# code: 9ferno

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```/* derived from /netlib/fdlibm */

/* @(#)e_asin.c 1.3 95/01/18 */
/*
* ====================================================
*
* Developed at SunSoft, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/

/* __ieee754_asin(x)
* Method :
*	Since  asin(x) = x + x^3/6 + x^5*3/40 + x^7*15/336 + ...
*	we approximate asin(x) on [0,0.5] by
*		asin(x) = x + x*x^2*R(x^2)
*	where
*		R(x^2) is a rational approximation of (asin(x)-x)/x^3
*	and its remez error is bounded by
*		|(asin(x)-x)/x^3 - R(x^2)| < 2^(-58.75)
*
*	For x in [0.5,1]
*		asin(x) = pi/2-2*asin(sqrt((1-x)/2))
*	Let y = (1-x), z = y/2, s := sqrt(z), and pio2_hi+pio2_lo=pi/2;
*	then for x>0.98
*		asin(x) = pi/2 - 2*(s+s*z*R(z))
*			= pio2_hi - (2*(s+s*z*R(z)) - pio2_lo)
*	For x<=0.98, let pio4_hi = pio2_hi/2, then
*		f = hi part of s;
*		c = sqrt(z) - f = (z-f*f)/(s+f) 	...f+c=sqrt(z)
*	and
*		asin(x) = pi/2 - 2*(s+s*z*R(z))
*			= pio4_hi+(pio4-2s)-(2s*z*R(z)-pio2_lo)
*			= pio4_hi+(pio4-2f)-(2s*z*R(z)-(pio2_lo+2c))
*
* Special cases:
*	if x is NaN, return x itself;
*	if |x|>1, return NaN with invalid signal.
*
*/

#include "fdlibm.h"

static const double
one =  1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
Huge =  1.000e+300,
pio2_hi =  1.57079632679489655800e+00, /* 0x3FF921FB, 0x54442D18 */
pio2_lo =  6.12323399573676603587e-17, /* 0x3C91A626, 0x33145C07 */
pio4_hi =  7.85398163397448278999e-01, /* 0x3FE921FB, 0x54442D18 */
/* coefficient for R(x^2) */
pS0 =  1.66666666666666657415e-01, /* 0x3FC55555, 0x55555555 */
pS1 = -3.25565818622400915405e-01, /* 0xBFD4D612, 0x03EB6F7D */
pS2 =  2.01212532134862925881e-01, /* 0x3FC9C155, 0x0E884455 */
pS3 = -4.00555345006794114027e-02, /* 0xBFA48228, 0xB5688F3B */
pS4 =  7.91534994289814532176e-04, /* 0x3F49EFE0, 0x7501B288 */
pS5 =  3.47933107596021167570e-05, /* 0x3F023DE1, 0x0DFDF709 */
qS1 = -2.40339491173441421878e+00, /* 0xC0033A27, 0x1C8A2D4B */
qS2 =  2.02094576023350569471e+00, /* 0x40002AE5, 0x9C598AC8 */
qS3 = -6.88283971605453293030e-01, /* 0xBFE6066C, 0x1B8D0159 */
qS4 =  7.70381505559019352791e-02; /* 0x3FB3B8C5, 0xB12E9282 */

double __ieee754_asin(double x)
{
double t,w,p,q,c,r,s;
int hx,ix;
hx = __HI(x);
ix = hx&0x7fffffff;
if(ix>= 0x3ff00000) {		/* |x|>= 1 */
if(((ix-0x3ff00000)|__LO(x))==0)
/* asin(1)=+-pi/2 with inexact */
return x*pio2_hi+x*pio2_lo;
return (x-x)/(x-x);		/* asin(|x|>1) is NaN */
} else if (ix<0x3fe00000) {	/* |x|<0.5 */
if(ix<0x3e400000) {		/* if |x| < 2**-27 */
if(Huge+x>one) return x;/* return x with inexact if x!=0*/
}
t = x*x;
p = t*(pS0+t*(pS1+t*(pS2+t*(pS3+t*(pS4+t*pS5)))));
q = one+t*(qS1+t*(qS2+t*(qS3+t*qS4)));
w = p/q;
return x+x*w;
}
/* 1> |x|>= 0.5 */
w = one-fabs(x);
t = w*0.5;
p = t*(pS0+t*(pS1+t*(pS2+t*(pS3+t*(pS4+t*pS5)))));
q = one+t*(qS1+t*(qS2+t*(qS3+t*qS4)));
s = sqrt(t);
if(ix>=0x3FEF3333) { 	/* if |x| > 0.975 */
w = p/q;
t = pio2_hi-(2.0*(s+s*w)-pio2_lo);
} else {
w  = s;
__LO(w) = 0;
c  = (t-w*w)/(s+w);
r  = p/q;
p  = 2.0*s*r-(pio2_lo-2.0*c);
q  = pio4_hi-2.0*w;
t  = pio4_hi-(p-q);
}
if(hx>0) return t; else return -t;
}
```