1 files changed, 34 insertions, 0 deletions
diff --git a/winsup/cygwin/math/atanhl.c b/winsup/cygwin/math/atanhl.c
new file mode 100644
index 0000000..59eb1bd
--- /dev/null
+++ b/winsup/cygwin/math/atanhl.c
@@ -0,0 +1,34 @@
+/**
+ * This file has no copyright assigned and is placed in the Public Domain.
+ * This file is part of the mingw-w64 runtime package.
+ * No warranty is given; refer to the file DISCLAIMER.PD within this package.
+ */
+#include <math.h>
+#include <errno.h>
+#include "fastmath.h"
+
+/* atanh (x) = 0.5 * log ((1.0 + x)/(1.0 - x)) */
+long double atanhl (long double x)
+{
+  long double z;
+  if (isnan (x))
+    return x;
+  z = fabsl (x);
+  if (z == 1.0L)
+    {
+      errno  = ERANGE;
+      return (x > 0 ? INFINITY : -INFINITY);
+    }
+  if ( z > 1.0L)
+    {
+      errno = EDOM;
+      return nanl("");
+    }
+  /* Rearrange formula to avoid precision loss for small x.
+  atanh(x) = 0.5 * log ((1.0 + x)/(1.0 - x))
+ 	   = 0.5 * log1p ((1.0 + x)/(1.0 - x) - 1.0)
+           = 0.5 * log1p ((1.0 + x - 1.0 + x) /(1.0 - x)) 
+           = 0.5 * log1p ((2.0 * x ) / (1.0 - x))  */
+  z = 0.5L * __fast_log1pl ((z + z) / (1.0L - z));
+  return x >= 0 ? z : -z;
+}