Merge pull request #543 from rshura/fix-clip-empty-rings

Skip empty rings when clipping polygons.

internal/clip/clip_test.go

@@ -13,8 +13,8 @@ func LO(points []geometry.Point) *geojson.LineString {
 
 func RO(minX, minY, maxX, maxY float64) *geojson.Rect {
 	return geojson.NewRect(geometry.Rect{
-		Min: geometry.Point{X: 1.5, Y: 0.5},
-		Max: geometry.Point{X: 2.5, Y: 1.8},
+		Min: geometry.Point{X: minX, Y: minY},
+		Max: geometry.Point{X: maxX, Y: maxY},
 	})
 }
 
@@ -61,8 +61,43 @@ func TestClipPolygonSimple(t *testing.T) {
 	if !ok {
 		t.Fatal("wrong type")
 	}
-	if !cp.Base().Exterior.Empty() && len(cp.Base().Holes) != 1 {
-		t.Fatal("result must have two parts in Polygon")
+	if cp.Base().Exterior.Empty() {
+		t.Fatal("Empty result.")
+	}
+	if len(cp.Base().Holes) != 1 {
+		t.Fatal("result must be a two-ring Polygon")
+	}
+}
+
+func TestClipPolygon2(t *testing.T) {
+	exterior := []geometry.Point{
+		{X: 2, Y: 2},
+		{X: 1, Y: 2},
+		{X: 1.5, Y: 1.5},
+		{X: 1, Y: 1},
+		{X: 2, Y: 1},
+		{X: 2, Y: 2},
+	}
+	holes := [][]geometry.Point{
+		[]geometry.Point{
+			{X: 1.9, Y: 1.9},
+			{X: 1.2, Y: 1.9},
+			{X: 1.45, Y: 1.65},
+			{X: 1.9, Y: 1.5},
+			{X: 1.9, Y: 1.9},
+		},
+	}
+	polygon := PPO(exterior, holes)
+	clipped := Clip(polygon, RO(1.1, 0.8, 1.15, 2.1))
+	cp, ok := clipped.(*geojson.Polygon)
+	if !ok {
+		t.Fatal("wrong type")
+	}
+	if cp.Base().Exterior.Empty() {
+		t.Fatal("Empty result.")
+	}
+	if len(cp.Base().Holes) != 0 {
+		t.Fatal("result must be a single-ring Polygon")
 	}
 }
 

internal/clip/polygon.go

@@ -18,7 +18,9 @@ func clipPolygon(
 		for i := 0; i < len(ringPoints); i++ {
 			ringPoints[i] = ring.PointAt(i)
 		}
-		newPoints = append(newPoints, clipRing(ringPoints, rect))
+		if clippedRing := clipRing(ringPoints, rect); len(clippedRing) > 0 {
+			newPoints = append(newPoints, clippedRing)
+		}
 	}
 
 	var exterior []geometry.Point