complex divide: match C99 implementation

R=iant, ken2, r, r2, ken3
CC=golang-dev
http://codereview.appspot.com/1686044

1 files changed, 47 insertions, 23 deletions

diff --git a/src/pkg/runtime/complex.c b/src/pkg/runtime/complex.c
index ca6ed79ba..2240d9fb8 100644
--- a/src/pkg/runtime/complex.c
+++ b/src/pkg/runtime/complex.c
@@ -4,33 +4,57 @@
 
 #include "runtime.h"
 
-// complex128div(num, den complex128) (quo complex128)
+typedef struct Complex128 Complex128;
+
 void
-·complex128div(float64 numreal, float64 numimag,
-	float64 denreal, float64 denimag,
-	float64 quoreal, float64 quoimag)
+·complex128div(Complex128 n, Complex128 d, Complex128 q)
 {
+	int32 ninf, dinf, nnan, dnan;
 	float64 a, b, ratio, denom;
 
-	a = denreal;
-	if(a < 0)
-		a = -a;
-	b = denimag;
-	if(b < 0)
-		b = -b;
-	if(a <= b) {
-		if(b == 0)
-			panicstring("complex divide by zero");
-		ratio = denreal/denimag;
-		denom = denreal*ratio + denimag;
-		quoreal = (numreal*ratio + numimag) / denom;
-		quoimag = (numimag*ratio - numreal) / denom;
+	// Special cases as in C99.
+	ninf = isInf(n.real, 0) || isInf(n.imag, 0);
+	dinf = isInf(d.real, 0) || isInf(d.imag, 0);
+
+	nnan = !ninf && (isNaN(n.real) || isNaN(n.imag));
+	dnan = !dinf && (isNaN(d.real) || isNaN(d.imag));
+
+	if(nnan || dnan) {
+		q.real = NaN();
+		q.imag = NaN();
+	} else if(ninf && !dinf && !dnan) {
+		q.real = Inf(0);
+		q.imag = Inf(0);
+	} else if(!ninf && !nnan && dinf) {
+		q.real = 0;
+		q.imag = 0;
+	} else if(d.real == 0 && d.imag == 0) {
+		if(n.real == 0 && n.imag == 0) {
+			q.real = NaN();
+			q.imag = NaN();
+		} else {
+			q.real = Inf(0);
+			q.imag = Inf(0);
+		}
 	} else {
-		ratio = denimag/denreal;
-		denom = denimag*ratio + denreal;
-		quoreal = (numimag*ratio + numreal) / denom;
-		quoimag = (numimag - numreal*ratio) / denom;
+		// Standard complex arithmetic, factored to avoid unnecessary overflow.
+		a = d.real;
+		if(a < 0)
+			a = -a;
+		b = d.imag;
+		if(b < 0)
+			b = -b;
+		if(a <= b) {
+			ratio = d.real/d.imag;
+			denom = d.real*ratio + d.imag;
+			q.real = (n.real*ratio + n.imag) / denom;
+			q.imag = (n.imag*ratio - n.real) / denom;
+		} else {
+			ratio = d.imag/d.real;
+			denom = d.imag*ratio + d.real;
+			q.real = (n.imag*ratio + n.real) / denom;
+			q.imag = (n.imag - n.real*ratio) / denom;
+		}
 	}
-	FLUSH(&quoreal);
-	FLUSH(&quoimag);
+	FLUSH(&q);
 }