mirror of
https://github.com/vale981/arb
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78 lines
2.1 KiB
C
78 lines
2.1 KiB
C
/*=============================================================================
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This file is part of ARB.
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ARB 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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ARB 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 ARB; 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) 2016 Fredrik Johansson
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******************************************************************************/
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#include "mag.h"
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void
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mag_expinv(mag_t res, const mag_t x)
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{
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if (mag_is_zero(x))
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{
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mag_one(res);
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}
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else if (mag_is_inf(x))
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{
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mag_zero(res);
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}
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else if (fmpz_sgn(MAG_EXPREF(x)) <= 0)
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{
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mag_one(res);
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}
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else if (fmpz_cmp_ui(MAG_EXPREF(x), 2 * MAG_BITS) > 0)
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{
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fmpz_t t;
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fmpz_init(t);
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/* If x > 2^60, exp(-x) < 2^(-2^60 / log(2)) */
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/* -1/log(2) < -369/256 */
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fmpz_set_si(t, -369);
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fmpz_mul_2exp(t, t, 2 * MAG_BITS - 8);
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mag_one(res);
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mag_mul_2exp_fmpz(res, res, t);
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fmpz_clear(t);
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}
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else
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{
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fmpz_t t;
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slong e = MAG_EXP(x);
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fmpz_init(t);
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fmpz_set_ui(t, MAG_MAN(x));
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if (e >= MAG_BITS)
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fmpz_mul_2exp(t, t, e - MAG_BITS);
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else
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fmpz_tdiv_q_2exp(t, t, MAG_BITS - e);
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/* upper bound for 1/e */
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mag_set_ui_2exp_si(res, 395007543, -30);
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mag_pow_fmpz(res, res, t);
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fmpz_clear(t);
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}
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}
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