s2: Add type tags to the Shape interface.

This begins the support for different encoding mechanisms and options,
mirroring C++.

Signed-off-by: David Symonds <dsymonds@golang.org>

s2/edge_vector_shape_test.go

@@ -51,4 +51,5 @@ func (e *edgeVectorShape) ChainPosition(edgeID int) ChainPosition { return Chain
 func (e *edgeVectorShape) IsEmpty() bool                          { return defaultShapeIsEmpty(e) }
 func (e *edgeVectorShape) IsFull() bool                           { return defaultShapeIsFull(e) }
 func (e *edgeVectorShape) Dimension() int                         { return 1 }
+func (e *edgeVectorShape) typeTag() typeTag                       { return typeTagNone }
 func (e *edgeVectorShape) privateInterface()                      {}

s2/lax_loop_test.go

@@ -79,4 +79,5 @@ func (l *laxLoop) ChainEdge(i, j int) Edge {
 func (l *laxLoop) ChainPosition(e int) ChainPosition { return ChainPosition{0, e} }
 func (l *laxLoop) IsEmpty() bool                     { return defaultShapeIsEmpty(l) }
 func (l *laxLoop) IsFull() bool                      { return defaultShapeIsFull(l) }
+func (l *laxLoop) typeTag() typeTag                  { return typeTagNone }
 func (l *laxLoop) privateInterface()                 {}

s2/lax_polygon_test.go

@@ -167,6 +167,7 @@ func (p *laxPolygon) Edge(e int) Edge {
 }
 
 func (p *laxPolygon) Dimension() int                 { return 2 }
+func (p *laxPolygon) typeTag() typeTag               { return typeTagLaxPolygon }
 func (p *laxPolygon) privateInterface()              {}
 func (p *laxPolygon) IsEmpty() bool                  { return defaultShapeIsEmpty(p) }
 func (p *laxPolygon) IsFull() bool                   { return defaultShapeIsFull(p) }

s2/lax_polyline_test.go

@@ -46,4 +46,5 @@ func (l *laxPolyline) ChainPosition(e int) ChainPosition { return ChainPosition{
 func (l *laxPolyline) Dimension() int                    { return 1 }
 func (l *laxPolyline) IsEmpty() bool                     { return defaultShapeIsEmpty(l) }
 func (l *laxPolyline) IsFull() bool                      { return defaultShapeIsFull(l) }
+func (l *laxPolyline) typeTag() typeTag                  { return typeTagLaxPolyline }
 func (l *laxPolyline) privateInterface()                 {}

s2/loop.go

@@ -506,6 +506,8 @@ func (l *Loop) ChainPosition(edgeID int) ChainPosition {
 // Dimension returns the dimension of the geometry represented by this Loop.
 func (l *Loop) Dimension() int { return 2 }
 
+func (l *Loop) typeTag() typeTag { return typeTagNone }
+
 func (l *Loop) privateInterface() {}
 
 // IsEmpty reports true if this is the special empty loop that contains no points.

s2/point_vector.go

@@ -38,4 +38,5 @@ func (p *PointVector) ChainPosition(e int) ChainPosition { return ChainPosition{
 func (p *PointVector) Dimension() int                    { return 0 }
 func (p *PointVector) IsEmpty() bool                     { return defaultShapeIsEmpty(p) }
 func (p *PointVector) IsFull() bool                      { return defaultShapeIsFull(p) }
+func (p *PointVector) typeTag() typeTag                  { return typeTagPointVector }
 func (p *PointVector) privateInterface()                 {}

s2/polygon.go

@@ -814,6 +814,8 @@ func (p *Polygon) ChainPosition(edgeID int) ChainPosition {
 // Dimension returns the dimension of the geometry represented by this Polygon.
 func (p *Polygon) Dimension() int { return 2 }
 
+func (p *Polygon) typeTag() typeTag { return typeTagPolygon }
+
 func (p *Polygon) privateInterface() {}
 
 // Contains reports whether this polygon contains the other polygon.

s2/polyline.go

@@ -217,6 +217,8 @@ func (p *Polyline) IsEmpty() bool { return defaultShapeIsEmpty(p) }
 // IsFull reports whether this shape contains all points on the sphere.
 func (p *Polyline) IsFull() bool { return defaultShapeIsFull(p) }
 
+func (p *Polyline) typeTag() typeTag { return typeTagPolyline }
+
 func (p *Polyline) privateInterface() {}
 
 // findEndVertex reports the maximal end index such that the line segment between

s2/shape.go

@@ -114,6 +114,23 @@ func OriginReferencePoint(contained bool) ReferencePoint {
 	return ReferencePoint{Point: OriginPoint(), Contained: contained}
 }
 
+// typeTag is a 32-bit tag that can be used to identify the type of an encoded
+// Shape. All encodable types have a non-zero type tag. The tag associated with
+type typeTag uint32
+
+const (
+	// Indicates that a given Shape type cannot be encoded.
+	typeTagNone        typeTag = 0
+	typeTagPolygon     typeTag = 1
+	typeTagPolyline    typeTag = 2
+	typeTagPointVector typeTag = 3
+	typeTagLaxPolyline typeTag = 4
+	typeTagLaxPolygon  typeTag = 5
+
+	// The minimum allowable tag for future user-defined Shape types.
+	typeTagMinUser typeTag = 8192
+)
+
 // Shape represents polygonal geometry in a flexible way. It is organized as a
 // collection of edges that optionally defines an interior. All geometry
 // represented by a given Shape must have the same dimension, which means that
@@ -220,6 +237,10 @@ type Shape interface {
 	// IsFull reports whether the Shape contains all points on the sphere.
 	IsFull() bool
 
+	// typeTag returns a value that can be used to identify the type of an
+	// encoded Shape.
+	typeTag() typeTag
+
 	// We do not support implementations of this interface outside this package.
 	privateInterface()
 }

s2/shapeindex.go

@@ -1514,7 +1514,3 @@ func maxLevelForEdge(edge Edge) int {
 func (s *ShapeIndex) removeShapeInternal(removed *removedShape, allEdges [][]faceEdge, t *tracker) {
 	// TODO(roberts): finish the implementation of this.
 }
-
-// TODO(roberts): Differences from C++.
-// ShapeContainsPoint
-// FindContainingShapes