s_tanh.cpp (2012B)
1 /* @(#)s_tanh.c 5.1 93/09/24 */ 2 /* 3 * ==================================================== 4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 5 * 6 * Developed at SunPro, a Sun Microsystems, Inc. business. 7 * Permission to use, copy, modify, and distribute this 8 * software is freely granted, provided that this notice 9 * is preserved. 10 * ==================================================== 11 */ 12 13 //#include <sys/cdefs.h> 14 //__FBSDID("$FreeBSD$"); 15 16 /* Tanh(x) 17 * Return the Hyperbolic Tangent of x 18 * 19 * Method : 20 * x -x 21 * e - e 22 * 0. tanh(x) is defined to be ----------- 23 * x -x 24 * e + e 25 * 1. reduce x to non-negative by tanh(-x) = -tanh(x). 26 * 2. 0 <= x < 2**-28 : tanh(x) := x with inexact if x != 0 27 * -t 28 * 2**-28 <= x < 1 : tanh(x) := -----; t = expm1(-2x) 29 * t + 2 30 * 2 31 * 1 <= x < 22 : tanh(x) := 1 - -----; t = expm1(2x) 32 * t + 2 33 * 22 <= x <= INF : tanh(x) := 1. 34 * 35 * Special cases: 36 * tanh(NaN) is NaN; 37 * only tanh(0)=0 is exact for finite argument. 38 */ 39 40 #include <float.h> 41 #include <math.h> 42 43 #include "math_private.h" 44 45 static const volatile double tiny = 1.0e-300; 46 static const double one = 1.0, two = 2.0, huge = 1.0e300; 47 48 double 49 tanh(double x) 50 { 51 double t,z; 52 int32_t jx,ix; 53 54 GET_HIGH_WORD(jx,x); 55 ix = jx&0x7fffffff; 56 57 /* x is INF or NaN */ 58 if(ix>=0x7ff00000) { 59 if (jx>=0) return one/x+one; /* tanh(+-inf)=+-1 */ 60 else return one/x-one; /* tanh(NaN) = NaN */ 61 } 62 63 /* |x| < 22 */ 64 if (ix < 0x40360000) { /* |x|<22 */ 65 if (ix<0x3e300000) { /* |x|<2**-28 */ 66 if(huge+x>one) return x; /* tanh(tiny) = tiny with inexact */ 67 } 68 if (ix>=0x3ff00000) { /* |x|>=1 */ 69 t = expm1(two*fabs(x)); 70 z = one - two/(t+two); 71 } else { 72 t = expm1(-two*fabs(x)); 73 z= -t/(t+two); 74 } 75 /* |x| >= 22, return +-1 */ 76 } else { 77 z = one - tiny; /* raise inexact flag */ 78 } 79 return (jx>=0)? z: -z; 80 }