nsz Git - libm/blob - src/math/atan.c

   1 /* origin: FreeBSD /usr/src/lib/msun/src/s_atan.c */
   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 /* atan(x)
  13  * Method
  14  *   1. Reduce x to positive by atan(x) = -atan(-x).
  15  *   2. According to the integer k=4t+0.25 chopped, t=x, the argument
  16  *      is further reduced to one of the following intervals and the
  17  *      arctangent of t is evaluated by the corresponding formula:
  18  *
  19  *      [0,7/16]      atan(x) = t-t^3*(a1+t^2*(a2+...(a10+t^2*a11)...)
  20  *      [7/16,11/16]  atan(x) = atan(1/2) + atan( (t-0.5)/(1+t/2) )
  21  *      [11/16.19/16] atan(x) = atan( 1 ) + atan( (t-1)/(1+t) )
  22  *      [19/16,39/16] atan(x) = atan(3/2) + atan( (t-1.5)/(1+1.5t) )
  23  *      [39/16,INF]   atan(x) = atan(INF) + atan( -1/t )
  24  *
  25  * Constants:
  26  * The hexadecimal values are the intended ones for the following
  27  * constants. The decimal values may be used, provided that the
  28  * compiler will convert from decimal to binary accurately enough
  29  * to produce the hexadecimal values shown.
  30  */
  31
  32
  33 #include "libm.h"
  34
  35 static const double atanhi[] = {
  36   4.63647609000806093515e-01, /* atan(0.5)hi 0x3FDDAC67, 0x0561BB4F */
  37   7.85398163397448278999e-01, /* atan(1.0)hi 0x3FE921FB, 0x54442D18 */
  38   9.82793723247329054082e-01, /* atan(1.5)hi 0x3FEF730B, 0xD281F69B */
  39   1.57079632679489655800e+00, /* atan(inf)hi 0x3FF921FB, 0x54442D18 */
  40 };
  41
  42 static const double atanlo[] = {
  43   2.26987774529616870924e-17, /* atan(0.5)lo 0x3C7A2B7F, 0x222F65E2 */
  44   3.06161699786838301793e-17, /* atan(1.0)lo 0x3C81A626, 0x33145C07 */
  45   1.39033110312309984516e-17, /* atan(1.5)lo 0x3C700788, 0x7AF0CBBD */
  46   6.12323399573676603587e-17, /* atan(inf)lo 0x3C91A626, 0x33145C07 */
  47 };
  48
  49 static const double aT[] = {
  50   3.33333333333329318027e-01, /* 0x3FD55555, 0x5555550D */
  51  -1.99999999998764832476e-01, /* 0xBFC99999, 0x9998EBC4 */
  52   1.42857142725034663711e-01, /* 0x3FC24924, 0x920083FF */
  53  -1.11111104054623557880e-01, /* 0xBFBC71C6, 0xFE231671 */
  54   9.09088713343650656196e-02, /* 0x3FB745CD, 0xC54C206E */
  55  -7.69187620504482999495e-02, /* 0xBFB3B0F2, 0xAF749A6D */
  56   6.66107313738753120669e-02, /* 0x3FB10D66, 0xA0D03D51 */
  57  -5.83357013379057348645e-02, /* 0xBFADDE2D, 0x52DEFD9A */
  58   4.97687799461593236017e-02, /* 0x3FA97B4B, 0x24760DEB */
  59  -3.65315727442169155270e-02, /* 0xBFA2B444, 0x2C6A6C2F */
  60   1.62858201153657823623e-02, /* 0x3F90AD3A, 0xE322DA11 */
  61 };
  62
  63 static const double
  64 one = 1.0,
  65 huge = 1.0e300;
  66
  67 double atan(double x)
  68 {
  69         double w,s1,s2,z;
  70         int32_t ix,hx,id;
  71
  72         GET_HIGH_WORD(hx, x);
  73         ix = hx & 0x7fffffff;
  74         if (ix >= 0x44100000) {   /* if |x| >= 2^66 */
  75                 uint32_t low;
  76
  77                 GET_LOW_WORD(low, x);
  78                 if (ix > 0x7ff00000 ||
  79                     (ix == 0x7ff00000 && low != 0))  /* NaN */
  80                         return x+x;
  81                 if (hx > 0)
  82                         return  atanhi[3] + *(volatile double *)&atanlo[3];
  83                 else
  84                         return -atanhi[3] - *(volatile double *)&atanlo[3];
  85         }
  86         if (ix < 0x3fdc0000) {    /* |x| < 0.4375 */
  87                 if (ix < 0x3e400000) {  /* |x| < 2^-27 */
  88                         /* raise inexact */
  89                         if (huge+x > one)
  90                                 return x;
  91                 }
  92                 id = -1;
  93         } else {
  94                 x = fabs(x);
  95                 if (ix < 0x3ff30000) {  /* |x| < 1.1875 */
  96                         if (ix < 0x3fe60000) {  /*  7/16 <= |x| < 11/16 */
  97                                 id = 0;
  98                                 x = (2.0*x-one)/(2.0+x);
  99                         } else {                /* 11/16 <= |x| < 19/16 */
 100                                 id = 1;
 101                                 x = (x-one)/(x+one);
 102                         }
 103                 } else {
 104                         if (ix < 0x40038000) {  /* |x| < 2.4375 */
 105                                 id = 2;
 106                                 x = (x-1.5)/(one+1.5*x);
 107                         } else {                /* 2.4375 <= |x| < 2^66 */
 108                                 id = 3;
 109                                 x = -1.0/x;
 110                         }
 111                 }
 112         }
 113         /* end of argument reduction */
 114         z = x*x;
 115         w = z*z;
 116         /* break sum from i=0 to 10 aT[i]z**(i+1) into odd and even poly */
 117         s1 = z*(aT[0]+w*(aT[2]+w*(aT[4]+w*(aT[6]+w*(aT[8]+w*aT[10])))));
 118         s2 = w*(aT[1]+w*(aT[3]+w*(aT[5]+w*(aT[7]+w*aT[9]))));
 119         if (id < 0)
 120                 return x - x*(s1+s2);
 121         z = atanhi[id] - (x*(s1+s2) - atanlo[id] - x);
 122         return hx < 0 ? -z : z;
 123 }