* Makefile (conftest, conftest2): Link math

library so we can test for math functions.

* arith.c (log2_init): New static function.
(log2, logtwo): New functions.
(l2): New static variable.
(arith_init): Call log2_init.

* configure (lang_flags): Switching _XOPEN_SOURCE
from 500 to 600 to reveal log2.
Adding test for log2.

* eval.c (eval_init): Register log2 intrinsic.

* lib.h (logtwo): Declared.

* txr.1: Documented log2.

1 files changed, 28 insertions, 0 deletions

diff --git a/arith.c b/arith.c
index 76d00d1d..14d3d0e2 100644
--- a/arith.c
+++ b/arith.c
@@ -1444,6 +1444,33 @@ val logten(val num)
   return flo(log10(c_flo(to_float(lit("log"), num))));
 }
 
+#if HAVE_LOG2
+
+static void log2_init(void)
+{
+}
+
+#else
+
+static double l2;
+
+static void log2_init(void)
+{
+  l2 = log(2.0);
+}
+
+double log2(double x)
+{
+  return log(x)/l2;
+}
+
+#endif
+
+val logtwo(val num)
+{
+  return flo(log2(c_flo(to_float(lit("log"), num))));
+}
+
 val expo(val num)
 {
   return flo(exp(c_flo(to_float(lit("exp"), num))));
@@ -2038,4 +2065,5 @@ void arith_init(void)
   mp_set_intptr(&NUM_MAX_MP, NUM_MAX);
   mp_init(&INT_PTR_MAX_MP);
   mp_set_intptr(&INT_PTR_MAX_MP, INT_PTR_MAX);
+  log2_init();
 }