1 files changed, 69 insertions, 0 deletions
diff --git a/src/base/math/pow.c b/src/base/math/pow.c
new file mode 100644
index 0000000..912a4e1
--- /dev/null
+++ b/src/base/math/pow.c
@@ -0,0 +1,69 @@
+#include <u.h>
+#include <base.h>
+
+double
+math·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 = math·modf(y, &ye);
+	if(y1 != 0.0){
+		if(x <= 0.)
+			goto zreturn;
+		if(y1 > 0.5) {
+			y1 -= 1.;
+			ye += 1.;
+		}
+		xy = math·exp(y1 * math·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 math·NaN();
+		}
+		if(flip){
+			if(y == .5)
+				return 1./math·sqrt(x);
+			y = -y;
+		}else if(y == .5)
+			return math·sqrt(x);
+		return math·exp(y * math·log(x));
+	}
+	x = math·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 math·ldexp(xy, ey);
+}