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+// Copyright 2009 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 were from http://netlib.sandia.gov/cephes/cmath/sin.c,
+// available from http://www.netlib.org/cephes/cmath.tgz.
+// The go code is a simplified version of the original C.
+// tanh.c
+//
+// Hyperbolic tangent
+//
+// SYNOPSIS:
+//
+// double x, y, tanh();
+//
+// y = tanh( x );
+//
+// DESCRIPTION:
+//
+// Returns hyperbolic tangent of argument in the range MINLOG to MAXLOG.
+// MAXLOG = 8.8029691931113054295988e+01 = log(2**127)
+// MINLOG = -8.872283911167299960540e+01 = log(2**-128)
+//
+// A rational function is used for |x| < 0.625. The form
+// x + x**3 P(x)/Q(x) of Cody & Waite is employed.
+// Otherwise,
+// tanh(x) = sinh(x)/cosh(x) = 1 - 2/(exp(2x) + 1).
+//
+// ACCURACY:
+//
+// Relative error:
+// arithmetic domain # trials peak rms
+// IEEE -2,2 30000 2.5e-16 5.8e-17
+//
+// Cephes Math Library Release 2.8: June, 2000
+// Copyright 1984, 1987, 1989, 1992, 2000 by Stephen L. Moshier
+//
+// The readme file at http://netlib.sandia.gov/cephes/ says:
+// Some software in this archive may be from the book _Methods and
+// Programs for Mathematical Functions_ (Prentice-Hall or Simon & Schuster
+// International, 1989) or from the Cephes Mathematical Library, a
+// commercial product. In either event, it is copyrighted by the author.
+// What you see here may be used freely but it comes with no support or
+// guarantee.
+//
+// The two known misprints in the book are repaired here in the
+// source listings for the gamma function and the incomplete beta
+// integral.
+//
+// Stephen L. Moshier
+// moshier@na-net.ornl.gov
+//
+
+var tanhP = [...]float64{
+ -9.64399179425052238628e-1,
+ -9.92877231001918586564e1,
+ -1.61468768441708447952e3,
+}
+var tanhQ = [...]float64{
+ 1.12811678491632931402e2,
+ 2.23548839060100448583e3,
+ 4.84406305325125486048e3,
+}
+
+// Tanh returns the hyperbolic tangent of x.
+//
+// Special cases are:
+//
+// Tanh(±0) = ±0
+// Tanh(±Inf) = ±1
+// Tanh(NaN) = NaN
+func Tanh(x float64) float64 {
+ if haveArchTanh {
+ return archTanh(x)
+ }
+ return tanh(x)
+}
+
+func tanh(x float64) float64 {
+ const MAXLOG = 8.8029691931113054295988e+01 // log(2**127)
+ z := Abs(x)
+ switch {
+ case z > 0.5*MAXLOG:
+ if x < 0 {
+ return -1
+ }
+ return 1
+ case z >= 0.625:
+ s := Exp(2 * z)
+ z = 1 - 2/(s+1)
+ if x < 0 {
+ z = -z
+ }
+ default:
+ if x == 0 {
+ return x
+ }
+ s := x * x
+ z = x + x*s*((tanhP[0]*s+tanhP[1])*s+tanhP[2])/(((s+tanhQ[0])*s+tanhQ[1])*s+tanhQ[2])
+ }
+ return z
+}