1 files changed, 35 insertions, 0 deletions
diff --git a/lib/libm/atanhl.c b/lib/libm/atanhl.c
new file mode 100644
index 00000000..8afd5a03
--- /dev/null
+++ b/lib/libm/atanhl.c
@@ -0,0 +1,35 @@
+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double atanhl(long double x)
+{
+	return atanh(x);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
+long double atanhl(long double x)
+{
+	union ldshape u = { x };
+	unsigned e = u.i.se & 0x7fff;
+	unsigned s = u.i.se >> 15;
+
+	/* |x| */
+	u.i.se = e;
+	x = u.f;
+
+	if (e < 0x3ff - 1) {
+		if (e < 0x3ff - LDBL_MANT_DIG / 2) {
+			/* handle underflow */
+			if (e == 0)
+				FORCE_EVAL((float)x);
+		} else {
+			/* |x| < 0.5, up to 1.7ulp error */
+			x = 0.5 * log1pl(2 * x + 2 * x * x / (1 - x));
+		}
+	} else {
+		/* avoid overflow */
+		x = 0.5 * log1pl(2 * (x / (1 - x)));
+	}
+	return s ? -x : x;
+}
+#endif