1 /* @(#)k_tan.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 #if defined(LIBM_SCCS) && !defined(lint) 14 static char rcsid[] = "$NetBSD: k_tan.c,v 1.8 1995/05/10 20:46:37 jtc Exp $"; 15 #endif 16 17 /* __kernel_tan( x, y, k ) 18 * kernel tan function on [-pi/4, pi/4], pi/4 ~ 0.7854 19 * Input x is assumed to be bounded by ~pi/4 in magnitude. 20 * Input y is the tail of x. 21 * Input k indicates whether tan (if k=1) or 22 * -1/tan (if k= -1) is returned. 23 * 24 * Algorithm 25 * 1. Since tan(-x) = -tan(x), we need only to consider positive x. 26 * 2. if x < 2^-28 (hx<0x3e300000 0), return x with inexact if x!=0. 27 * 3. tan(x) is approximated by a odd polynomial of degree 27 on 28 * [0,0.67434] 29 * 3 27 30 * tan(x) ~ x + T1*x + ... + T13*x 31 * where 32 * 33 * |tan(x) 2 4 26 | -59.2 34 * |----- - (1+T1*x +T2*x +.... +T13*x )| <= 2 35 * | x | 36 * 37 * Note: tan(x+y) = tan(x) + tan'(x)*y 38 * ~ tan(x) + (1+x*x)*y 39 * Therefore, for better accuracy in computing tan(x+y), let 40 * 3 2 2 2 2 41 * r = x *(T2+x *(T3+x *(...+x *(T12+x *T13)))) 42 * then 43 * 3 2 44 * tan(x+y) = x + (T1*x + (x *(r+y)+y)) 45 * 46 * 4. For x in [0.67434,pi/4], let y = pi/4 - x, then 47 * tan(x) = tan(pi/4-y) = (1-tan(y))/(1+tan(y)) 48 * = 1 - 2*(tan(y) - (tan(y)^2)/(1+tan(y))) 49 */ 50 51 #include "math.h" 52 #include "math_private.h" 53 54 static const double xxx[] = { 55 3.33333333333334091986e-01, /* 3FD55555, 55555563 */ 56 1.33333333333201242699e-01, /* 3FC11111, 1110FE7A */ 57 5.39682539762260521377e-02, /* 3FABA1BA, 1BB341FE */ 58 2.18694882948595424599e-02, /* 3F9664F4, 8406D637 */ 59 8.86323982359930005737e-03, /* 3F8226E3, E96E8493 */ 60 3.59207910759131235356e-03, /* 3F6D6D22, C9560328 */ 61 1.45620945432529025516e-03, /* 3F57DBC8, FEE08315 */ 62 5.88041240820264096874e-04, /* 3F4344D8, F2F26501 */ 63 2.46463134818469906812e-04, /* 3F3026F7, 1A8D1068 */ 64 7.81794442939557092300e-05, /* 3F147E88, A03792A6 */ 65 7.14072491382608190305e-05, /* 3F12B80F, 32F0A7E9 */ 66 -1.85586374855275456654e-05, /* BEF375CB, DB605373 */ 67 2.59073051863633712884e-05, /* 3EFB2A70, 74BF7AD4 */ 68 /* one */ 1.00000000000000000000e+00, /* 3FF00000, 00000000 */ 69 /* pio4 */ 7.85398163397448278999e-01, /* 3FE921FB, 54442D18 */ 70 /* pio4lo */ 3.06161699786838301793e-17 /* 3C81A626, 33145C07 */ 71 }; 72 #define one xxx[13] 73 #define pio4 xxx[14] 74 #define pio4lo xxx[15] 75 #define T xxx 76 77 double 78 __kernel_tan(double x, double y, int iy) 79 { 80 double z, r, v, w, s; 81 int32_t ix, hx; 82 83 GET_HIGH_WORD(hx, x); /* high word of x */ 84 ix = hx & 0x7fffffff; /* high word of |x| */ 85 if (ix < 0x3e300000) { /* x < 2**-28 */ 86 if ((int) x == 0) { /* generate inexact */ 87 u_int32_t low; 88 GET_LOW_WORD(low, x); 89 if(((ix | low) | (iy + 1)) == 0) 90 return one / fabs(x); 91 else { 92 if (iy == 1) 93 return x; 94 else { /* compute -1 / (x+y) carefully */ 95 double a, t; 96 97 z = w = x + y; 98 SET_LOW_WORD(z, 0); 99 v = y - (z - x); 100 t = a = -one / w; 101 SET_LOW_WORD(t, 0); 102 s = one + t * z; 103 return t + a * (s + t * v); 104 } 105 } 106 } 107 } 108 if (ix >= 0x3FE59428) { /* |x| >= 0.6744 */ 109 if (hx < 0) { 110 x = -x; 111 y = -y; 112 } 113 z = pio4 - x; 114 w = pio4lo - y; 115 x = z + w; 116 y = 0.0; 117 } 118 z = x * x; 119 w = z * z; 120 /* 121 * Break x^5*(T[1]+x^2*T[2]+...) into 122 * x^5(T[1]+x^4*T[3]+...+x^20*T[11]) + 123 * x^5(x^2*(T[2]+x^4*T[4]+...+x^22*[T12])) 124 */ 125 r = T[1] + w * (T[3] + w * (T[5] + w * (T[7] + w * (T[9] + 126 w * T[11])))); 127 v = z * (T[2] + w * (T[4] + w * (T[6] + w * (T[8] + w * (T[10] + 128 w * T[12]))))); 129 s = z * x; 130 r = y + z * (s * (r + v) + y); 131 r += T[0] * s; 132 w = x + r; 133 if (ix >= 0x3FE59428) { 134 v = (double) iy; 135 return (double) (1 - ((hx >> 30) & 2)) * 136 (v - 2.0 * (x - (w * w / (w + v) - r))); 137 } 138 if (iy == 1) 139 return w; 140 else { 141 /* 142 * if allow error up to 2 ulp, simply return 143 * -1.0 / (x+r) here 144 */ 145 /* compute -1.0 / (x+r) accurately */ 146 double a, t; 147 z = w; 148 SET_LOW_WORD(z, 0); 149 v = r - (z - x); /* z+v = r+x */ 150 t = a = -1.0 / w; /* a = -1.0/w */ 151 SET_LOW_WORD(t, 0); 152 s = 1.0 + t * z; 153 return t + a * (s + t * v); 154 } 155 } 156