Copyright 2010 The Go Authors. All rights reserved. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.

package math
The original C code, the long comment, and the constants below are from FreeBSD's /usr/src/lib/msun/src/e_acosh.c and came with this notice. The go code is a simplified version of the original C. ==================================================== Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. Developed at SunPro, a Sun Microsystems, Inc. business. Permission to use, copy, modify, and distribute this software is freely granted, provided that this notice is preserved. ==================================================== __ieee754_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.
Acosh returns the inverse hyperbolic cosine of x. Special cases are: Acosh(+Inf) = +Inf Acosh(x) = NaN if x < 1 Acosh(NaN) = NaN
func ( float64) float64

func ( float64) float64 {
first case is special case
	switch {
	case  < 1 || IsNaN():
		return NaN()
	case  == 1:
		return 0
	case  >= :
		return Log() + Ln2 // x > 2**28
	case  > 2:
		return Log(2* - 1/(+Sqrt(*-1))) // 2**28 > x > 2
	}
	 :=  - 1
	return Log1p( + Sqrt(2*+*)) // 2 >= x > 1