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1906 | serge | 1 | /* @(#)s_tanh.c 5.1 93/09/24 */ |
2 | /* |
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3 | * ==================================================== |
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4 | * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. |
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5 | * |
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6 | * Developed at SunPro, a Sun Microsystems, Inc. business. |
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7 | * Permission to use, copy, modify, and distribute this |
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8 | * software is freely granted, provided that this notice |
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9 | * is preserved. |
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10 | * ==================================================== |
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11 | */ |
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12 | |||
13 | |||
14 | /* Tanh(x) |
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15 | * Return the Hyperbolic Tangent of x |
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16 | * |
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17 | * Method : |
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18 | * x -x |
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19 | * e - e |
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20 | * 0. tanh(x) is defined to be ----------- |
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21 | * x -x |
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22 | * e + e |
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23 | * 1. reduce x to non-negative by tanh(-x) = -tanh(x). |
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24 | * 2. 0 <= x <= 2**-55 : tanh(x) := x*(one+x) |
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25 | * -t |
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26 | * 2**-55 < x <= 1 : tanh(x) := -----; t = expm1(-2x) |
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27 | * t + 2 |
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28 | * 2 |
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29 | * 1 <= x <= 22.0 : tanh(x) := 1- ----- ; t=expm1(2x) |
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30 | * t + 2 |
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31 | * 22.0 < x <= INF : tanh(x) := 1. |
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32 | * |
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33 | * Special cases: |
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34 | * tanh(NaN) is NaN; |
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35 | * only tanh(0)=0 is exact for finite argument. |
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36 | */ |
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37 | |||
38 | #include |
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39 | #include "fdlibm.h" |
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40 | |||
41 | #ifdef __STDC__ |
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42 | static const double one=1.0, two=2.0, tiny = 1.0e-300; |
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43 | #else |
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44 | static double one=1.0, two=2.0, tiny = 1.0e-300; |
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45 | #endif |
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46 | |||
47 | double tanh(double x) |
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48 | { |
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49 | double t,z; |
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50 | int jx,ix,lx; |
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51 | |||
52 | /* High word of |x|. */ |
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53 | EXTRACT_WORDS(jx,lx,x); |
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54 | ix = jx&0x7fffffff; |
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55 | |||
56 | /* x is INF or NaN */ |
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57 | if(ix>=0x7ff00000) { |
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58 | if (jx>=0) return one/x+one; /* tanh(+-inf)=+-1 */ |
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59 | else return one/x-one; /* tanh(NaN) = NaN */ |
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60 | } |
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61 | |||
62 | /* |x| < 22 */ |
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63 | if (ix < 0x40360000) { /* |x|<22 */ |
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64 | if ((ix | lx) == 0) |
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65 | return x; /* x == +-0 */ |
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66 | if (ix<0x3c800000) /* |x|<2**-55 */ |
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67 | return x*(one+x); /* tanh(small) = small */ |
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68 | if (ix>=0x3ff00000) { /* |x|>=1 */ |
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69 | t = __expm1(two*fabs(x)); |
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70 | z = one - two/(t+two); |
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71 | } else { |
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72 | t = __expm1(-two*fabs(x)); |
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73 | z= -t/(t+two); |
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74 | } |
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75 | /* |x| > 22, return +-1 */ |
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76 | } else { |
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77 | z = one - tiny; /* raised inexact flag */ |
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78 | } |
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79 | return (jx>=0)? z: -z; |
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80 | }2**-55>0x3c800000)>22>>>=>>=>=>=>>=>=> |