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/* mpfr_log10 -- logarithm in base 10.
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 r=log10(a)
r=log10(a)=log(a)/log(10)
*/
int
mpfr_log10 (mpfr_ptr r, mpfr_srcptr a, mpfr_rnd_t rnd_mode)
{
int inexact;
MPFR_SAVE_EXPO_DECL (expo);
MPFR_LOG_FUNC
(("a[%Pu]=%.*Rg rnd=%d", mpfr_get_prec (a), mpfr_log_prec, a, rnd_mode),
("r[%Pu]=%.*Rg inexact=%d",
mpfr_get_prec (r), mpfr_log_prec, r, inexact));
/* If a is NaN, the result is NaN */
if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (a)))
{
if (MPFR_IS_NAN (a))
{
MPFR_SET_NAN (r);
MPFR_RET_NAN;
}
/* check for infinity before zero */
else if (MPFR_IS_INF (a))
{
if (MPFR_IS_NEG (a))
/* log10(-Inf) = NaN */
{
MPFR_SET_NAN (r);
MPFR_RET_NAN;
}
else /* log10(+Inf) = +Inf */
{
MPFR_SET_INF (r);
MPFR_SET_POS (r);
MPFR_RET (0); /* exact */
}
}
else /* a = 0 */
{
MPFR_ASSERTD (MPFR_IS_ZERO (a));
MPFR_SET_INF (r);
MPFR_SET_NEG (r);
MPFR_SET_DIVBY0 ();
MPFR_RET (0); /* log10(0) is an exact -infinity */
}
}
/* If a is negative, the result is NaN */
if (MPFR_UNLIKELY (MPFR_IS_NEG (a)))
{
MPFR_SET_NAN (r);
MPFR_RET_NAN;
}
/* If a is 1, the result is 0 */
if (mpfr_cmp_ui (a, 1) == 0)
{
MPFR_SET_ZERO (r);
MPFR_SET_POS (r);
MPFR_RET (0); /* result is exact */
}
MPFR_SAVE_EXPO_MARK (expo);
/* General case */
{
/* Declaration of the intermediary variable */
mpfr_t t, tt;
MPFR_ZIV_DECL (loop);
/* Declaration of the size variable */
mpfr_prec_t Ny = MPFR_PREC(r); /* Precision of output variable */
mpfr_prec_t Nt; /* Precision of the intermediary variable */
mpfr_exp_t err; /* Precision of error */
/* compute the precision of intermediary variable */
/* the optimal number of bits : see algorithms.tex */
Nt = Ny + 4 + MPFR_INT_CEIL_LOG2 (Ny);
/* initialize of intermediary variables */
mpfr_init2 (t, Nt);
mpfr_init2 (tt, Nt);
/* First computation of log10 */
MPFR_ZIV_INIT (loop, Nt);
for (;;)
{
/* compute log10 */
mpfr_set_ui (t, 10, MPFR_RNDN); /* 10 */
mpfr_log (t, t, MPFR_RNDD); /* log(10) */
mpfr_log (tt, a, MPFR_RNDN); /* log(a) */
mpfr_div (t, tt, t, MPFR_RNDN); /* log(a)/log(10) */
/* estimation of the error */
err = Nt - 4;
if (MPFR_LIKELY (MPFR_CAN_ROUND (t, err, Ny, rnd_mode)))
break;
/* log10(10^n) is exact:
FIXME: Can we have 10^n exactly representable as a mpfr_t
but n can't fit an unsigned long? */
if (MPFR_IS_POS (t)
&& mpfr_integer_p (t) && mpfr_fits_ulong_p (t, MPFR_RNDN)
&& !mpfr_ui_pow_ui (tt, 10, mpfr_get_ui (t, MPFR_RNDN), MPFR_RNDN)
&& mpfr_cmp (a, tt) == 0)
break;
/* actualization of the precision */
MPFR_ZIV_NEXT (loop, Nt);
mpfr_set_prec (t, Nt);
mpfr_set_prec (tt, Nt);
}
MPFR_ZIV_FREE (loop);
inexact = mpfr_set (r, t, rnd_mode);
mpfr_clear (t);
mpfr_clear (tt);
}
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (r, inexact, rnd_mode);
}
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