1 /* @(#)e_asin.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 #ifndef lint 14 static char rcsid[] = "$Id: e_asin.c,v 1.4 1994/03/03 17:04:06 jtc Exp $"; 15 #endif 16 17 /* __ieee754_asin(x) 18 * Method : 19 * Since asin(x) = x + x^3/6 + x^5*3/40 + x^7*15/336 + ... 20 * we approximate asin(x) on [0,0.5] by 21 * asin(x) = x + x*x^2*R(x^2) 22 * where 23 * R(x^2) is a rational approximation of (asin(x)-x)/x^3 24 * and its remez error is bounded by 25 * |(asin(x)-x)/x^3 - R(x^2)| < 2^(-58.75) 26 * 27 * For x in [0.5,1] 28 * asin(x) = pi/2-2*asin(sqrt((1-x)/2)) 29 * Let y = (1-x), z = y/2, s := sqrt(z), and pio2_hi+pio2_lo=pi/2; 30 * then for x>0.98 31 * asin(x) = pi/2 - 2*(s+s*z*R(z)) 32 * = pio2_hi - (2*(s+s*z*R(z)) - pio2_lo) 33 * For x<=0.98, let pio4_hi = pio2_hi/2, then 34 * f = hi part of s; 35 * c = sqrt(z) - f = (z-f*f)/(s+f) ...f+c=sqrt(z) 36 * and 37 * asin(x) = pi/2 - 2*(s+s*z*R(z)) 38 * = pio4_hi+(pio4-2s)-(2s*z*R(z)-pio2_lo) 39 * = pio4_hi+(pio4-2f)-(2s*z*R(z)-(pio2_lo+2c)) 40 * 41 * Special cases: 42 * if x is NaN, return x itself; 43 * if |x|>1, return NaN with invalid signal. 44 * 45 */ 46 47 48 #include <math.h> 49 #include <machine/endian.h> 50 51 #if BYTE_ORDER == LITTLE_ENDIAN 52 #define n0 1 53 #else 54 #define n0 0 55 #endif 56 57 #ifdef __STDC__ 58 static const double 59 #else 60 static double 61 #endif 62 one = 1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */ 63 huge = 1.000e+300, 64 pio2_hi = 1.57079632679489655800e+00, /* 0x3FF921FB, 0x54442D18 */ 65 pio2_lo = 6.12323399573676603587e-17, /* 0x3C91A626, 0x33145C07 */ 66 pio4_hi = 7.85398163397448278999e-01, /* 0x3FE921FB, 0x54442D18 */ 67 /* coefficient for R(x^2) */ 68 pS0 = 1.66666666666666657415e-01, /* 0x3FC55555, 0x55555555 */ 69 pS1 = -3.25565818622400915405e-01, /* 0xBFD4D612, 0x03EB6F7D */ 70 pS2 = 2.01212532134862925881e-01, /* 0x3FC9C155, 0x0E884455 */ 71 pS3 = -4.00555345006794114027e-02, /* 0xBFA48228, 0xB5688F3B */ 72 pS4 = 7.91534994289814532176e-04, /* 0x3F49EFE0, 0x7501B288 */ 73 pS5 = 3.47933107596021167570e-05, /* 0x3F023DE1, 0x0DFDF709 */ 74 qS1 = -2.40339491173441421878e+00, /* 0xC0033A27, 0x1C8A2D4B */ 75 qS2 = 2.02094576023350569471e+00, /* 0x40002AE5, 0x9C598AC8 */ 76 qS3 = -6.88283971605453293030e-01, /* 0xBFE6066C, 0x1B8D0159 */ 77 qS4 = 7.70381505559019352791e-02; /* 0x3FB3B8C5, 0xB12E9282 */ 78 79 #ifdef __STDC__ 80 double __ieee754_asin(double x) 81 #else 82 double __ieee754_asin(x) 83 double x; 84 #endif 85 { 86 double t,w,p,q,c,r,s; 87 int hx,ix; 88 89 hx = *(n0+(int*)&x); 90 ix = hx&0x7fffffff; 91 if(ix>= 0x3ff00000) { /* |x|>= 1 */ 92 if(((ix-0x3ff00000)|*(1-n0+(int*)&x))==0) 93 /* asin(1)=+-pi/2 with inexact */ 94 return x*pio2_hi+x*pio2_lo; 95 return (x-x)/(x-x); /* asin(|x|>1) is NaN */ 96 } else if (ix<0x3fe00000) { /* |x|<0.5 */ 97 if(ix<0x3e400000) { /* if |x| < 2**-27 */ 98 if(huge+x>one) return x;/* return x with inexact if x!=0*/ 99 } else 100 t = x*x; 101 p = t*(pS0+t*(pS1+t*(pS2+t*(pS3+t*(pS4+t*pS5))))); 102 q = one+t*(qS1+t*(qS2+t*(qS3+t*qS4))); 103 w = p/q; 104 return x+x*w; 105 } 106 /* 1> |x|>= 0.5 */ 107 w = one-fabs(x); 108 t = w*0.5; 109 p = t*(pS0+t*(pS1+t*(pS2+t*(pS3+t*(pS4+t*pS5))))); 110 q = one+t*(qS1+t*(qS2+t*(qS3+t*qS4))); 111 s = sqrt(t); 112 if(ix>=0x3FEF3333) { /* if |x| > 0.975 */ 113 w = p/q; 114 t = pio2_hi-(2.0*(s+s*w)-pio2_lo); 115 } else { 116 w = s; 117 *(1-n0+(int*)&w) = 0; 118 c = (t-w*w)/(s+w); 119 r = p/q; 120 p = 2.0*s*r-(pio2_lo-2.0*c); 121 q = pio4_hi-2.0*w; 122 t = pio4_hi-(p-q); 123 } 124 if(hx>0) return t; else return -t; 125 } 126