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/* mpfr_expm1 -- Compute exp(x)-1
Copyright 2001-2016 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
http://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 expm1 is done by
expm1(x)=exp(x)-1
*/
int
mpfr_expm1 (mpfr_ptr y, mpfr_srcptr x , mpfr_rnd_t rnd_mode)
{
int inexact;
mpfr_exp_t ex;
MPFR_SAVE_EXPO_DECL (expo);
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));
if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
{
if (MPFR_IS_NAN (x))
{
MPFR_SET_NAN (y);
MPFR_RET_NAN;
}
/* check for inf or -inf (expm1(-inf)=-1) */
else if (MPFR_IS_INF (x))
{
if (MPFR_IS_POS (x))
{
MPFR_SET_INF (y);
MPFR_SET_POS (y);
MPFR_RET (0);
}
else
return mpfr_set_si (y, -1, rnd_mode);
}
else
{
MPFR_ASSERTD (MPFR_IS_ZERO (x));
MPFR_SET_ZERO (y); /* expm1(+/- 0) = +/- 0 */
MPFR_SET_SAME_SIGN (y, x);
MPFR_RET (0);
}
}
ex = MPFR_GET_EXP (x);
if (ex < 0)
{
/* For -1 < x < 0, abs(expm1(x)-x) < x^2/2.
For 0 < x < 1, abs(expm1(x)-x) < x^2. */
if (MPFR_IS_POS (x))
MPFR_FAST_COMPUTE_IF_SMALL_INPUT (y, x, - ex, 0, 1, rnd_mode, {});
else
MPFR_FAST_COMPUTE_IF_SMALL_INPUT (y, x, - ex, 1, 0, rnd_mode, {});
}
MPFR_SAVE_EXPO_MARK (expo);
if (MPFR_IS_NEG (x) && ex > 5) /* x <= -32 */
{
mpfr_t minus_one, t;
mpfr_exp_t err;
mpfr_init2 (minus_one, 2);
mpfr_init2 (t, 64);
mpfr_set_si (minus_one, -1, MPFR_RNDN);
mpfr_const_log2 (t, MPFR_RNDU); /* round upward since x is negative */
mpfr_div (t, x, t, MPFR_RNDU); /* > x / ln(2) */
err = mpfr_cmp_si (t, MPFR_EMIN_MIN >= -LONG_MAX ?
MPFR_EMIN_MIN : -LONG_MAX) <= 0 ?
- (MPFR_EMIN_MIN >= -LONG_MAX ? MPFR_EMIN_MIN : -LONG_MAX) :
- mpfr_get_si (t, MPFR_RNDU);
/* exp(x) = 2^(x/ln(2))
<= 2^max(MPFR_EMIN_MIN,-LONG_MAX,ceil(x/ln(2)+epsilon))
with epsilon > 0 */
mpfr_clear (t);
MPFR_SMALL_INPUT_AFTER_SAVE_EXPO (y, minus_one, err, 0, 0, rnd_mode,
expo, { mpfr_clear (minus_one); });
mpfr_clear (minus_one);
}
/* General case */
{
/* Declaration of the intermediary variable */
mpfr_t t;
/* Declaration of the size variable */
mpfr_prec_t Ny = MPFR_PREC(y); /* target precision */
mpfr_prec_t Nt; /* working precision */
mpfr_exp_t err, exp_te; /* error */
MPFR_ZIV_DECL (loop);
/* compute the precision of intermediary variable */
/* the optimal number of bits : see algorithms.tex */
Nt = Ny + MPFR_INT_CEIL_LOG2 (Ny) + 6;
/* if |x| is smaller than 2^(-e), we will loose about e bits in the
subtraction exp(x) - 1 */
if (ex < 0)
Nt += - ex;
/* initialize auxiliary variable */
mpfr_init2 (t, Nt);
/* First computation of expm1 */
MPFR_ZIV_INIT (loop, Nt);
for (;;)
{
MPFR_BLOCK_DECL (flags);
/* exp(x) may overflow and underflow */
MPFR_BLOCK (flags, mpfr_exp (t, x, MPFR_RNDN));
if (MPFR_OVERFLOW (flags))
{
inexact = mpfr_overflow (y, rnd_mode, MPFR_SIGN_POS);
MPFR_SAVE_EXPO_UPDATE_FLAGS (expo, MPFR_FLAGS_OVERFLOW);
break;
}
else if (MPFR_UNDERFLOW (flags))
{
inexact = mpfr_set_si (y, -1, rnd_mode);
MPFR_ASSERTD (inexact == 0);
inexact = -1;
if (MPFR_IS_LIKE_RNDZ (rnd_mode, 1))
{
inexact = 1;
mpfr_nexttozero (y);
}
break;
}
exp_te = MPFR_GET_EXP (t); /* FIXME: exp(x) may overflow! */
mpfr_sub_ui (t, t, 1, MPFR_RNDN); /* exp(x)-1 */
/* error estimate */
/*err=Nt-(__gmpfr_ceil_log2(1+pow(2,MPFR_EXP(te)-MPFR_EXP(t))));*/
err = Nt - (MAX (exp_te - MPFR_GET_EXP (t), 0) + 1);
if (MPFR_LIKELY (MPFR_CAN_ROUND (t, err, Ny, rnd_mode)))
{
inexact = mpfr_set (y, t, rnd_mode);
break;
}
/* increase the precision */
MPFR_ZIV_NEXT (loop, Nt);
mpfr_set_prec (t, Nt);
}
MPFR_ZIV_FREE (loop);
mpfr_clear (t);
}
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (y, inexact, rnd_mode);
}
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