Fix #119 arc glitch.

Author: mbostock

src/arc.js

@@ -25,7 +25,9 @@ function arcPadAngle(d) {
 function intersect(x0, y0, x1, y1, x2, y2, x3, y3) {
   var x10 = x1 - x0, y10 = y1 - y0,
       x32 = x3 - x2, y32 = y3 - y2,
-      t = (x32 * (y0 - y2) - y32 * (x0 - x2)) / (y32 * x10 - x32 * y10);
+      t = y32 * x10 - x32 * y10;
+  if (t * t < epsilon) return;
+  t = (x32 * (y0 - y2) - y32 * (x0 - x2)) / t;
   return [x0 + t * x10, y0 + t * y10];
 }
 
@@ -146,12 +148,12 @@ export default function() {
         var x11 = r1 * cos(a11),
             y11 = r1 * sin(a11),
             x00 = r0 * cos(a00),
-            y00 = r0 * sin(a00);
+            y00 = r0 * sin(a00),
+            oc;
 
         // Restrict the corner radius according to the sector angle.
-        if (da < pi) {
-          var oc = da0 > epsilon ? intersect(x01, y01, x00, y00, x11, y11, x10, y10) : [x10, y10],
-              ax = x01 - oc[0],
+        if (da <= tau - epsilon && (oc = intersect(x01, y01, x00, y00, x11, y11, x10, y10))) {
+          var ax = x01 - oc[0],
               ay = y01 - oc[1],
               bx = x11 - oc[0],
               by = y11 - oc[1],