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#include <math.h>
#include <errno.h>
#include <limits.h>
double
pow(double x, double y) /* return x ^ y (exponentiation) */
{
double xy, y1, ye;
long i;
int ex, ey, flip;
if(y == 0.0)
return 1.0;
flip = 0;
if(y < 0.){
y = -y;
flip = 1;
}
y1 = modf(y, &ye);
if(y1 != 0.0){
if(x <= 0.)
goto zreturn;
if(y1 > 0.5) {
y1 -= 1.;
ye += 1.;
}
xy = exp(y1 * log(x));
}else
xy = 1.0;
if(ye > LONG_MAX){
if(x <= 0.){
zreturn:
if(x || flip)
errno = EDOM;
return 0.;
}
if(flip){
if(y == .5)
return 1./sqrt(x);
y = -y;
}else if(y == .5)
return sqrt(x);
return exp(y * log(x));
}
x = frexp(x, &ex);
ey = 0;
i = ye;
if(i)
for(;;){
if(i & 1){
xy *= x;
ey += ex;
}
i >>= 1;
if(i == 0)
break;
x *= x;
ex <<= 1;
if(x < .5){
x += x;
ex -= 1;
}
}
if(flip){
xy = 1. / xy;
ey = -ey;
}
errno = 0;
x = ldexp(xy, ey);
if(errno && ey < 0) {
errno = 0;
x = 0.;
}
return x;
}
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