1/* __gmpfr_ceil_log2 - returns ceil(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 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*/
29long
30__gmpfr_ceil_log2 (double d)
31{
32  long exp;
33#if _MPFR_IEEE_FLOATS
34  union mpfr_ieee_double_extract x;
35
36  x.d = d;
37  /* The cast below is useless in theory, but let us not depend on the
38     integer promotion rules (for instance, tcc is currently wrong). */
39  exp = (long) x.s.exp - 1023;
40  MPFR_ASSERTN (exp < 1023);  /* fail on infinities */
41  x.s.exp = 1023; /* value for 1 <= d < 2 */
42  if (x.d != 1.0) /* d: not a power of two? */
43    exp++;
44  return exp;
45#else /* _MPFR_IEEE_FLOATS */
46  double m;
47
48  if (d < 0.0)
49    return __gmpfr_floor_log2 (-d) + 1;
50  else if (d == 0.0)
51    return -1023;
52  else if (d >= 1.0)
53    {
54      exp = 0;
55      for (m = 1.0; m < d; m *= 2.0)
56        exp++;
57    }
58  else
59    {
60      exp = 1;
61      for (m = 1.0; m >= d; m *= 0.5)
62        exp--;
63    }
64#endif /* _MPFR_IEEE_FLOATS */
65  return exp;
66}
67
68