1 
2 /* @(#)e_acos.c 1.3 95/01/18 */
3 /*
4  * ====================================================
5  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
6  *
7  * Developed at SunSoft, a Sun Microsystems, Inc. business.
8  * Permission to use, copy, modify, and distribute this
9  * software is freely granted, provided that this notice
10  * is preserved.
11  * ====================================================
12  */
13 
14 #include <sys/cdefs.h>
15 __FBSDID("$FreeBSD$");
16 
17 /* __ieee754_acos(x)
18  * Method :
19  *	acos(x)  = pi/2 - asin(x)
20  *	acos(-x) = pi/2 + asin(x)
21  * For |x|<=0.5
22  *	acos(x) = pi/2 - (x + x*x^2*R(x^2))	(see asin.c)
23  * For x>0.5
24  * 	acos(x) = pi/2 - (pi/2 - 2asin(sqrt((1-x)/2)))
25  *		= 2asin(sqrt((1-x)/2))
26  *		= 2s + 2s*z*R(z) 	...z=(1-x)/2, s=sqrt(z)
27  *		= 2f + (2c + 2s*z*R(z))
28  *     where f=hi part of s, and c = (z-f*f)/(s+f) is the correction term
29  *     for f so that f+c ~ sqrt(z).
30  * For x<-0.5
31  *	acos(x) = pi - 2asin(sqrt((1-|x|)/2))
32  *		= pi - 0.5*(s+s*z*R(z)), where z=(1-|x|)/2,s=sqrt(z)
33  *
34  * Special cases:
35  *	if x is NaN, return x itself;
36  *	if |x|>1, return NaN with invalid signal.
37  *
38  * Function needed: sqrt
39  */
40 
41 #include <float.h>
42 
43 #include "math.h"
44 #include "math_private.h"
45 
46 static const double
47 one=  1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
48 pi =  3.14159265358979311600e+00, /* 0x400921FB, 0x54442D18 */
49 pio2_hi =  1.57079632679489655800e+00; /* 0x3FF921FB, 0x54442D18 */
50 static volatile double
51 pio2_lo =  6.12323399573676603587e-17; /* 0x3C91A626, 0x33145C07 */
52 static const double
53 pS0 =  1.66666666666666657415e-01, /* 0x3FC55555, 0x55555555 */
54 pS1 = -3.25565818622400915405e-01, /* 0xBFD4D612, 0x03EB6F7D */
55 pS2 =  2.01212532134862925881e-01, /* 0x3FC9C155, 0x0E884455 */
56 pS3 = -4.00555345006794114027e-02, /* 0xBFA48228, 0xB5688F3B */
57 pS4 =  7.91534994289814532176e-04, /* 0x3F49EFE0, 0x7501B288 */
58 pS5 =  3.47933107596021167570e-05, /* 0x3F023DE1, 0x0DFDF709 */
59 qS1 = -2.40339491173441421878e+00, /* 0xC0033A27, 0x1C8A2D4B */
60 qS2 =  2.02094576023350569471e+00, /* 0x40002AE5, 0x9C598AC8 */
61 qS3 = -6.88283971605453293030e-01, /* 0xBFE6066C, 0x1B8D0159 */
62 qS4 =  7.70381505559019352791e-02; /* 0x3FB3B8C5, 0xB12E9282 */
63 
64 double
__ieee754_acos(double x)65 __ieee754_acos(double x)
66 {
67 	double z,p,q,r,w,s,c,df;
68 	int32_t hx,ix;
69 	GET_HIGH_WORD(hx,x);
70 	ix = hx&0x7fffffff;
71 	if(ix>=0x3ff00000) {	/* |x| >= 1 */
72 	    u_int32_t lx;
73 	    GET_LOW_WORD(lx,x);
74 	    if(((ix-0x3ff00000)|lx)==0) {	/* |x|==1 */
75 		if(hx>0) return 0.0;		/* acos(1) = 0  */
76 		else return pi+2.0*pio2_lo;	/* acos(-1)= pi */
77 	    }
78 	    return (x-x)/(x-x);		/* acos(|x|>1) is NaN */
79 	}
80 	if(ix<0x3fe00000) {	/* |x| < 0.5 */
81 	    if(ix<=0x3c600000) return pio2_hi+pio2_lo;/*if|x|<2**-57*/
82 	    z = x*x;
83 	    p = z*(pS0+z*(pS1+z*(pS2+z*(pS3+z*(pS4+z*pS5)))));
84 	    q = one+z*(qS1+z*(qS2+z*(qS3+z*qS4)));
85 	    r = p/q;
86 	    return pio2_hi - (x - (pio2_lo-x*r));
87 	} else  if (hx<0) {		/* x < -0.5 */
88 	    z = (one+x)*0.5;
89 	    p = z*(pS0+z*(pS1+z*(pS2+z*(pS3+z*(pS4+z*pS5)))));
90 	    q = one+z*(qS1+z*(qS2+z*(qS3+z*qS4)));
91 	    s = sqrt(z);
92 	    r = p/q;
93 	    w = r*s-pio2_lo;
94 	    return pi - 2.0*(s+w);
95 	} else {			/* x > 0.5 */
96 	    z = (one-x)*0.5;
97 	    s = sqrt(z);
98 	    df = s;
99 	    SET_LOW_WORD(df,0);
100 	    c  = (z-df*df)/(s+df);
101 	    p = z*(pS0+z*(pS1+z*(pS2+z*(pS3+z*(pS4+z*pS5)))));
102 	    q = one+z*(qS1+z*(qS2+z*(qS3+z*qS4)));
103 	    r = p/q;
104 	    w = r*s+c;
105 	    return 2.0*(df+w);
106 	}
107 }
108 
109 #if LDBL_MANT_DIG == 53
110 __weak_reference(acos, acosl);
111 #endif
112