opendirat: don’t depend on openat-safer
[gnulib.git] / lib / log2.c
blob710ac456a0b58f1d04c1ad405b72e0f0807ac05a
1 /* Base 2 logarithm.
2 Copyright (C) 2012-2024 Free Software Foundation, Inc.
4 This file is free software: you can redistribute it and/or modify
5 it under the terms of the GNU Lesser General Public License as
6 published by the Free Software Foundation, either version 3 of the
7 License, or (at your option) any later version.
9 This file is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 GNU Lesser General Public License for more details.
14 You should have received a copy of the GNU Lesser General Public License
15 along with this program. If not, see <https://www.gnu.org/licenses/>. */
17 #include <config.h>
19 /* Specification. */
20 #include <math.h>
22 /* Best possible approximation of log(2) as a 'double'. */
23 #define LOG2 0.693147180559945309417232121458176568075
25 /* Best possible approximation of 1/log(2) as a 'double'. */
26 #define LOG2_INVERSE 1.44269504088896340735992468100189213743
28 /* sqrt(0.5). */
29 #define SQRT_HALF 0.707106781186547524400844362104849039284
31 double
32 log2 (double x)
34 if (isnand (x))
35 return x;
37 if (x <= 0.0)
39 if (x == 0.0)
40 /* Return -Infinity. */
41 return - HUGE_VAL;
42 else
44 /* Return NaN. */
45 #if defined _MSC_VER || (defined __sgi && !defined __GNUC__)
46 static double zero;
47 return zero / zero;
48 #else
49 return 0.0 / 0.0;
50 #endif
54 /* Decompose x into
55 x = 2^e * y
56 where
57 e is an integer,
58 1/2 < y < 2.
59 Then log2(x) = e + log2(y) = e + log(y)/log(2). */
61 int e;
62 double y;
64 y = frexp (x, &e);
65 if (y < SQRT_HALF)
67 y = 2.0 * y;
68 e = e - 1;
71 return (double) e + log (y) * LOG2_INVERSE;