Don’t generate empty polygons during clipping.

Fixes #1823; spurious closePath events were being generated for
degenerate polygons due to generation of empty polygons and rings in
rare cases.

d3.js

@@ -3224,7 +3224,6 @@
           clip.lineEnd = ringEnd;
           segments = [];
           polygon = [];
-          listener.polygonStart();
         },
         polygonEnd: function() {
           clip.point = point;
@@ -3233,13 +3232,15 @@
           segments = d3.merge(segments);
           var clipStartInside = d3_geo_pointInPolygon(rotatedClipStart, polygon);
           if (segments.length) {
+            if (!polygonStarted) listener.polygonStart(), polygonStarted = true;
             d3_geo_clipPolygon(segments, d3_geo_clipSort, clipStartInside, interpolate, listener);
           } else if (clipStartInside) {
+            if (!polygonStarted) listener.polygonStart(), polygonStarted = true;
             listener.lineStart();
             interpolate(null, null, 1, listener);
             listener.lineEnd();
           }
-          listener.polygonEnd();
+          if (polygonStarted) listener.polygonEnd(), polygonStarted = false;
           segments = polygon = null;
         },
         sphere: function() {
@@ -3267,7 +3268,7 @@
         line.lineEnd();
       }
       var segments;
-      var buffer = d3_geo_clipBufferListener(), ringListener = clipLine(buffer), polygon, ring;
+      var buffer = d3_geo_clipBufferListener(), ringListener = clipLine(buffer), polygonStarted = false, polygon, ring;
       function pointRing(λ, φ) {
         ring.push([ λ, φ ]);
         var point = rotate(λ, φ);
@@ -3288,9 +3289,12 @@
         if (clean & 1) {
           segment = ringSegments[0];
           var n = segment.length - 1, i = -1, point;
-          listener.lineStart();
-          while (++i < n) listener.point((point = segment[i])[0], point[1]);
-          listener.lineEnd();
+          if (n > 0) {
+            if (!polygonStarted) listener.polygonStart(), polygonStarted = true;
+            listener.lineStart();
+            while (++i < n) listener.point((point = segment[i])[0], point[1]);
+            listener.lineEnd();
+          }
           return;
         }
         if (n > 1 && clean & 2) ringSegments.push(ringSegments.pop().concat(ringSegments.shift()));

src/geo/clip.js

@@ -18,7 +18,6 @@ function d3_geo_clip(pointVisible, clipLine, interpolate, clipStart) {
         clip.lineEnd = ringEnd;
         segments = [];
         polygon = [];
-        listener.polygonStart();
       },
       polygonEnd: function() {
         clip.point = point;
@@ -28,13 +27,15 @@ function d3_geo_clip(pointVisible, clipLine, interpolate, clipStart) {
         segments = d3.merge(segments);
         var clipStartInside = d3_geo_pointInPolygon(rotatedClipStart, polygon);
         if (segments.length) {
+          if (!polygonStarted) listener.polygonStart(), polygonStarted = true;
           d3_geo_clipPolygon(segments, d3_geo_clipSort, clipStartInside, interpolate, listener);
         } else if (clipStartInside) {
+          if (!polygonStarted) listener.polygonStart(), polygonStarted = true;
           listener.lineStart();
           interpolate(null, null, 1, listener);
           listener.lineEnd();
         }
-        listener.polygonEnd();
+        if (polygonStarted) listener.polygonEnd(), polygonStarted = false;
         segments = polygon = null;
       },
       sphere: function() {
@@ -61,6 +62,7 @@ function d3_geo_clip(pointVisible, clipLine, interpolate, clipStart) {
 
     var buffer = d3_geo_clipBufferListener(),
         ringListener = clipLine(buffer),
+        polygonStarted = false,
         polygon,
         ring;
 
@@ -96,9 +98,12 @@ function d3_geo_clip(pointVisible, clipLine, interpolate, clipStart) {
         var n = segment.length - 1,
             i = -1,
             point;
-        listener.lineStart();
-        while (++i < n) listener.point((point = segment[i])[0], point[1]);
-        listener.lineEnd();
+        if (n > 0) {
+          if (!polygonStarted) listener.polygonStart(), polygonStarted = true;
+          listener.lineStart();
+          while (++i < n) listener.point((point = segment[i])[0], point[1]);
+          listener.lineEnd();
+        }
         return;
       }
 

test/geo/path-test.js

@@ -386,6 +386,10 @@ suite.addBatch({
       "renders a small circle of 120°": function(p) {
         p(_.geo.circle().angle(120)());
         assert.deepEqual(testContext.buffer().filter(function(d) { return d.type === "moveTo"; }), [{type: "moveTo", x: 87, y: 700}]);
+      },
+      "degenerate polygon": function(p) {
+        p({type: "Polygon", coordinates: [[[0, 0], [0, 0], [0, 0], [0, 0]]]});
+        assert.deepEqual(testContext.buffer(), []);
       }
     },