20060303a

1 files changed, 68 insertions, 0 deletions

diff --git a/libkern/pow.c b/libkern/pow.c
new file mode 100644
index 00000000..2dd39143
--- /dev/null
+++ b/libkern/pow.c
@@ -0,0 +1,68 @@
+#include <lib9.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 > 0x7FFFFFFF){	/* should be ~0UL but compiler can't convert double to ulong */
+		if(x <= 0.){
+ zreturn:
+			if(x==0. && !flip)
+				return 0.;
+			return NaN();
+		}
+		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;
+	}
+	return ldexp(xy, ey);
+}