xref: /dflybsd-src/contrib/mpfr/src/log10.c (revision 2786097444a0124b5d33763854de247e230c6629)

/* mpfr_log10 -- logarithm in base 10.

Copyright 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
Contributed by the AriC and Caramel 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 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);

    /* initialise 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;

        /* actualisation 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);
}