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| 2 | /* @(#)k_tan.c 5.1 93/09/24 */
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| 3 | /*
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| 4 | * ====================================================
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| 5 | * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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| 6 | *
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| 7 | * Developed at SunPro, a Sun Microsystems, Inc. business.
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| 8 | * Permission to use, copy, modify, and distribute this
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| 9 | * software is freely granted, provided that this notice
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| 10 | * is preserved.
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| 11 | * ====================================================
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| 12 | */
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| 13 |
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| 14 | /* __kernel_tan( x, y, k )
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| 15 | * kernel tan function on [-pi/4, pi/4], pi/4 ~ 0.7854
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| 16 | * Input x is assumed to be bounded by ~pi/4 in magnitude.
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| 17 | * Input y is the tail of x.
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| 18 | * Input k indicates whether tan (if k=1) or
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| 19 | * -1/tan (if k= -1) is returned.
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| 20 | *
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| 21 | * Algorithm
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| 22 | * 1. Since tan(-x) = -tan(x), we need only to consider positive x.
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| 23 | * 2. if x < 2^-28 (hx<0x3e300000 0), return x with inexact if x!=0.
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| 24 | * 3. tan(x) is approximated by a odd polynomial of degree 27 on
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| 25 | * [0,0.67434]
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| 26 | * 3 27
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| 27 | * tan(x) ~ x + T1*x + ... + T13*x
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| 28 | * where
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| 29 | *
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| 30 | * |tan(x) 2 4 26 | -59.2
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| 31 | * |----- - (1+T1*x +T2*x +.... +T13*x )| <= 2
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| 32 | * | x |
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| 33 | *
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| 34 | * Note: tan(x+y) = tan(x) + tan'(x)*y
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| 35 | * ~ tan(x) + (1+x*x)*y
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| 36 | * Therefore, for better accuracy in computing tan(x+y), let
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| 37 | * 3 2 2 2 2
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| 38 | * r = x *(T2+x *(T3+x *(...+x *(T12+x *T13))))
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| 39 | * then
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| 40 | * 3 2
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| 41 | * tan(x+y) = x + (T1*x + (x *(r+y)+y))
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| 42 | *
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| 43 | * 4. For x in [0.67434,pi/4], let y = pi/4 - x, then
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| 44 | * tan(x) = tan(pi/4-y) = (1-tan(y))/(1+tan(y))
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| 45 | * = 1 - 2*(tan(y) - (tan(y)^2)/(1+tan(y)))
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| 46 | */
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| 47 |
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| 48 | #include "fdlibm.h"
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| 49 |
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| 50 | #ifndef _DOUBLE_IS_32BITS
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| 51 |
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| 52 | #ifdef __STDC__
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| 53 | static const double
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| 54 | #else
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| 55 | static double
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| 56 | #endif
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| 57 | one = 1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
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| 58 | pio4 = 7.85398163397448278999e-01, /* 0x3FE921FB, 0x54442D18 */
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| 59 | pio4lo= 3.06161699786838301793e-17, /* 0x3C81A626, 0x33145C07 */
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| 60 | T[] = {
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| 61 | 3.33333333333334091986e-01, /* 0x3FD55555, 0x55555563 */
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| 62 | 1.33333333333201242699e-01, /* 0x3FC11111, 0x1110FE7A */
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| 63 | 5.39682539762260521377e-02, /* 0x3FABA1BA, 0x1BB341FE */
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| 64 | 2.18694882948595424599e-02, /* 0x3F9664F4, 0x8406D637 */
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| 65 | 8.86323982359930005737e-03, /* 0x3F8226E3, 0xE96E8493 */
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| 66 | 3.59207910759131235356e-03, /* 0x3F6D6D22, 0xC9560328 */
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| 67 | 1.45620945432529025516e-03, /* 0x3F57DBC8, 0xFEE08315 */
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| 68 | 5.88041240820264096874e-04, /* 0x3F4344D8, 0xF2F26501 */
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| 69 | 2.46463134818469906812e-04, /* 0x3F3026F7, 0x1A8D1068 */
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| 70 | 7.81794442939557092300e-05, /* 0x3F147E88, 0xA03792A6 */
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| 71 | 7.14072491382608190305e-05, /* 0x3F12B80F, 0x32F0A7E9 */
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| 72 | -1.85586374855275456654e-05, /* 0xBEF375CB, 0xDB605373 */
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| 73 | 2.59073051863633712884e-05, /* 0x3EFB2A70, 0x74BF7AD4 */
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| 74 | };
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| 75 |
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| 76 | #ifdef __STDC__
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| 77 | double __kernel_tan(double x, double y, int iy)
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| 78 | #else
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| 79 | double __kernel_tan(x, y, iy)
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| 80 | double x,y; int iy;
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| 81 | #endif
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| 82 | {
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| 83 | double z,r,v,w,s;
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| 84 | int32_t ix,hx;
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| 85 | GET_HIGH_WORD(hx,x);
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| 86 | ix = hx&0x7fffffff; /* high word of |x| */
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| 87 | if(ix<0x3e300000) /* x < 2**-28 */
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| 88 | {if((int)x==0) { /* generate inexact */
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| 89 | uint32_t low;
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| 90 | GET_LOW_WORD(low,x);
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| 91 | if(((ix|low)|(iy+1))==0) return one/fabs(x);
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| 92 | else return (iy==1)? x: -one/x;
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| 93 | }
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| 94 | }
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| 95 | if(ix>=0x3FE59428) { /* |x|>=0.6744 */
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| 96 | if(hx<0) {x = -x; y = -y;}
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| 97 | z = pio4-x;
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| 98 | w = pio4lo-y;
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| 99 | x = z+w; y = 0.0;
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| 100 | }
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| 101 | z = x*x;
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| 102 | w = z*z;
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| 103 | /* Break x^5*(T[1]+x^2*T[2]+...) into
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| 104 | * x^5(T[1]+x^4*T[3]+...+x^20*T[11]) +
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| 105 | * x^5(x^2*(T[2]+x^4*T[4]+...+x^22*[T12]))
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| 106 | */
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| 107 | r = T[1]+w*(T[3]+w*(T[5]+w*(T[7]+w*(T[9]+w*T[11]))));
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| 108 | v = z*(T[2]+w*(T[4]+w*(T[6]+w*(T[8]+w*(T[10]+w*T[12])))));
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| 109 | s = z*x;
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| 110 | r = y + z*(s*(r+v)+y);
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| 111 | r += T[0]*s;
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| 112 | w = x+r;
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| 113 | if(ix>=0x3FE59428) {
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| 114 | v = (double)iy;
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| 115 | return (double)(1-((hx>>30)&2))*(v-2.0*(x-(w*w/(w+v)-r)));
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| 116 | }
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| 117 | if(iy==1) return w;
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| 118 | else { /* if allow error up to 2 ulp,
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| 119 | simply return -1.0/(x+r) here */
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| 120 | /* compute -1.0/(x+r) accurately */
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| 121 | double a,t;
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| 122 | z = w;
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| 123 | SET_LOW_WORD(z,0);
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| 124 | v = r-(z - x); /* z+v = r+x */
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| 125 | t = a = -1.0/w; /* a = -1.0/w */
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| 126 | SET_LOW_WORD(t,0);
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| 127 | s = 1.0+t*z;
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| 128 | return t+a*(s+t*v);
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| 129 | }
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| 130 | }
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| 131 |
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| 132 | #endif /* defined(_DOUBLE_IS_32BITS) */
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