1/* __gmpfr_floor_log2 - returns floor(log(d)/log(2))
2
3Copyright 1999-2004, 2006-2023 Free Software Foundation, Inc.
4Contributed by the AriC and Caramba projects, INRIA.
5
6This file is part of the GNU MPFR Library.
7
8The GNU MPFR Library is free software; you can redistribute it and/or modify
9it under the terms of the GNU Lesser General Public License as published by
10the Free Software Foundation; either version 3 of the License, or (at your
11option) any later version.
12
13The GNU MPFR Library is distributed in the hope that it will be useful, but
14WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
15or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
16License for more details.
17
18You should have received a copy of the GNU Lesser General Public License
19along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
20https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
2151 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
22
23#include "mpfr-impl.h"
24
25/* returns floor(log2(d)) */
26long
27__gmpfr_floor_log2 (double d)
28{
29  long exp;
30#if _MPFR_IEEE_FLOATS
31  union mpfr_ieee_double_extract x;
32
33  x.d = d;
34  /* The cast below is useless in theory, but let us not depend on the
35     integer promotion rules (for instance, tcc is currently wrong). */
36  exp = (long) x.s.exp - 1023;
37  MPFR_ASSERTN (exp < 1023);  /* fail on infinities */
38  return exp;
39#else /* _MPFR_IEEE_FLOATS */
40  double m;
41
42  MPFR_ASSERTD (d >= 0);
43  if (d >= 1.0)
44    {
45      exp = -1;
46      for (m = 1.0; m <= d; m *= 2.0)
47        exp++;
48    }
49  else
50    {
51      exp = 0;
52      for (m = 1.0; m > d; m *= 0.5)
53        exp--;
54    }
55  return exp;
56#endif /* _MPFR_IEEE_FLOATS */
57}
58