Imported Upstream version 4.8.1

[platform/upstream/gcc48.git] / libgo / go / math / tanh.go

diff --git a/libgo/go/math/tanh.go b/libgo/go/math/tanh.go
index 03a641b..7305be6 100644 (file)
--- a/libgo/go/math/tanh.go
+++ b/libgo/go/math/tanh.go
@@ -4,12 +4,66 @@
 
 package math
 
-/*
-       Floating-point hyperbolic tangent.
+// 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
+//
 
-       Sinh and Cosh are called except for large arguments, which
-       would cause overflow improperly.
-*/
+var tanhP = [...]float64{
+       -9.64399179425052238628E-1,
+       -9.92877231001918586564E1,
+       -1.61468768441708447952E3,
+}
+var tanhQ = [...]float64{
+       1.12811678491632931402E2,
+       2.23548839060100448583E3,
+       4.84406305325125486048E3,
+}
 
 // Tanh computes the hyperbolic tangent of x.
 //
@@ -18,15 +72,26 @@ package math
 //     Tanh(±Inf) = ±1
 //     Tanh(NaN) = NaN
 func Tanh(x float64) float64 {
-       if x < 0 {
-               x = -x
-               if x > 21 {
+       const MAXLOG = 8.8029691931113054295988e+01 // log(2**127)
+       z := Abs(x)
+       switch {
+       case z > 0.5*MAXLOG:
+               if x < 0 {
                        return -1
                }
-               return -Sinh(x) / Cosh(x)
-       }
-       if x > 21 {
                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 Sinh(x) / Cosh(x)
+       return z
 }