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9f4a7cc1 ZAL |
1 | /* |
2 | * Copyright (c) 1985 Regents of the University of California. | |
3 | * | |
4 | * Use and reproduction of this software are granted in accordance with | |
5 | * the terms and conditions specified in the Berkeley Software License | |
6 | * Agreement (in particular, this entails acknowledgement of the programs' | |
7 | * source, and inclusion of this notice) with the additional understanding | |
8 | * that all recipients should regard themselves as participants in an | |
9 | * ongoing research project and hence should feel obligated to report | |
10 | * their experiences (good or bad) with these elementary function codes, | |
11 | * using "sendbug 4bsd-bugs@BERKELEY", to the authors. | |
12 | */ | |
13 | ||
14 | #ifndef lint | |
a62df508 | 15 | static char sccsid[] = |
859dc438 ZAL |
16 | "@(#)pow.c 4.5 (Berkeley) 8/21/85; 1.7 (ucb.elefunt) %G%"; |
17 | #endif /* not lint */ | |
9f4a7cc1 ZAL |
18 | |
19 | /* POW(X,Y) | |
20 | * RETURN X**Y | |
21 | * DOUBLE PRECISION (VAX D format 56 bits, IEEE DOUBLE 53 BITS) | |
22 | * CODED IN C BY K.C. NG, 1/8/85; | |
23 | * REVISED BY K.C. NG on 7/10/85. | |
24 | * | |
25 | * Required system supported functions: | |
26 | * scalb(x,n) | |
27 | * logb(x) | |
28 | * copysign(x,y) | |
29 | * finite(x) | |
30 | * drem(x,y) | |
31 | * | |
32 | * Required kernel functions: | |
33 | * exp__E(a,c) ...return exp(a+c) - 1 - a*a/2 | |
34 | * log__L(x) ...return (log(1+x) - 2s)/s, s=x/(2+x) | |
35 | * pow_p(x,y) ...return +(anything)**(finite non zero) | |
36 | * | |
37 | * Method | |
38 | * 1. Compute and return log(x) in three pieces: | |
39 | * log(x) = n*ln2 + hi + lo, | |
40 | * where n is an integer. | |
41 | * 2. Perform y*log(x) by simulating muti-precision arithmetic and | |
42 | * return the answer in three pieces: | |
43 | * y*log(x) = m*ln2 + hi + lo, | |
44 | * where m is an integer. | |
45 | * 3. Return x**y = exp(y*log(x)) | |
46 | * = 2^m * ( exp(hi+lo) ). | |
47 | * | |
48 | * Special cases: | |
49 | * (anything) ** 0 is 1 ; | |
50 | * (anything) ** 1 is itself; | |
51 | * (anything) ** NaN is NaN; | |
52 | * NaN ** (anything except 0) is NaN; | |
53 | * +-(anything > 1) ** +INF is +INF; | |
54 | * +-(anything > 1) ** -INF is +0; | |
55 | * +-(anything < 1) ** +INF is +0; | |
56 | * +-(anything < 1) ** -INF is +INF; | |
57 | * +-1 ** +-INF is NaN and signal INVALID; | |
58 | * +0 ** +(anything except 0, NaN) is +0; | |
59 | * -0 ** +(anything except 0, NaN, odd integer) is +0; | |
60 | * +0 ** -(anything except 0, NaN) is +INF and signal DIV-BY-ZERO; | |
61 | * -0 ** -(anything except 0, NaN, odd integer) is +INF with signal; | |
62 | * -0 ** (odd integer) = -( +0 ** (odd integer) ); | |
63 | * +INF ** +(anything except 0,NaN) is +INF; | |
64 | * +INF ** -(anything except 0,NaN) is +0; | |
65 | * -INF ** (odd integer) = -( +INF ** (odd integer) ); | |
66 | * -INF ** (even integer) = ( +INF ** (even integer) ); | |
67 | * -INF ** -(anything except integer,NaN) is NaN with signal; | |
68 | * -(x=anything) ** (k=integer) is (-1)**k * (x ** k); | |
69 | * -(anything except 0) ** (non-integer) is NaN with signal; | |
70 | * | |
71 | * Accuracy: | |
72 | * pow(x,y) returns x**y nearly rounded. In particular, on a SUN, a VAX, | |
73 | * and a Zilog Z8000, | |
74 | * pow(integer,integer) | |
75 | * always returns the correct integer provided it is representable. | |
76 | * In a test run with 100,000 random arguments with 0 < x, y < 20.0 | |
77 | * on a VAX, the maximum observed error was 1.79 ulps (units in the | |
78 | * last place). | |
79 | * | |
80 | * Constants : | |
81 | * The hexadecimal values are the intended ones for the following constants. | |
82 | * The decimal values may be used, provided that the compiler will convert | |
83 | * from decimal to binary accurately enough to produce the hexadecimal values | |
84 | * shown. | |
85 | */ | |
86 | ||
859dc438 | 87 | #if defined(vax)||defined(tahoe) /* VAX D format */ |
9f4a7cc1 ZAL |
88 | #include <errno.h> |
89 | extern double infnan(); | |
859dc438 | 90 | #ifdef vax |
0e01cbea | 91 | #define _0x(A,B) 0x/**/A/**/B |
859dc438 | 92 | #else /* vax */ |
0e01cbea | 93 | #define _0x(A,B) 0x/**/B/**/A |
859dc438 | 94 | #endif /* vax */ |
62b65e15 | 95 | /* static double */ |
9f4a7cc1 ZAL |
96 | /* ln2hi = 6.9314718055829871446E-1 , Hex 2^ 0 * .B17217F7D00000 */ |
97 | /* ln2lo = 1.6465949582897081279E-12 , Hex 2^-39 * .E7BCD5E4F1D9CC */ | |
98 | /* invln2 = 1.4426950408889634148E0 , Hex 2^ 1 * .B8AA3B295C17F1 */ | |
99 | /* sqrt2 = 1.4142135623730950622E0 ; Hex 2^ 1 * .B504F333F9DE65 */ | |
0e01cbea ZAL |
100 | static long ln2hix[] = { _0x(7217,4031), _0x(0000,f7d0)}; |
101 | static long ln2lox[] = { _0x(bcd5,2ce7), _0x(d9cc,e4f1)}; | |
102 | static long invln2x[] = { _0x(aa3b,40b8), _0x(17f1,295c)}; | |
103 | static long sqrt2x[] = { _0x(04f3,40b5), _0x(de65,33f9)}; | |
9f4a7cc1 ZAL |
104 | #define ln2hi (*(double*)ln2hix) |
105 | #define ln2lo (*(double*)ln2lox) | |
106 | #define invln2 (*(double*)invln2x) | |
107 | #define sqrt2 (*(double*)sqrt2x) | |
859dc438 | 108 | #else /* defined(vax)||defined(tahoe) */ |
62b65e15 | 109 | static double |
9f4a7cc1 ZAL |
110 | ln2hi = 6.9314718036912381649E-1 , /*Hex 2^ -1 * 1.62E42FEE00000 */ |
111 | ln2lo = 1.9082149292705877000E-10 , /*Hex 2^-33 * 1.A39EF35793C76 */ | |
112 | invln2 = 1.4426950408889633870E0 , /*Hex 2^ 0 * 1.71547652B82FE */ | |
113 | sqrt2 = 1.4142135623730951455E0 ; /*Hex 2^ 0 * 1.6A09E667F3BCD */ | |
859dc438 | 114 | #endif /* defined(vax)||defined(tahoe) */ |
9f4a7cc1 | 115 | |
62b65e15 | 116 | static double zero=0.0, half=1.0/2.0, one=1.0, two=2.0, negone= -1.0; |
9f4a7cc1 ZAL |
117 | |
118 | double pow(x,y) | |
119 | double x,y; | |
120 | { | |
121 | double drem(),pow_p(),copysign(),t; | |
122 | int finite(); | |
123 | ||
124 | if (y==zero) return(one); | |
125 | else if(y==one | |
859dc438 | 126 | #if !defined(vax)&&!defined(tahoe) |
9f4a7cc1 | 127 | ||x!=x |
859dc438 | 128 | #endif /* !defined(vax)&&!defined(tahoe) */ |
9f4a7cc1 | 129 | ) return( x ); /* if x is NaN or y=1 */ |
859dc438 | 130 | #if !defined(vax)&&!defined(tahoe) |
9f4a7cc1 | 131 | else if(y!=y) return( y ); /* if y is NaN */ |
859dc438 | 132 | #endif /* !defined(vax)&&!defined(tahoe) */ |
9f4a7cc1 ZAL |
133 | else if(!finite(y)) /* if y is INF */ |
134 | if((t=copysign(x,one))==one) return(zero/zero); | |
135 | else if(t>one) return((y>zero)?y:zero); | |
136 | else return((y<zero)?-y:zero); | |
137 | else if(y==two) return(x*x); | |
138 | else if(y==negone) return(one/x); | |
139 | ||
140 | /* sign(x) = 1 */ | |
141 | else if(copysign(one,x)==one) return(pow_p(x,y)); | |
142 | ||
143 | /* sign(x)= -1 */ | |
144 | /* if y is an even integer */ | |
145 | else if ( (t=drem(y,two)) == zero) return( pow_p(-x,y) ); | |
146 | ||
147 | /* if y is an odd integer */ | |
148 | else if (copysign(t,one) == one) return( -pow_p(-x,y) ); | |
149 | ||
150 | /* Henceforth y is not an integer */ | |
151 | else if(x==zero) /* x is -0 */ | |
152 | return((y>zero)?-x:one/(-x)); | |
153 | else { /* return NaN */ | |
859dc438 | 154 | #if defined(vax)||defined(tahoe) |
9f4a7cc1 | 155 | return (infnan(EDOM)); /* NaN */ |
859dc438 | 156 | #else /* defined(vax)||defined(tahoe) */ |
9f4a7cc1 | 157 | return(zero/zero); |
859dc438 | 158 | #endif /* defined(vax)||defined(tahoe) */ |
9f4a7cc1 ZAL |
159 | } |
160 | } | |
161 | ||
162 | /* pow_p(x,y) return x**y for x with sign=1 and finite y */ | |
163 | static double pow_p(x,y) | |
164 | double x,y; | |
165 | { | |
166 | double logb(),scalb(),copysign(),log__L(),exp__E(); | |
167 | double c,s,t,z,tx,ty; | |
859dc438 | 168 | #ifdef tahoe |
471c7555 | 169 | double tahoe_tmp; |
859dc438 | 170 | #endif /* tahoe */ |
9f4a7cc1 ZAL |
171 | float sx,sy; |
172 | long k=0; | |
173 | int n,m; | |
174 | ||
175 | if(x==zero||!finite(x)) { /* if x is +INF or +0 */ | |
859dc438 | 176 | #if defined(vax)||defined(tahoe) |
9f4a7cc1 | 177 | return((y>zero)?x:infnan(ERANGE)); /* if y<zero, return +INF */ |
859dc438 | 178 | #else /* defined(vax)||defined(tahoe) */ |
9f4a7cc1 | 179 | return((y>zero)?x:one/x); |
859dc438 | 180 | #endif /* defined(vax)||defined(tahoe) */ |
9f4a7cc1 ZAL |
181 | } |
182 | if(x==1.0) return(x); /* if x=1.0, return 1 since y is finite */ | |
183 | ||
184 | /* reduce x to z in [sqrt(1/2)-1, sqrt(2)-1] */ | |
185 | z=scalb(x,-(n=logb(x))); | |
859dc438 | 186 | #if !defined(vax)&&!defined(tahoe) /* IEEE double; subnormal number */ |
9f4a7cc1 | 187 | if(n <= -1022) {n += (m=logb(z)); z=scalb(z,-m);} |
859dc438 | 188 | #endif /* !defined(vax)&&!defined(tahoe) */ |
9f4a7cc1 ZAL |
189 | if(z >= sqrt2 ) {n += 1; z *= half;} z -= one ; |
190 | ||
191 | /* log(x) = nlog2+log(1+z) ~ nlog2 + t + tx */ | |
192 | s=z/(two+z); c=z*z*half; tx=s*(c+log__L(s*s)); | |
193 | t= z-(c-tx); tx += (z-t)-c; | |
194 | ||
195 | /* if y*log(x) is neither too big nor too small */ | |
196 | if((s=logb(y)+logb(n+t)) < 12.0) | |
197 | if(s>-60.0) { | |
198 | ||
199 | /* compute y*log(x) ~ mlog2 + t + c */ | |
200 | s=y*(n+invln2*t); | |
201 | m=s+copysign(half,s); /* m := nint(y*log(x)) */ | |
202 | k=y; | |
203 | if((double)k==y) { /* if y is an integer */ | |
204 | k = m-k*n; | |
205 | sx=t; tx+=(t-sx); } | |
206 | else { /* if y is not an integer */ | |
207 | k =m; | |
208 | tx+=n*ln2lo; | |
209 | sx=(c=n*ln2hi)+t; tx+=(c-sx)+t; } | |
210 | /* end of checking whether k==y */ | |
211 | ||
212 | sy=y; ty=y-sy; /* y ~ sy + ty */ | |
859dc438 | 213 | #ifdef tahoe |
471c7555 | 214 | s = (tahoe_tmp = sx)*sy-k*ln2hi; |
859dc438 | 215 | #else /* tahoe */ |
9f4a7cc1 | 216 | s=(double)sx*sy-k*ln2hi; /* (sy+ty)*(sx+tx)-kln2 */ |
859dc438 | 217 | #endif /* tahoe */ |
9f4a7cc1 ZAL |
218 | z=(tx*ty-k*ln2lo); |
219 | tx=tx*sy; ty=sx*ty; | |
220 | t=ty+z; t+=tx; t+=s; | |
221 | c= -((((t-s)-tx)-ty)-z); | |
222 | ||
223 | /* return exp(y*log(x)) */ | |
224 | t += exp__E(t,c); return(scalb(one+t,m)); | |
225 | } | |
226 | /* end of if log(y*log(x)) > -60.0 */ | |
227 | ||
228 | else | |
229 | /* exp(+- tiny) = 1 with inexact flag */ | |
230 | {ln2hi+ln2lo; return(one);} | |
231 | else if(copysign(one,y)*(n+invln2*t) <zero) | |
232 | /* exp(-(big#)) underflows to zero */ | |
233 | return(scalb(one,-5000)); | |
234 | else | |
235 | /* exp(+(big#)) overflows to INF */ | |
236 | return(scalb(one, 5000)); | |
237 | ||
238 | } |