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/* Base 2 logarithm.
Copyright (C) 2012-2017 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#include <config.h>
/* Specification. */
#include <math.h>
/* Best possible approximation of log(2) as a 'double'. */
#define LOG2 0.693147180559945309417232121458176568075
/* Best possible approximation of 1/log(2) as a 'double'. */
#define LOG2_INVERSE 1.44269504088896340735992468100189213743
/* sqrt(0.5). */
#define SQRT_HALF 0.707106781186547524400844362104849039284
double
log2 (double x)
{
if (isnand (x))
return x;
if (x <= 0.0)
{
if (x == 0.0)
/* Return -Infinity. */
return - HUGE_VAL;
else
{
/* Return NaN. */
#if defined _MSC_VER || (defined __sgi && !defined __GNUC__)
static double zero;
return zero / zero;
#else
return 0.0 / 0.0;
#endif
}
}
/* Decompose x into
x = 2^e * y
where
e is an integer,
1/2 < y < 2.
Then log2(x) = e + log2(y) = e + log(y)/log(2). */
{
int e;
double y;
y = frexp (x, &e);
if (y < SQRT_HALF)
{
y = 2.0 * y;
e = e - 1;
}
return (double) e + log (y) * LOG2_INVERSE;
}
}
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