add support to convert from HilbertQuadkey to s2Cell.

src/s2geometry.js

@@ -200,6 +200,30 @@ S2.IJToST = function(ij,order,offsets) {
   ];
 };
 
+
+
+var rotateAndFlipQuadrant = function(n, point, rx, ry)
+{
+	var newX, newY;
+	if(ry == 0)
+	{
+		if(rx == 1){
+			point.x = n - 1 - point.x;
+			point.y = n - 1 - point.y
+
+		}
+
+    var x = point.x;
+		point.x = point.y
+		point.y = x;
+	}
+
+}
+
+
+
+
+
 // hilbert space-filling curve
 // based on http://blog.notdot.net/2009/11/Damn-Cool-Algorithms-Spatial-indexing-with-Quadtrees-and-Hilbert-Curves
 // note: rather then calculating the final integer hilbert position, we just return the list of quads
@@ -240,6 +264,55 @@ var pointToHilbertQuadList = function(x,y,order,face) {
 
 S2.S2Cell = function(){};
 
+S2.S2Cell.FromHilbertQuadKey = function(hilbertQuadkey) {
+  var parts = hilbertQuadkey.split('/');
+  var face = parseInt(parts[0]);
+  var position = parts[1];
+  var maxLevel = position.length;
+  var point = {
+    x : 0,
+    y: 0
+  }
+
+	for(var i = maxLevel - 1; i >= 0; i--)
+	{
+		
+		var level = maxLevel - i;
+		var bit = position[i];
+		var rx = 0, ry = 0;
+		if(bit == '1')
+		{
+			ry = 1;
+		}	
+		else if(bit == '2')
+		{
+			rx = 1;
+			ry = 1;
+		}	
+		else if(bit == '3')
+		{
+			rx = 1;
+		}
+		
+		var val = Math.pow(2, level - 1)
+		rotateAndFlipQuadrant(val, point, rx, ry);
+		
+		point.x += val * rx;
+		point.y += val * ry;
+
+	}
+  
+  if(face % 2 == 1)
+  {
+    var t = point.x;
+    point.x = point.y;
+    point.y = t;
+  }
+
+
+  return S2.S2Cell.FromFaceIJ(parseInt(face),[point.x, point.y],level)
+
+}
 //static method to construct
 S2.S2Cell.FromLatLng = function(latLng, level) {
   if (latLng.lat == null ||  latLng.lng == null) {

tests/hilbertToCell.js

@@ -0,0 +1,43 @@
+var S2 = require('../src/s2geometry.js').S2;
+
+
+
+
+for(var level = 1; level <= 20; level++)
+{
+	var success = 0;
+	var total = 0;
+	for (var x = -180.0; x < 180; x += 0.5) {
+		for (var y = -180.0; y < 180; y += 0.5) {
+
+				var latlng = { lat: x, lng: y };
+				var cell = S2.S2Cell.FromLatLng(latlng, level);
+				var quadKey = cell.toHilbertQuadkey();
+				var cell2 = S2.S2Cell.FromHilbertQuadKey(quadKey);
+
+				if(cell.face != cell2.face || 
+					cell.ij[0] != cell2.ij[0] ||
+					cell.ij[1] != cell2.ij[1] ||
+					cell.level != cell2.level)
+					{
+						/*console.log({
+							cell: cell,
+							cell2: cell2})*/
+						
+					}
+					else
+					{
+						success++;
+					}
+					total++;
+				// check equal
+		}
+	}
+
+	console.log("level:" + level + "\t total:" + total + "\t success:" + success)
+}
+
+
+
+
+