contrib/mpfr/uceil_log2.c

   1 /* __gmpfr_ceil_log2 - returns ceil(log(d)/log(2))
   2
   3 Copyright 1999, 2000, 2001, 2002, 2003, 2004, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
   4 Contributed by the Arenaire and Cacao projects, INRIA.
   5
   6 This file is part of the GNU MPFR Library.
   7
   8 The GNU MPFR Library is free software; you can redistribute it and/or modify
   9 it under the terms of the GNU Lesser General Public License as published by
  10 the Free Software Foundation; either version 2.1 of the License, or (at your
  11 option) any later version.
  12
  13 The GNU MPFR Library is distributed in the hope that it will be useful, but
  14 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  15 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  16 License for more details.
  17
  18 You should have received a copy of the GNU Lesser General Public License
  19 along with the GNU MPFR Library; see the file COPYING.LIB.  If not, write to
  20 the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
  21 MA 02110-1301, USA. */
  22
  23 #include "mpfr-impl.h"
  24
  25 /* returns ceil(log(d)/log(2)) if d > 0,
  26    -1023 if d = +0,
  27    and floor(log(-d)/log(2))+1 if d < 0*/
  28 long
  29 __gmpfr_ceil_log2 (double d)
  30 {
  31   long exp;
  32 #if _GMP_IEEE_FLOATS
  33   union ieee_double_extract x;
  34
  35   x.d = d;
  36   exp = x.s.exp - 1023;
  37   x.s.exp = 1023; /* value for 1 <= d < 2 */
  38   if (x.d != 1.0) /* d: not a power of two? */
  39     exp++;
  40   return exp;
  41 #else
  42   double m;
  43
  44   if (d < 0.0)
  45     return __gmpfr_floor_log2(-d)+1;
  46   else if (d == 0.0)
  47     return -1023;
  48   else if (d >= 1.0)
  49     {
  50       exp = 0;
  51       for( m= 1.0 ; m < d ; m *=2.0 )
  52         exp++;
  53     }
  54   else
  55     {
  56       exp = 1;
  57       for( m= 1.0 ; m >= d ; m *= (1.0/2.0) )
  58         exp--;
  59     }
  60 #endif
  61   return exp;
  62 }
  63