
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g = n_gcdinv(inv, ua, b);
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if (a < WORD(0))
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*inv = n_submod(UWORD(0), *inv, b);
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return g;
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}
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int
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fmpz_invmod(fmpz_t f, const fmpz_t g, const fmpz_t h)
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{
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fmpz c1 = *g;
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fmpz c2 = *h;
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int val;
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if (fmpz_is_zero(h))
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{
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flint_printf("Exception (fmpz_invmod). Division by zero.\n");
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abort();
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}
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if (!COEFF_IS_MPZ(c1)) /* g is small */
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{
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if (!COEFF_IS_MPZ(c2)) /* h is also small */
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{
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ulong inv, gcd;
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if (c2 < WORD(0))
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c2 = -c2;
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if (c2 == WORD(1))
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return 0; /* special case not handled by n_invmod */
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gcd = z_gcdinv(&inv, c1, c2);
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return (gcd == UWORD(1) ? fmpz_set_si(f, inv), 1 : 0);
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}
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else /* h is large and g is small */
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{
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__mpz_struct temp; /* put g into a temporary mpz_t */
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__mpz_struct *mpz_ptr;
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if (c1 < WORD(0))
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{
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c1 = -c1;
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temp._mp_d = (mp_limb_t *) & c1;
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temp._mp_size = -1;
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}
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else if (c1 == WORD(0))
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temp._mp_size = 0;
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else
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{
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temp._mp_d = (mp_limb_t *) & c1;
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temp._mp_size = 1;
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}
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mpz_ptr = _fmpz_promote(f);
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val = mpz_invert(mpz_ptr, &temp, COEFF_TO_PTR(c2));
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_fmpz_demote_val(f); /* inverse mod h may result in small value */
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return val;
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}
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}
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else /* g is large */
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{
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if (!COEFF_IS_MPZ(c2)) /* h is small */
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{
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ulong gcd, inv, r;
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if (c2 < WORD(0))
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c2 = -c2;
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if (c2 == WORD(1))
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return 0; /* special case not handled by z_gcd_invert */
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/* reduce g mod h first */
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r = flint_mpz_fdiv_ui(COEFF_TO_PTR(c1), c2);
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gcd = z_gcdinv(&inv, r, c2);
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return (gcd == UWORD(1) ? fmpz_set_si(f, inv), 1 : 0);
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}
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else /* both are large */
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{
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__mpz_struct *mpz_ptr = _fmpz_promote(f);
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val = mpz_invert(mpz_ptr, COEFF_TO_PTR(c1), COEFF_TO_PTR(c2));
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_fmpz_demote_val(f); /* reduction mod h may result in small value */
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return val;
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}
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}
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}
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