Fix catan, catanh inaccuracy through use of log (bug 15394).

1 files changed, 8 insertions, 1 deletions

diff --git a/math/s_catanh.c b/math/s_catanh.c
index 5371f44b67..0ee8c64b6f 100644
--- a/math/s_catanh.c
+++ b/math/s_catanh.c
@@ -64,7 +64,14 @@ __catanh (__complex__ double x)
       double den = 1.0 - __real__ x;
       den = i2 + den * den;
 
-      __real__ res = 0.25 * (__ieee754_log (num) - __ieee754_log (den));
+      double f = num / den;
+      if (f < 0.5)
+	__real__ res = 0.25 * __ieee754_log (f);
+      else
+	{
+	  num = 4.0 * __real__ x;
+	  __real__ res = 0.25 * __log1p (num / den);
+	}
 
       den = 1 - __real__ x * __real__ x - i2;