1 files changed, 29 insertions, 0 deletions
diff --git a/lib/libm/acoshf.c b/lib/libm/acoshf.c
new file mode 100644
index 00000000..515ca313
--- /dev/null
+++ b/lib/libm/acoshf.c
@@ -0,0 +1,29 @@
+#include "libm.h"
+
+#if FLT_EVAL_METHOD == 2
+#undef sqrtf
+#define sqrtf sqrtl
+#elif FLT_EVAL_METHOD == 1
+#undef sqrtf
+#define sqrtf sqrt
+#endif
+
+/* acosh(x) = log(x + sqrt(x*x-1)) */
+float acoshf(float x)
+{
+	union {
+		float f;
+		uint32_t i;
+	} u = { x };
+	uint32_t a = u.i & 0x7fffffff;
+
+	if (a < 0x3f800000 + (1 << 23))
+		/* |x| < 2, invalid if x < 1 */
+		/* up to 2ulp error in [1,1.125] */
+		return log1pf(x - 1 + sqrtf((x - 1) * (x - 1) + 2 * (x - 1)));
+	if (u.i < 0x3f800000 + (12 << 23))
+		/* 2 <= x < 0x1p12 */
+		return logf(2 * x - 1 / (x + sqrtf(x * x - 1)));
+	/* x >= 0x1p12 or x <= -2 or nan */
+	return logf(x) + 0.693147180559945309417232121458176568f;
+}