cellPolygons filter out empty cells and have the cell index as a prop…

…erty

fixes #106

(Also fixes the test case, in which the clipped cell had 0 point but was not null, resulting in [undefined, undefined] values.)

README.md

@@ -189,7 +189,7 @@ Renders the cell with the specified index *i* to the specified *context*. The sp
 
 <a href="#voronoi_cellPolygons" name="voronoi_cellPolygons">#</a> <i>voronoi</i>.<b>cellPolygons</b>() [<>](https://github.com/d3/d3-delaunay/blob/master/src/voronoi.js "Source")
 
-Returns an iterable over the [polygons for each cell](#voronoi_cellPolygon), in order.
+Returns an iterable over the non-empty [polygons for each cell](#voronoi_cellPolygon), with the cell index as property.
 
 <a href="#voronoi_cellPolygon" name="voronoi_cellPolygon">#</a> <i>voronoi</i>.<b>cellPolygon</b>(<i>i</i>) [<>](https://github.com/d3/d3-delaunay/blob/master/src/voronoi.js "Source")
 

src/voronoi.js

@@ -107,7 +107,7 @@ export default class Voronoi {
   renderCell(i, context) {
     const buffer = context == null ? context = new Path : undefined;
     const points = this._clip(i);
-    if (points === null) return;
+    if (points === null || !points.length) return;
     context.moveTo(points[0], points[1]);
     let n = points.length;
     while (points[0] === points[n-2] && points[1] === points[n-1] && n > 1) n -= 2;
@@ -122,7 +122,7 @@ export default class Voronoi {
     const {delaunay: {points}} = this;
     for (let i = 0, n = points.length / 2; i < n; ++i) {
       const cell = this.cellPolygon(i);
-      if (cell) yield cell;
+      if (cell) cell.index = i, yield cell;
     }
   }
   cellPolygon(i) {

test/voronoi-test.js

@@ -78,3 +78,12 @@ tape("a degenerate triangle is avoided", test => {
   const voronoi = Delaunay.from(pts).voronoi([10, 10, 960, 500]);
   test.equal(voronoi.cellPolygon(0).length, 4);
 });
+
+tape("cellPolygons filter out empty cells and have the cell index as a property", test => {
+  const pts = [[0, 0], [3, 3], [1, 1], [-3, -2]];
+  const voronoi = Delaunay.from(pts).voronoi([0, 0, 2, 2]);
+  test.deepEqual([...voronoi.cellPolygons()], [
+    Object.assign([[0, 0], [1, 0], [0, 1], [0, 0]], {index:0, }),
+    Object.assign([[0, 1], [1, 0], [2, 0], [2, 2], [0, 2], [0, 1]], { index: 2 })
+  ]);
+});