1 files changed, 31 insertions, 0 deletions
diff --git a/lib/libm/atanhf.c b/lib/libm/atanhf.c
new file mode 100644
index 00000000..d1026974
--- /dev/null
+++ b/lib/libm/atanhf.c
@@ -0,0 +1,31 @@
+#include "libm.h"
+
+/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
+float atanhf(float x)
+{
+	union {
+		float f;
+		uint32_t i;
+	} u = { .f = x };
+	unsigned s = u.i >> 31;
+	float_t y;
+
+	/* |x| */
+	u.i &= 0x7fffffff;
+	y = u.f;
+
+	if (u.i < 0x3f800000 - (1 << 23)) {
+		if (u.i < 0x3f800000 - (32 << 23)) {
+			/* handle underflow */
+			if (u.i < (1 << 23))
+				FORCE_EVAL((float)(y * y));
+		} else {
+			/* |x| < 0.5, up to 1.7ulp error */
+			y = 0.5f * log1pf(2 * y + 2 * y * y / (1 - y));
+		}
+	} else {
+		/* avoid overflow */
+		y = 0.5f * log1pf(2 * (y / (1 - y)));
+	}
+	return s ? -y : y;
+}