niceSOFT/mpfr | Gitverse

src/get_d.c
182 строки5 KB
Vincent Lefèvre
03 янв 2018, 13:53
03 янв 2018, 13:532fce388
/* mpfr_get_d, mpfr_get_d_2exp -- convert a multiple precision floating-point
                                  number to a machine double precision float

Copyright 1999-2018 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. */

#include <float.h>

#define MPFR_NEED_LONGLONG_H
#include "mpfr-impl.h"

#include "ieee_floats.h"

/* Assumes IEEE-754 double precision; otherwise, only an approximated
   result will be returned, without any guaranty (and special cases
   such as NaN must be avoided if not supported). */

double
mpfr_get_d (mpfr_srcptr src, mpfr_rnd_t rnd_mode)
{
  double d;
  int negative;
  mpfr_exp_t e;

  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (src)))
    {
      if (MPFR_IS_NAN (src))
        return MPFR_DBL_NAN;

      negative = MPFR_IS_NEG (src);

      if (MPFR_IS_INF (src))
        return negative ? MPFR_DBL_INFM : MPFR_DBL_INFP;

      MPFR_ASSERTD (MPFR_IS_ZERO(src));
      return negative ? DBL_NEG_ZERO : 0.0;
    }

  e = MPFR_GET_EXP (src);
  negative = MPFR_IS_NEG (src);

  if (MPFR_UNLIKELY(rnd_mode == MPFR_RNDA))
    rnd_mode = negative ? MPFR_RNDD : MPFR_RNDU;

  /* the smallest normalized number is 2^(-1022)=0.1e-1021, and the smallest
     subnormal is 2^(-1074)=0.1e-1073 */
  if (MPFR_UNLIKELY (e < -1073))
    {
      /* Note: Avoid using a constant expression DBL_MIN * DBL_EPSILON
         as this gives 0 instead of the correct result with gcc on some
         Alpha machines. */
      d = negative ?
        (rnd_mode == MPFR_RNDD ||
         (rnd_mode == MPFR_RNDN && mpfr_cmp_si_2exp(src, -1, -1075) < 0)
         ? -DBL_MIN : DBL_NEG_ZERO) :
        (rnd_mode == MPFR_RNDU ||
         (rnd_mode == MPFR_RNDN && mpfr_cmp_si_2exp(src, 1, -1075) > 0)
         ? DBL_MIN : 0.0);
      if (d != 0.0) /* we multiply DBL_MIN = 2^(-1022) by DBL_EPSILON = 2^(-52)
                       to get +-2^(-1074) */
        d *= DBL_EPSILON;
    }
  /* the largest normalized number is 2^1024*(1-2^(-53))=0.111...111e1024 */
  else if (MPFR_UNLIKELY (e > 1024))
    {
      d = negative ?
        (rnd_mode == MPFR_RNDZ || rnd_mode == MPFR_RNDU ?
         -DBL_MAX : MPFR_DBL_INFM) :
        (rnd_mode == MPFR_RNDZ || rnd_mode == MPFR_RNDD ?
         DBL_MAX : MPFR_DBL_INFP);
    }
  else
    {
      int nbits;
      mp_size_t np, i;
      mp_limb_t tp[ MPFR_LIMBS_PER_DOUBLE ];
      int carry;

      nbits = IEEE_DBL_MANT_DIG; /* 53 */
      if (MPFR_UNLIKELY (e < -1021))
        /*In the subnormal case, compute the exact number of significant bits*/
        {
          nbits += (1021 + e);
          MPFR_ASSERTD (nbits >= 1);
        }
      np = MPFR_PREC2LIMBS (nbits);
      MPFR_ASSERTD ( np <= MPFR_LIMBS_PER_DOUBLE );
      carry = mpfr_round_raw_4 (tp, MPFR_MANT(src), MPFR_PREC(src), negative,
                                nbits, rnd_mode);
      if (MPFR_UNLIKELY(carry))
        d = 1.0;
      else
        {
          /* The following computations are exact thanks to the previous
             mpfr_round_raw. */
          d = (double) tp[0] / MP_BASE_AS_DOUBLE;
          for (i = 1 ; i < np ; i++)
            d = (d + tp[i]) / MP_BASE_AS_DOUBLE;
          /* d is the mantissa (between 1/2 and 1) of the argument rounded
             to 53 bits */
        }
      d = mpfr_scale2 (d, e);
      if (negative)
        d = -d;
    }

  return d;
}

#undef mpfr_get_d1
double
mpfr_get_d1 (mpfr_srcptr src)
{
  return mpfr_get_d (src, __gmpfr_default_rounding_mode);
}

double
mpfr_get_d_2exp (long *expptr, mpfr_srcptr src, mpfr_rnd_t rnd_mode)
{
  double ret;
  mpfr_exp_t exp;
  mpfr_t tmp;

  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (src)))
    {
      int negative;
      *expptr = 0;
      if (MPFR_IS_NAN (src))
        return MPFR_DBL_NAN;
      negative = MPFR_IS_NEG (src);
      if (MPFR_IS_INF (src))
        return negative ? MPFR_DBL_INFM : MPFR_DBL_INFP;
      MPFR_ASSERTD (MPFR_IS_ZERO(src));
      return negative ? DBL_NEG_ZERO : 0.0;
    }

  MPFR_ALIAS (tmp, src, MPFR_SIGN (src), 0);
  ret = mpfr_get_d (tmp, rnd_mode);

  if (MPFR_IS_PURE_FP(src))
    {
      exp = MPFR_GET_EXP (src);

      /* rounding can give 1.0, adjust back to 0.5 <= abs(ret) < 1.0 */
      if (ret == 1.0)
        {
          ret = 0.5;
          exp++;
        }
      else if (ret == -1.0)
        {
          ret = -0.5;
          exp++;
        }

      MPFR_ASSERTN ((ret >= 0.5 && ret < 1.0)
                    || (ret <= -0.5 && ret > -1.0));
      MPFR_ASSERTN (exp >= LONG_MIN && exp <= LONG_MAX);
    }
  else
    exp = 0;

  *expptr = exp;
  return ret;
}