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/* mpfr_acosh -- inverse hyperbolic cosine
Copyright 2001-2020 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"
/* The computation of acosh is done by *
* acosh= ln(x + sqrt(x^2-1)) */
int
mpfr_acosh (mpfr_ptr y, mpfr_srcptr x , mpfr_rnd_t rnd_mode)
{
MPFR_SAVE_EXPO_DECL (expo);
int inexact;
int comp;
MPFR_LOG_FUNC (
("x[%Pu]=%.*Rg rnd=%d", mpfr_get_prec (x), mpfr_log_prec, x, rnd_mode),
("y[%Pu]=%.*Rg inexact=%d", mpfr_get_prec (y), mpfr_log_prec, y,
inexact));
/* Deal with special cases */
if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
{
/* Nan, or zero or -Inf */
if (MPFR_IS_INF (x) && MPFR_IS_POS (x))
{
MPFR_SET_INF (y);
MPFR_SET_POS (y);
MPFR_RET (0);
}
else /* Nan, or zero or -Inf */
{
MPFR_SET_NAN (y);
MPFR_RET_NAN;
}
}
comp = mpfr_cmp_ui (x, 1);
if (MPFR_UNLIKELY (comp < 0))
{
MPFR_SET_NAN (y);
MPFR_RET_NAN;
}
else if (MPFR_UNLIKELY (comp == 0))
{
MPFR_SET_ZERO (y); /* acosh(1) = +0 */
MPFR_SET_POS (y);
MPFR_RET (0);
}
MPFR_SAVE_EXPO_MARK (expo);
/* General case */
{
/* Declaration of the intermediary variables */
mpfr_t t;
/* Declaration of the size variables */
mpfr_prec_t Ny = MPFR_PREC(y); /* Precision of output variable */
mpfr_prec_t Nt; /* Precision of the intermediary variable */
mpfr_exp_t err, exp_te, d; /* Precision of error */
MPFR_ZIV_DECL (loop);
/* compute the precision of intermediary variable */
/* the optimal number of bits : see algorithms.tex */
Nt = Ny + 4 + MPFR_INT_CEIL_LOG2 (Ny);
/* initialization of intermediary variables */
mpfr_init2 (t, Nt);
/* First computation of acosh */
MPFR_ZIV_INIT (loop, Nt);
for (;;)
{
MPFR_BLOCK_DECL (flags);
/* compute acosh */
MPFR_BLOCK (flags, mpfr_sqr (t, x, MPFR_RNDD)); /* x^2 */
if (MPFR_OVERFLOW (flags))
{
mpfr_t ln2;
mpfr_prec_t pln2;
/* As x is very large and the precision is not too large, we
assume that we obtain the same result by evaluating ln(2x).
We need to compute ln(x) + ln(2) as 2x can overflow. TODO:
write a proof and add an MPFR_ASSERTN. */
mpfr_log (t, x, MPFR_RNDN); /* err(log) < 1/2 ulp(t) */
pln2 = Nt - MPFR_PREC_MIN < MPFR_GET_EXP (t) ?
MPFR_PREC_MIN : Nt - MPFR_GET_EXP (t);
mpfr_init2 (ln2, pln2);
mpfr_const_log2 (ln2, MPFR_RNDN); /* err(ln2) < 1/2 ulp(t) */
mpfr_add (t, t, ln2, MPFR_RNDN); /* err <= 3/2 ulp(t) */
mpfr_clear (ln2);
err = 1;
}
else
{
exp_te = MPFR_GET_EXP (t);
mpfr_sub_ui (t, t, 1, MPFR_RNDD); /* x^2-1 */
if (MPFR_UNLIKELY (MPFR_IS_ZERO (t)))
{
/* This means that x is very close to 1: x = 1 + t with
t < 2^(-Nt). We have: acosh(x) = sqrt(2t) (1 - eps(t))
with 0 < eps(t) < t / 12. */
mpfr_sub_ui (t, x, 1, MPFR_RNDD); /* t = x - 1 */
mpfr_mul_2ui (t, t, 1, MPFR_RNDN); /* 2t */
mpfr_sqrt (t, t, MPFR_RNDN); /* sqrt(2t) */
err = 1;
}
else
{
d = exp_te - MPFR_GET_EXP (t);
mpfr_sqrt (t, t, MPFR_RNDN); /* sqrt(x^2-1) */
mpfr_add (t, t, x, MPFR_RNDN); /* sqrt(x^2-1)+x */
mpfr_log (t, t, MPFR_RNDN); /* ln(sqrt(x^2-1)+x) */
/* error estimate -- see algorithms.tex */
err = 3 + MAX (1, d) - MPFR_GET_EXP (t);
/* error is bounded by 1/2 + 2^err <= 2^(max(0,1+err)) */
err = MAX (0, 1 + err);
}
}
if (MPFR_LIKELY (MPFR_CAN_ROUND (t, Nt - err, Ny, rnd_mode)))
break;
/* reactualisation of the precision */
MPFR_ZIV_NEXT (loop, Nt);
mpfr_set_prec (t, Nt);
}
MPFR_ZIV_FREE (loop);
inexact = mpfr_set (y, t, rnd_mode);
mpfr_clear (t);
}
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (y, inexact, rnd_mode);
}
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