Introduce new APIs,

Polygon APIs,
-Project,
-ProjectToBoundary

Edge Query APIs,
-GetEdge
-Project

Author: Sreekanth Sivasankaran

s2/edge_query.go

@@ -798,6 +798,19 @@ func (e *EdgeQuery) processOrEnqueue(id CellID, indexCell *ShapeIndexCell) {
 	})
 }
 
+func (e *EdgeQuery) GetEdge(result EdgeQueryResult) Edge {
+	return e.index.Shape(result.shapeID).Edge(int(result.edgeID))
+}
+
+func (e *EdgeQuery) Project(point Point, result EdgeQueryResult) Point {
+	if result.edgeID < 0 {
+		return point
+	}
+
+	edge := e.GetEdge(result)
+	return Project(point, edge.V0, edge.V1)
+}
+
 // TODO(roberts): Remaining pieces
 // GetEdge
 // Project

s2/polygon.go

@@ -1184,6 +1184,21 @@ func (p *Polygon) decodeCompressed(d *decoder) {
 	p.initLoopProperties()
 }
 
+func (p *Polygon) Project(point *Point) Point {
+	if p.ContainsPoint(*point) {
+		return *point
+	}
+	return p.ProjectToBoundary(point)
+}
+
+func (p *Polygon) ProjectToBoundary(point *Point) Point {
+	options := NewClosestEdgeQueryOptions().MaxResults(1).IncludeInteriors(false)
+	q := NewClosestEdgeQuery(p.index, options)
+	target := NewMinDistanceToPointTarget(*point)
+	edges := q.FindEdges(target)
+	return q.Project(*point, edges[0])
+}
+
 // TODO(roberts): Differences from C++
 // Centroid
 // SnapLevel

s2/polygon_test.go

@@ -1124,6 +1124,29 @@ func TestPolygonInvert(t *testing.T) {
 	}
 }
 
+func TestPolygonProject(t *testing.T) {
+	polygon := makePolygon(nearLoop0+nearLoop2, true)
+	point := PointFromLatLng(LatLngFromDegrees(1.1, 0))
+	projected := polygon.Project(&point)
+	if !projected.ApproxEqual(point) {
+		t.Errorf("projection %v failed for %v", projected, point)
+	}
+
+	point = PointFromLatLng(LatLngFromDegrees(5.1, -2))
+	projected = polygon.Project(&point)
+	expected := PointFromLatLng(LatLngFromDegrees(5, -2))
+	if !projected.ApproxEqual(expected) {
+		t.Errorf("projection %v failed for %v", projected, expected)
+	}
+
+	point = PointFromLatLng(LatLngFromDegrees(0, -3))
+	projected = polygon.Project(&point)
+	expected = PointFromLatLng(LatLngFromDegrees(0, -2))
+	if !projected.ApproxEqual(expected) {
+		t.Errorf("projection %v failed for %v", projected, expected)
+	}
+}
+
 // TODO(roberts): Remaining Tests
 // TestInit
 // TestMultipleInit