xref: /dflybsd-src/contrib/mpfr/src/scale2.c (revision 2786097444a0124b5d33763854de247e230c6629)

/* mpfr_scale2 -- multiply a double float by 2^exp

Copyright 1999, 2000, 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. */

#include <float.h> /* for DBL_EPSILON */
#include "mpfr-impl.h"

/* Note: we could use the ldexp function, but since we want not to depend on
   math.h, we write our own implementation. */

/* multiplies 1/2 <= d <= 1 by 2^exp */
double
mpfr_scale2 (double d, int exp)
{
#if _GMP_IEEE_FLOATS
  {
    union ieee_double_extract x;

    if (MPFR_UNLIKELY (d == 1.0))
      {
        d = 0.5;
        exp ++;
      }

    /* now 1/2 <= d < 1 */

    /* infinities and zeroes have already been checked */
    MPFR_ASSERTD (-1073 <= exp && exp <= 1025);

    x.d = d;
    if (MPFR_UNLIKELY (exp < -1021)) /* subnormal case */
      {
        x.s.exp += exp + 52;
        x.d *= DBL_EPSILON;
      }
    else /* normalized case */
      {
        x.s.exp += exp;
      }
    return x.d;
  }
#else /* _GMP_IEEE_FLOATS */
  {
    double factor;

    /* An overflow may occurs (example: 0.5*2^1024) */
    if (d < 1.0)
      {
        d += d;
        exp--;
      }
    /* Now 1.0 <= d < 2.0 */

    if (exp < 0)
      {
        factor = 0.5;
        exp = -exp;
      }
    else
      {
        factor = 2.0;
      }
    while (exp != 0)
      {
        if ((exp & 1) != 0)
          d *= factor;
        exp >>= 1;
        factor *= factor;
      }
    return d;
  }
#endif
}