New module 'log2l'.

[gnulib.git] / lib / log2l.c

diff --git a/lib/log2l.c b/lib/log2l.c
new file mode 100644 (file)
index 0000000..cc69605
--- /dev/null
+++ b/lib/log2l.c
@@ -0,0 +1,85 @@
+/* Base 2 logarithm.
+   Copyright (C) 2011-2012 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>
+
+#if HAVE_SAME_LONG_DOUBLE_AS_DOUBLE
+
+long double
+log2l (long double x)
+{
+  return log2 (x);
+}
+
+#else
+
+/* Best possible approximation of log(2) as a 'long double'.  */
+#define LOG2 0.693147180559945309417232121458176568075L
+
+/* Best possible approximation of 1/log(2) as a 'long double'.  */
+#define LOG2_INVERSE 1.44269504088896340735992468100189213743L
+
+/* sqrt(0.5).  */
+#define SQRT_HALF 0.707106781186547524400844362104849039284L
+
+long double
+log2l (long double x)
+{
+  if (isnanl (x))
+    return x;
+
+  if (x <= 0.0L)
+    {
+      if (x == 0.0L)
+        /* Return -Infinity.  */
+        return - HUGE_VALL;
+      else
+        {
+          /* Return NaN.  */
+#if defined _MSC_VER || (defined __sgi && !defined __GNUC__)
+          static long double zero;
+          return zero / zero;
+#else
+          return 0.0L / 0.0L;
+#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;
+    long double y;
+
+    y = frexpl (x, &e);
+    if (y < SQRT_HALF)
+      {
+        y = 2.0L * y;
+        e = e - 1;
+      }
+
+    return (long double) e + logl (y) * LOG2_INVERSE;
+  }
+}
+
+#endif