about summary refs log tree commit diff
path: root/sysdeps/ieee754/ldbl-96/k_tanl.c
blob: ae6821d9844fd134c576068bd94b0acecadbe290 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */

/*
  Long double expansions are
  Copyright (C) 2001 Stephen L. Moshier <moshier@na-net.ornl.gov>
  and are incorporated herein by permission of the author.  The author
  reserves the right to distribute this material elsewhere under different
  copying permissions.  These modifications are distributed here under
  the following terms:

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, see
    <http://www.gnu.org/licenses/>.  */

/* __kernel_tanl( x, y, k )
 * kernel tan function on [-pi/4, pi/4], pi/4 ~ 0.7854
 * Input x is assumed to be bounded by ~pi/4 in magnitude.
 * Input y is the tail of x.
 * Input k indicates whether tan (if k=1) or
 * -1/tan (if k= -1) is returned.
 *
 * Algorithm
 *	1. Since tan(-x) = -tan(x), we need only to consider positive x.
 *	2. if x < 2^-33, return x with inexact if x!=0.
 *	3. tan(x) is approximated by a rational form x + x^3 / 3 + x^5 R(x^2)
 *          on [0,0.67433].
 *
 *	   Note: tan(x+y) = tan(x) + tan'(x)*y
 *		          ~ tan(x) + (1+x*x)*y
 *	   Therefore, for better accuracy in computing tan(x+y), let
 *		r = x^3 * R(x^2)
 *	   then
 *		tan(x+y) = x + (x^3 / 3 + (x^2 *(r+y)+y))
 *
 *      4. For x in [0.67433,pi/4],  let y = pi/4 - x, then
 *		tan(x) = tan(pi/4-y) = (1-tan(y))/(1+tan(y))
 *		       = 1 - 2*(tan(y) - (tan(y)^2)/(1+tan(y)))
 */

#include <float.h>
#include <math.h>
#include <math_private.h>
static const long double
  one = 1.0L,
  pio4hi = 0xc.90fdaa22168c235p-4L,
  pio4lo = -0x3.b399d747f23e32ecp-68L,

  /* tan x = x + x^3 / 3 + x^5 T(x^2)/U(x^2)
     0 <= x <= 0.6743316650390625
     Peak relative error 8.0e-36  */
 TH =  3.333333333333333333333333333333333333333E-1L,
 T0 = -1.813014711743583437742363284336855889393E7L,
 T1 =  1.320767960008972224312740075083259247618E6L,
 T2 = -2.626775478255838182468651821863299023956E4L,
 T3 =  1.764573356488504935415411383687150199315E2L,
 T4 = -3.333267763822178690794678978979803526092E-1L,

 U0 = -1.359761033807687578306772463253710042010E8L,
 U1 =  6.494370630656893175666729313065113194784E7L,
 U2 = -4.180787672237927475505536849168729386782E6L,
 U3 =  8.031643765106170040139966622980914621521E4L,
 U4 = -5.323131271912475695157127875560667378597E2L;
  /* 1.000000000000000000000000000000000000000E0 */


long double
__kernel_tanl (long double x, long double y, int iy)
{
  long double z, r, v, w, s;
  long double absx = fabsl (x);
  int sign;

  if (absx < 0x1p-33)
    {
      if ((int) x == 0)
	{			/* generate inexact */
	  if (x == 0 && iy == -1)
	    return one / fabsl (x);
	  else if (iy == 1)
	    {
	      if (absx < LDBL_MIN)
		{
		  long double force_underflow = x * x;
		  math_force_eval (force_underflow);
		}
	      return x;
	    }
	  else
	    return -one / x;
	}
    }
  if (absx >= 0.6743316650390625L)
    {
      if (signbit (x))
	{
	  x = -x;
	  y = -y;
	  sign = -1;
	}
      else
	sign = 1;
      z = pio4hi - x;
      w = pio4lo - y;
      x = z + w;
      y = 0.0;
    }
  z = x * x;
  r = T0 + z * (T1 + z * (T2 + z * (T3 + z * T4)));
  v = U0 + z * (U1 + z * (U2 + z * (U3 + z * (U4 + z))));
  r = r / v;

  s = z * x;
  r = y + z * (s * r + y);
  r += TH * s;
  w = x + r;
  if (absx >= 0.6743316650390625L)
    {
      v = (long double) iy;
      w = (v - 2.0 * (x - (w * w / (w + v) - r)));
      if (sign < 0)
	w = -w;
      return w;
    }
  if (iy == 1)
    return w;
  else
    return -1.0 / (x + r);
}