1 files changed, 27 insertions, 0 deletions
diff --git a/lib/libm/acosh.c b/lib/libm/acosh.c
new file mode 100644
index 00000000..a53e01ed
--- /dev/null
+++ b/lib/libm/acosh.c
@@ -0,0 +1,27 @@
+#include "libm.h"
+
+#if FLT_EVAL_METHOD == 2
+#undef sqrt
+#define sqrt sqrtl
+#endif
+
+/* acosh(x) = log(x + sqrt(x*x-1)) */
+double acosh(double x)
+{
+	union {
+		double f;
+		uint64_t i;
+	} u = { .f = x };
+	unsigned e = u.i >> 52 & 0x7ff;
+
+	/* x < 1 domain error is handled in the called functions */
+
+	if (e < 0x3ff + 1)
+		/* |x| < 2, up to 2ulp error in [1,1.125] */
+		return log1p(x - 1 + sqrt((x - 1) * (x - 1) + 2 * (x - 1)));
+	if (e < 0x3ff + 26)
+		/* |x| < 0x1p26 */
+		return log(2 * x - 1 / (x + sqrt(x * x - 1)));
+	/* |x| >= 0x1p26 or nan */
+	return log(x) + 0.693147180559945309417232121458176568;
+}