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107 lines
3.3 KiB
107 lines
3.3 KiB
/* origin: FreeBSD /usr/src/lib/msun/src/e_asin.c */
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunSoft, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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/* asin(x)
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* Method :
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* Since asin(x) = x + x^3/6 + x^5*3/40 + x^7*15/336 + ...
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* we approximate asin(x) on [0,0.5] by
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* asin(x) = x + x*x^2*R(x^2)
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* where
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* R(x^2) is a rational approximation of (asin(x)-x)/x^3
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* and its remez error is bounded by
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* |(asin(x)-x)/x^3 - R(x^2)| < 2^(-58.75)
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*
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* For x in [0.5,1]
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* asin(x) = pi/2-2*asin(sqrt((1-x)/2))
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* Let y = (1-x), z = y/2, s := sqrt(z), and pio2_hi+pio2_lo=pi/2;
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* then for x>0.98
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* asin(x) = pi/2 - 2*(s+s*z*R(z))
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* = pio2_hi - (2*(s+s*z*R(z)) - pio2_lo)
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* For x<=0.98, let pio4_hi = pio2_hi/2, then
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* f = hi part of s;
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* c = sqrt(z) - f = (z-f*f)/(s+f) ...f+c=sqrt(z)
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* and
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* asin(x) = pi/2 - 2*(s+s*z*R(z))
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* = pio4_hi+(pio4-2s)-(2s*z*R(z)-pio2_lo)
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* = pio4_hi+(pio4-2f)-(2s*z*R(z)-(pio2_lo+2c))
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*
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* Special cases:
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* if x is NaN, return x itself;
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* if |x|>1, return NaN with invalid signal.
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*
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*/
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#include "libm.h"
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static const double
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pio2_hi = 1.57079632679489655800e+00, /* 0x3FF921FB, 0x54442D18 */
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pio2_lo = 6.12323399573676603587e-17, /* 0x3C91A626, 0x33145C07 */
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/* coefficients for R(x^2) */
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pS0 = 1.66666666666666657415e-01, /* 0x3FC55555, 0x55555555 */
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pS1 = -3.25565818622400915405e-01, /* 0xBFD4D612, 0x03EB6F7D */
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pS2 = 2.01212532134862925881e-01, /* 0x3FC9C155, 0x0E884455 */
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pS3 = -4.00555345006794114027e-02, /* 0xBFA48228, 0xB5688F3B */
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pS4 = 7.91534994289814532176e-04, /* 0x3F49EFE0, 0x7501B288 */
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pS5 = 3.47933107596021167570e-05, /* 0x3F023DE1, 0x0DFDF709 */
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qS1 = -2.40339491173441421878e+00, /* 0xC0033A27, 0x1C8A2D4B */
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qS2 = 2.02094576023350569471e+00, /* 0x40002AE5, 0x9C598AC8 */
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qS3 = -6.88283971605453293030e-01, /* 0xBFE6066C, 0x1B8D0159 */
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qS4 = 7.70381505559019352791e-02; /* 0x3FB3B8C5, 0xB12E9282 */
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static double R(double z)
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{
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double_t p, q;
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p = z*(pS0+z*(pS1+z*(pS2+z*(pS3+z*(pS4+z*pS5)))));
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q = 1.0+z*(qS1+z*(qS2+z*(qS3+z*qS4)));
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return p/q;
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}
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double asin(double x)
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{
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double z,r,s;
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uint32_t hx,ix;
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GET_HIGH_WORD(hx, x);
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ix = hx & 0x7fffffff;
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/* |x| >= 1 or nan */
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if (ix >= 0x3ff00000) {
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uint32_t lx;
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GET_LOW_WORD(lx, x);
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if (((ix-0x3ff00000) | lx) == 0)
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/* asin(1) = +-pi/2 with inexact */
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return x*pio2_hi + 0x1p-120f;
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return 0/(x-x);
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}
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/* |x| < 0.5 */
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if (ix < 0x3fe00000) {
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/* if 0x1p-1022 <= |x| < 0x1p-26, avoid raising underflow */
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if (ix < 0x3e500000 && ix >= 0x00100000)
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return x;
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return x + x*R(x*x);
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}
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/* 1 > |x| >= 0.5 */
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z = (1 - fabs(x))*0.5;
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s = sqrt(z);
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r = R(z);
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if (ix >= 0x3fef3333) { /* if |x| > 0.975 */
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x = pio2_hi-(2*(s+s*r)-pio2_lo);
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} else {
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double f,c;
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/* f+c = sqrt(z) */
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f = s;
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SET_LOW_WORD(f,0);
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c = (z-f*f)/(s+f);
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x = 0.5*pio2_hi - (2*s*r - (pio2_lo-2*c) - (0.5*pio2_hi-2*f));
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}
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if (hx >> 31)
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return -x;
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return x;
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}
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