mpfr-2.4.1/get_ld.c

/* mpfr_get_ld, mpfr_get_ld_2exp -- convert a multiple precision floating-point
                                    number to a machine long double

Copyright 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
Contributed by the Arenaire and Cacao 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 2.1 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.LIB.  If not, write to
the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
MA 02110-1301, USA. */

#include <float.h>

#include "mpfr-impl.h"

#ifndef HAVE_LDOUBLE_IEEE_EXT_LITTLE

long double
mpfr_get_ld (mpfr_srcptr x, mp_rnd_t rnd_mode)
{

  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
    return (long double) mpfr_get_d (x, rnd_mode);
  else /* now x is a normal non-zero number */
    {
      long double r; /* result */
      long double m;
      double s; /* part of result */
      mp_exp_t sh; /* exponent shift, so that x/2^sh is in the double range */
      mpfr_t y, z;
      int sign;

      /* first round x to the target long double precision, so that
         all subsequent operations are exact (this avoids double rounding
         problems) */
      mpfr_init2 (y, MPFR_LDBL_MANT_DIG);
      mpfr_init2 (z, IEEE_DBL_MANT_DIG);

      mpfr_set (y, x, rnd_mode);
      sh = MPFR_GET_EXP (y);
      sign = MPFR_SIGN (y);
      MPFR_SET_EXP (y, 0);
      MPFR_SET_POS (y);

      r = 0.0;
      do {
        s = mpfr_get_d (y, GMP_RNDN); /* high part of y */
        r += (long double) s;
        mpfr_set_d (z, s, GMP_RNDN);  /* exact */
        mpfr_sub (y, y, z, GMP_RNDN); /* exact */
      } while (!MPFR_IS_ZERO (y));

      mpfr_clear (z);
      mpfr_clear (y);

      /* we now have to multiply back by 2^sh */
      MPFR_ASSERTD (r > 0);
      if (sh != 0)
        {
          /* An overflow may occurs (example: 0.5*2^1024) */
          while (r < 1.0)
            {
              r += r;
              sh--;
            }

          if (sh > 0)
            m = 2.0;
          else
            {
              m = 0.5;
              sh = -sh;
            }

          for (;;)
            {
              if (sh % 2)
                r = r * m;
              sh >>= 1;
              if (sh == 0)
                break;
              m = m * m;
            }
        }
      if (sign < 0)
        r = -r;
      return r;
    }
}

#else

long double
mpfr_get_ld (mpfr_srcptr x, mp_rnd_t rnd_mode)
{
  mpfr_long_double_t ld;
  mpfr_t tmp;
  MPFR_SAVE_EXPO_DECL (expo);

  MPFR_SAVE_EXPO_MARK (expo);
  mpfr_set_emin (-16382-63);
  mpfr_set_emax (16383);

  mpfr_init2 (tmp, MPFR_LDBL_MANT_DIG);
  mpfr_subnormalize(tmp, mpfr_set (tmp, x, rnd_mode), rnd_mode);
  mpfr_prec_round (tmp, 64, GMP_RNDZ); /* exact */
  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (tmp)))
    ld.ld = (long double) mpfr_get_d (tmp, rnd_mode);
  else
    {
      mp_limb_t *tmpmant;
      mp_exp_t e, denorm;

      tmpmant = MPFR_MANT (tmp);
      e = MPFR_GET_EXP (tmp);
      denorm = MPFR_UNLIKELY (e < -16382) ? - e - 16382 + 1 : 0;
#if BITS_PER_MP_LIMB >= 64
      ld.s.manl = (tmpmant[0] >> denorm);
      ld.s.manh = (tmpmant[0] >> denorm) >> 32;
#elif BITS_PER_MP_LIMB == 32
      if (MPFR_LIKELY (denorm == 0))
        {
          ld.s.manl = tmpmant[0];
          ld.s.manh = tmpmant[1];
        }
      else if (denorm < 32)
        {
          ld.s.manl = (tmpmant[0] >> denorm) | (tmpmant[1] << (32 - denorm));
          ld.s.manh = tmpmant[1] >> denorm;
        }
      else /* 32 <= denorm <= 64 */
        {
          ld.s.manl = tmpmant[1] >> (denorm - 32);
          ld.s.manh = 0;
        }
#else
# error "BITS_PER_MP_LIMB must be 32 or >= 64"
      /* Other values have never been supported anyway. */
#endif
      if (MPFR_LIKELY (denorm == 0))
        {
          ld.s.exph = (e + 0x3FFE) >> 8;
          ld.s.expl = (e + 0x3FFE);
        }
      else
        ld.s.exph = ld.s.expl = 0;
      ld.s.sign = MPFR_IS_NEG (x);
    }

  mpfr_clear (tmp);
  MPFR_SAVE_EXPO_FREE (expo);
  return ld.ld;
}

#endif

/* contributed by Damien Stehle */
long double
mpfr_get_ld_2exp (long *expptr, mpfr_srcptr src, mp_rnd_t rnd_mode)
{
  long double ret;
  mp_exp_t exp;
  mpfr_t tmp;

  if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (src)))
    return (long double) mpfr_get_d_2exp (expptr, src, rnd_mode);

  tmp[0] = *src;        /* Hack copy mpfr_t */
  MPFR_SET_EXP (tmp, 0);
  ret = mpfr_get_ld (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;
}