1 /* origin: FreeBSD /usr/src/lib/msun/src/k_tan.c */
3 * ====================================================
4 * Copyright 2004 Sun Microsystems, Inc. All Rights Reserved.
6 * Permission to use, copy, modify, and distribute this
7 * software is freely granted, provided that this notice
9 * ====================================================
12 * kernel tan function on ~[-pi/4, pi/4] (except on -0), pi/4 ~ 0.7854
13 * Input x is assumed to be bounded by ~pi/4 in magnitude.
14 * Input y is the tail of x.
15 * Input k indicates whether tan (if k = 1) or -1/tan (if k = -1) is returned.
18 * 1. Since tan(-x) = -tan(x), we need only to consider positive x.
19 * 2. Callers must return tan(-0) = -0 without calling here since our
20 * odd polynomial is not evaluated in a way that preserves -0.
21 * Callers may do the optimization tan(x) ~ x for tiny x.
22 * 3. tan(x) is approximated by a odd polynomial of degree 27 on
25 * tan(x) ~ x + T1*x + ... + T13*x
28 * |tan(x) 2 4 26 | -59.2
29 * |----- - (1+T1*x +T2*x +.... +T13*x )| <= 2
32 * Note: tan(x+y) = tan(x) + tan'(x)*y
33 * ~ tan(x) + (1+x*x)*y
34 * Therefore, for better accuracy in computing tan(x+y), let
36 * r = x *(T2+x *(T3+x *(...+x *(T12+x *T13))))
39 * tan(x+y) = x + (T1*x + (x *(r+y)+y))
41 * 4. For x in [0.67434,pi/4], let y = pi/4 - x, then
42 * tan(x) = tan(pi/4-y) = (1-tan(y))/(1+tan(y))
43 * = 1 - 2*(tan(y) - (tan(y)^2)/(1+tan(y)))
48 static const double T[] = {
49 3.33333333333334091986e-01, /* 3FD55555, 55555563 */
50 1.33333333333201242699e-01, /* 3FC11111, 1110FE7A */
51 5.39682539762260521377e-02, /* 3FABA1BA, 1BB341FE */
52 2.18694882948595424599e-02, /* 3F9664F4, 8406D637 */
53 8.86323982359930005737e-03, /* 3F8226E3, E96E8493 */
54 3.59207910759131235356e-03, /* 3F6D6D22, C9560328 */
55 1.45620945432529025516e-03, /* 3F57DBC8, FEE08315 */
56 5.88041240820264096874e-04, /* 3F4344D8, F2F26501 */
57 2.46463134818469906812e-04, /* 3F3026F7, 1A8D1068 */
58 7.81794442939557092300e-05, /* 3F147E88, A03792A6 */
59 7.14072491382608190305e-05, /* 3F12B80F, 32F0A7E9 */
60 -1.85586374855275456654e-05, /* BEF375CB, DB605373 */
61 2.59073051863633712884e-05, /* 3EFB2A70, 74BF7AD4 */
62 /* one */ 1.00000000000000000000e+00, /* 3FF00000, 00000000 */
63 /* pio4 */ 7.85398163397448278999e-01, /* 3FE921FB, 54442D18 */
64 /* pio4lo */ 3.06161699786838301793e-17 /* 3C81A626, 33145C07 */
70 double __tan(double x, double y, int iy)
72 double z, r, v, w, s, sign;
76 ix = hx & 0x7fffffff; /* high word of |x| */
77 if (ix >= 0x3FE59428) { /* |x| >= 0.6744 */
90 * Break x^5*(T[1]+x^2*T[2]+...) into
91 * x^5(T[1]+x^4*T[3]+...+x^20*T[11]) +
92 * x^5(x^2*(T[2]+x^4*T[4]+...+x^22*[T12]))
94 r = T[1] + w*(T[3] + w*(T[5] + w*(T[7] + w*(T[9] + w*T[11]))));
95 v = z*(T[2] + w*(T[4] + w*(T[6] + w*(T[8] + w*(T[10] + w*T[12])))));
97 r = y + z * (s * (r + v) + y);
100 if (ix >= 0x3FE59428) {
102 sign = 1 - ((hx >> 30) & 2);
103 return sign * (v - 2.0 * (x - (w * w / (w + v) - r)));
109 * if allow error up to 2 ulp, simply return
112 /* compute -1.0 / (x+r) accurately */
116 v = r - (z - x); /* z+v = r+x */
117 t = a = -1.0 / w; /* a = -1.0/w */
120 return t + a * (s + t * v);