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/* mpfr_tanh -- hyperbolic tangent
Copyright 2001-2021 Free Software Foundation, Inc.
Contributed by the AriC and Caramba projects, INRIA.
This file is part of the GNU MPFR Library.
The GNU MPFR Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at your
option) any later version.
The GNU MPFR Library 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 Lesser General Public
License for more details.
You should have received a copy of the GNU Lesser General Public License
along with the GNU MPFR Library; see the file COPYING.LESSER. If not, see
https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
#define MPFR_NEED_LONGLONG_H
#include "mpfr-impl.h"
int
mpfr_tanh (mpfr_ptr y, mpfr_srcptr xt , mpfr_rnd_t rnd_mode)
{
/****** Declaration ******/
mpfr_t x;
int inexact;
MPFR_SAVE_EXPO_DECL (expo);
MPFR_LOG_FUNC
(("x[%Pu]=%.*Rg rnd=%d", mpfr_get_prec (xt), mpfr_log_prec, xt, rnd_mode),
("y[%Pu]=%.*Rg inexact=%d",
mpfr_get_prec (y), mpfr_log_prec, y, inexact));
/* Special value checking */
if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (xt)))
{
if (MPFR_IS_NAN (xt))
{
MPFR_SET_NAN (y);
MPFR_RET_NAN;
}
else if (MPFR_IS_INF (xt))
{
/* tanh(inf) = 1 && tanh(-inf) = -1 */
return mpfr_set_si (y, MPFR_INT_SIGN (xt), rnd_mode);
}
else /* tanh (0) = 0 and xt is zero */
{
MPFR_ASSERTD (MPFR_IS_ZERO(xt));
MPFR_SET_ZERO (y);
MPFR_SET_SAME_SIGN (y, xt);
MPFR_RET (0);
}
}
/* tanh(x) = x - x^3/3 + ... so the error is < 2^(3*EXP(x)-1) */
MPFR_FAST_COMPUTE_IF_SMALL_INPUT (y, xt, -2 * MPFR_GET_EXP (xt), 1, 0,
rnd_mode, {});
MPFR_TMP_INIT_ABS (x, xt);
MPFR_SAVE_EXPO_MARK (expo);
/* General case */
{
/* Declaration of the intermediary variable */
mpfr_t t, te;
mpfr_exp_t d;
/* Declaration of the size variable */
mpfr_prec_t Ny = MPFR_PREC(y); /* target precision */
mpfr_prec_t Nt; /* working precision */
long int err; /* error */
int sign = MPFR_SIGN (xt);
MPFR_ZIV_DECL (loop);
MPFR_GROUP_DECL (group);
/* First check for BIG overflow of exp(2*x):
For x > 0, exp(2*x) > 2^(2*x)
If 2 ^(2*x) > 2^emax or x>emax/2, there is an overflow */
if (MPFR_UNLIKELY (mpfr_cmp_si (x, __gmpfr_emax/2) >= 0)) {
/* initialize of intermediary variables
since 'set_one' label assumes the variables have been
initialize */
MPFR_GROUP_INIT_2 (group, MPFR_PREC_MIN, t, te);
goto set_one;
}
/* Compute the precision of intermediary variable */
/* The optimal number of bits: see algorithms.tex */
Nt = Ny + MPFR_INT_CEIL_LOG2 (Ny) + 4;
/* if x is small, there will be a cancellation in exp(2x)-1 */
if (MPFR_GET_EXP (x) < 0)
Nt += -MPFR_GET_EXP (x);
/* The error analysis in algorithms.tex assumes that x is exactly
representable with working precision Nt.
FIXME: adapt the error analysis for the case Nt < PREC(x). */
if (Nt < MPFR_PREC(x))
Nt = MPFR_PREC(x);
/* initialize of intermediary variable */
MPFR_GROUP_INIT_2 (group, Nt, t, te);
MPFR_ZIV_INIT (loop, Nt);
for (;;)
{
/* tanh = (exp(2x)-1)/(exp(2x)+1) */
inexact = mpfr_mul_2ui (te, x, 1, MPFR_RNDN); /* 2x */
MPFR_ASSERTD(inexact == 0); /* see FIXME above */
/* since x > 0, we can only have an overflow */
mpfr_exp (te, te, MPFR_RNDN); /* exp(2x) */
if (MPFR_UNLIKELY (MPFR_IS_INF (te)))
{
set_one:
inexact = MPFR_FROM_SIGN_TO_INT (sign);
mpfr_set4 (y, __gmpfr_one, MPFR_RNDN, sign);
if (MPFR_IS_LIKE_RNDZ (rnd_mode, MPFR_IS_NEG_SIGN (sign)))
{
inexact = -inexact;
mpfr_nexttozero (y);
}
break;
}
d = MPFR_GET_EXP (te); /* For Error calculation */
mpfr_add_ui (t, te, 1, MPFR_RNDD); /* exp(2x) + 1 */
mpfr_sub_ui (te, te, 1, MPFR_RNDU); /* exp(2x) - 1 */
d = d - MPFR_GET_EXP (te);
mpfr_div (t, te, t, MPFR_RNDN); /* (exp(2x)-1)/(exp(2x)+1) */
/* Calculation of the error, see algorithms.tex; the current value
of d is k in algorithms.tex. */
d = MAX(3, d + 1); /* d = exponent in 2^(max(3,k+1)) */
err = Nt - (d + 1);
/* The inequality is the condition max(3,k+1) <= floor(p/2). */
if (MPFR_LIKELY (d <= Nt / 2 &&
MPFR_CAN_ROUND (t, err, Ny, rnd_mode)))
{
inexact = mpfr_set4 (y, t, rnd_mode, sign);
break;
}
/* if t=1, we still can round since |sinh(x)| < 1 */
if (MPFR_GET_EXP (t) == 1)
goto set_one;
/* Actualisation of the precision */
MPFR_ZIV_NEXT (loop, Nt);
MPFR_GROUP_REPREC_2 (group, Nt, t, te);
}
MPFR_ZIV_FREE (loop);
MPFR_GROUP_CLEAR (group);
}
MPFR_SAVE_EXPO_FREE (expo);
inexact = mpfr_check_range (y, inexact, rnd_mode);
return inexact;
}
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