1 files changed, 32 insertions, 0 deletions
diff --git a/lib/libm/atanh.c b/lib/libm/atanh.c
new file mode 100644
index 00000000..08a34d7e
--- /dev/null
+++ b/lib/libm/atanh.c
@@ -0,0 +1,32 @@
+#include "libm.h"
+
+/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
+double atanh(double x)
+{
+	union {
+		double f;
+		uint64_t i;
+	} u = { .f = x };
+	unsigned e = u.i >> 52 & 0x7ff;
+	unsigned s = u.i >> 63;
+	double_t y;
+
+	/* |x| */
+	u.i &= (uint64_t)-1 / 2;
+	y = u.f;
+
+	if (e < 0x3ff - 1) {
+		if (e < 0x3ff - 32) {
+			/* handle underflow */
+			if (e == 0)
+				FORCE_EVAL((float)y);
+		} else {
+			/* |x| < 0.5, up to 1.7ulp error */
+			y = 0.5 * log1p(2 * y + 2 * y * y / (1 - y));
+		}
+	} else {
+		/* avoid overflow */
+		y = 0.5 * log1p(2 * (y / (1 - y)));
+	}
+	return s ? -y : y;
+}