1 files changed, 55 insertions, 0 deletions
diff --git a/src/math/acosh.c b/src/math/acosh.c
new file mode 100644
index 00000000..a7c87e3c
--- /dev/null
+++ b/src/math/acosh.c
@@ -0,0 +1,55 @@
+/* origin: FreeBSD /usr/src/lib/msun/src/e_acosh.c */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunSoft, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ *
+ */
+/* acosh(x)
+ * Method :
+ *      Based on
+ *              acosh(x) = log [ x + sqrt(x*x-1) ]
+ *      we have
+ *              acosh(x) := log(x)+ln2, if x is large; else
+ *              acosh(x) := log(2x-1/(sqrt(x*x-1)+x)) if x>2; else
+ *              acosh(x) := log1p(t+sqrt(2.0*t+t*t)); where t=x-1.
+ *
+ * Special cases:
+ *      acosh(x) is NaN with signal if x<1.
+ *      acosh(NaN) is NaN without signal.
+ */
+
+#include "libm.h"
+
+static const double
+one = 1.0,
+ln2 = 6.93147180559945286227e-01; /* 0x3FE62E42, 0xFEFA39EF */
+
+double acosh(double x)
+{
+	double t;
+	int32_t hx;
+	uint32_t lx;
+
+	EXTRACT_WORDS(hx, lx, x);
+	if (hx < 0x3ff00000) {  /* x < 1 */
+		return (x-x)/(x-x);
+	} else if (hx >= 0x41b00000) {  /* x > 2**28 */
+		if (hx >= 0x7ff00000)  /* x is inf of NaN */
+			return x+x;
+		return log(x) + ln2;   /* acosh(huge) = log(2x) */
+	} else if ((hx-0x3ff00000 | lx) == 0) {
+		return 0.0;            /* acosh(1) = 0 */
+	} else if (hx > 0x40000000) {  /* 2**28 > x > 2 */
+		t = x*x;
+		return log(2.0*x - one/(x+sqrt(t-one)));
+	} else {                /* 1 < x < 2 */
+		t = x-one;
+		return log1p(t + sqrt(2.0*t+t*t));
+	}
+}