Mercurial > hg > octave-lojdl > gnulib-hg
annotate lib/log2f.c @ 17426:90f3d53e01f5
sig2str: port to C++
* lib/sig2str.h (sig2str, str2sig): Declare as extern "C".
Reported by Daniel J Sebald in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-06/msg00000.html>.
| author | Paul Eggert <eggert@cs.ucla.edu> |
|---|---|
| date | Sun, 02 Jun 2013 11:52:41 -0700 |
| parents | e542fd46ad6f |
| children |
| rev | line source |
|---|---|
| 16678 | 1 /* Base 2 logarithm. |
|
17249
e542fd46ad6f
maint: update all copyright year number ranges
Eric Blake <eblake@redhat.com>
parents:
16678
diff
changeset
|
2 Copyright (C) 2012-2013 Free Software Foundation, Inc. |
| 16678 | 3 |
| 4 This program is free software: you can redistribute it and/or modify | |
| 5 it under the terms of the GNU General Public License as published by | |
| 6 the Free Software Foundation; either version 3 of the License, or | |
| 7 (at your option) any later version. | |
| 8 | |
| 9 This program 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 General Public License for more details. | |
| 13 | |
| 14 You should have received a copy of the GNU General Public License | |
| 15 along with this program. If not, see <http://www.gnu.org/licenses/>. */ | |
| 16 | |
| 17 #include <config.h> | |
| 18 | |
| 19 /* Specification. */ | |
| 20 #include <math.h> | |
| 21 | |
| 22 #if HAVE_LOG2 | |
| 23 | |
| 24 float | |
| 25 log2f (float x) | |
| 26 { | |
| 27 return (float) log2 ((double) x); | |
| 28 } | |
| 29 | |
| 30 #else | |
| 31 | |
| 32 /* Best possible approximation of log(2) as a 'float'. */ | |
| 33 #define LOG2 0.693147180559945309417232121458176568075f | |
| 34 | |
| 35 /* Best possible approximation of 1/log(2) as a 'float'. */ | |
| 36 #define LOG2_INVERSE 1.44269504088896340735992468100189213743f | |
| 37 | |
| 38 /* sqrt(0.5). */ | |
| 39 #define SQRT_HALF 0.707106781186547524400844362104849039284f | |
| 40 | |
| 41 float | |
| 42 log2f (float x) | |
| 43 { | |
| 44 if (isnanf (x)) | |
| 45 return x; | |
| 46 | |
| 47 if (x <= 0.0f) | |
| 48 { | |
| 49 if (x == 0.0f) | |
| 50 /* Return -Infinity. */ | |
| 51 return - HUGE_VALF; | |
| 52 else | |
| 53 { | |
| 54 /* Return NaN. */ | |
| 55 #if defined _MSC_VER | |
| 56 static float zero; | |
| 57 return zero / zero; | |
| 58 #else | |
| 59 return 0.0f / 0.0f; | |
| 60 #endif | |
| 61 } | |
| 62 } | |
| 63 | |
| 64 /* Decompose x into | |
| 65 x = 2^e * y | |
| 66 where | |
| 67 e is an integer, | |
| 68 1/2 < y < 2. | |
| 69 Then log2(x) = e + log2(y) = e + log(y)/log(2). */ | |
| 70 { | |
| 71 int e; | |
| 72 float y; | |
| 73 | |
| 74 y = frexpf (x, &e); | |
| 75 if (y < SQRT_HALF) | |
| 76 { | |
| 77 y = 2.0f * y; | |
| 78 e = e - 1; | |
| 79 } | |
| 80 | |
| 81 return (float) e + logf (y) * LOG2_INVERSE; | |
| 82 } | |
| 83 } | |
| 84 | |
| 85 #endif |