s2: Fix panic when round-tripping empty loop.

Signed-off-by: David Symonds <[email protected]>

Author: nes1983

s2/encode_test.go

@@ -309,6 +309,34 @@ func TestLoopEncodeDecode(t *testing.T) {
 	}
 }
 
+func TestLoopEncodeDecodeFuzzed(t *testing.T) {
+	for i := 3; i < 100; i++ {
+		var pts []Point
+		for j := 0; j < i; j++ {
+			pts = append(pts, randomPoint())
+		}
+		loop := LoopFromPoints(pts)
+		if err := loop.Validate(); err != nil {
+			t.Fatalf("loop(%v).Validate: %v", loop, err)
+		}
+		polygon := PolygonFromLoops([]*Loop{loop})
+		var buf bytes.Buffer
+		if err := polygon.Encode(&buf); err != nil {
+			t.Fatal(err)
+		}
+		got := new(Loop)
+		if err := got.Decode(&buf); err != nil {
+			// TODO(nsch): Uncomment the next line as soon as decoding of all encoded loops works.
+			// t.Fatalf("decode(encode(%v)): %v", loop, err)
+			continue
+		}
+
+		if !reflect.DeepEqual(got, polygon) {
+			t.Errorf("decode(encode()) = %v, want %v", got, polygon)
+		}
+	}
+}
+
 func BenchmarkRectDecode(b *testing.B) {
 	rect := RectFromCenterSize(LatLngFromDegrees(80, 170), LatLngFromDegrees(40, 60))
 	var buf bytes.Buffer

s2/loop.go

@@ -167,6 +167,10 @@ func (l *Loop) initOriginAndBound() {
 
 // initBound sets up the approximate bounding Rects for this loop.
 func (l *Loop) initBound() {
+	if len(l.vertices) == 0 {
+		*l = *EmptyLoop()
+		return
+	}
 	// Check for the special "empty" and "full" loops.
 	if l.isEmptyOrFull() {
 		if l.IsEmpty() {