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/* Base 2 logarithm.
Copyright (C) 2012-2021 Free Software Foundation, Inc.
This file is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 3 of the
License, or (at your option) any later version.
This file 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 Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
#include <config.h>
/* Specification. */
#include <math.h>
#if HAVE_LOG2
float
log2f (float x)
{
return (float) log2 ((double) x);
}
#else
/* Best possible approximation of log(2) as a 'float'. */
#define LOG2 0.693147180559945309417232121458176568075f
/* Best possible approximation of 1/log(2) as a 'float'. */
#define LOG2_INVERSE 1.44269504088896340735992468100189213743f
/* sqrt(0.5). */
#define SQRT_HALF 0.707106781186547524400844362104849039284f
float
log2f (float x)
{
if (isnanf (x))
return x;
if (x <= 0.0f)
{
if (x == 0.0f)
/* Return -Infinity. */
return - HUGE_VALF;
else
{
/* Return NaN. */
#if defined _MSC_VER
static float zero;
return zero / zero;
#else
return 0.0f / 0.0f;
#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;
float y;
y = frexpf (x, &e);
if (y < SQRT_HALF)
{
y = 2.0f * y;
e = e - 1;
}
return (float) e + logf (y) * LOG2_INVERSE;
}
}
#endif
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