Improve ChildrenInZoomRange implementation

Author: peitilisc

maptile/tile.go

@@ -228,8 +228,11 @@ func (t Tile) Children() Tiles {
 
 // ChildrenInZoomRange returns all the children tiles of tile from ranges [zoomStart, zoomEnd], both ends inclusive.
 func ChildrenInZoomRange(tile Tile, zoomStart, zoomEnd Zoom) Tiles {
-	if zoomStart < tile.Z || zoomEnd < tile.Z {
-		panic("tile.Z is greater than zoomStart or zoomEnd")
+	if !(zoomStart <= zoomEnd) {
+		panic("zoomStart must be <= zoomEnd")
+	}
+	if !(tile.Z <= zoomStart) {
+		panic("tile.Z is must be <= zoomStart")
 	}
 
 	zDeltaStart := zoomStart - tile.Z
@@ -238,11 +241,11 @@ func ChildrenInZoomRange(tile Tile, zoomStart, zoomEnd Zoom) Tiles {
 	res := make([]Tile, 0)
 
 	for d := zDeltaStart; d <= zDeltaEnd; d++ {
-		xTopLeft := tile.X << d
-		yTopLeft := tile.Y << d
-		dim := uint32(math.Pow(2, float64(d)))
-		for x := xTopLeft; x < xTopLeft+dim; x++ {
-			for y := yTopLeft; y < yTopLeft+dim; y++ {
+		xStart := tile.X << d
+		yStart := tile.Y << d
+		dim := uint32(1 << d)
+		for x := xStart; x < xStart+dim; x++ {
+			for y := yStart; y < yStart+dim; y++ {
 				res = append(res, New(x, y, tile.Z+d))
 			}
 		}

maptile/tile_test.go

@@ -235,6 +235,24 @@ func TestChildrenTilesSubRange(t *testing.T) {
 	}
 }
 
+func TestChildrenTilesSubRangeInvalidParams_ZoomStart_Larger_Than_ZoomEnd(t *testing.T) {
+	// No need to check whether `recover()` is nil. Just turn off the panic.
+	defer func() { _ = recover() }()
+	tile := New(0, 0, 0)
+	ChildrenInZoomRange(tile, 10, 8)
+	// Never reaches here if `ChildrenInZoomRange` panics.
+	t.Errorf("did not panic")
+}
+
+func TestChildrenTilesSubRangeInvalidParams_TileZ_Larger_Than_ZoomStart(t *testing.T) {
+	// No need to check whether `recover()` is nil. Just turn off the panic.
+	defer func() { _ = recover() }()
+	tile := New(0, 0, 10)
+	ChildrenInZoomRange(tile, 9, 12)
+	// Never reaches here if `ChildrenInZoomRange` panics.
+	t.Errorf("did not panic")
+}
+
 func TestRange(t *testing.T) {
 	tile := New(4, 4, 5)
 	min, max := tile.Range(3)