add id conversion

README.md

@@ -1,7 +1,7 @@
 s2-geometry (JavaScript/ES5.1)
 ======================
 
-A pure JavaScript/ES5.1 port of Google/Niantic's S2 Geometry library (used by Ingress, Pokemon GO)
+A pure JavaScript/ES5.1 port of Google/Niantic's S2 Geometry library (used by **Ingress**, **Pokemon GO**)
 
 Currently contains basic support for S2Cell
 
@@ -17,3 +17,38 @@ cell.getNeighbors();  // [ cellLeft, cellDown, cellRight, cellUp ]
 
 cell.getLatLng();     // { lat: 40.2574448, lng: -111.7089464 }
 ```
+
+convert Cell Id to Quadkey
+------------------
+
+Convert from base 10 (decimal) `S2 Cell Id` to base 4 `quadkey` (aka hilbert curve quadtree id)
+
+Example '4/032212303102210' becomes '9749618446378729472'
+
+```
+'use strict';
+
+var quadkey = '4/032212303102210'
+var parts = quadkey.split('/');
+var face = parts[0];                  // 4
+var position = parts[1];              // '032212303102210';
+var level = '032212303102210'.length; // 15
+
+var cellId = S2.fromFacePosLevel(face, position, level);
+
+console.log(cellId);
+```
+
+Convert from hilbert quadtree id to s2 cell id:
+
+Example '9749618446378729472' becomes '4/032212303102210'
+
+```
+'use strict';
+
+var cellId = '9749618446378729472';
+
+var hilbertQuadkey = S2.toHilbertQuadkey(cellId);
+
+console.log(hilbertQuadkey);
+```

package.json

@@ -1,6 +1,6 @@
 {
   "name": "s2-geometry",
-  "version": "1.0.0",
+  "version": "1.1.0",
   "description": "A pure JavaScript/ES5.1 port of Google/Niantic's S2 Geometry library (used by Ingress, Pokemon GO)",
   "main": "src/s2geometry.js",
   "scripts": {
@@ -32,5 +32,8 @@
   "bugs": {
     "url": "https://github.com/coolaj86/s2-geometry-javascript/issues"
   },
-  "homepage": "https://github.com/coolaj86/s2-geometry-javascript#readme"
+  "homepage": "https://github.com/coolaj86/s2-geometry-javascript#readme",
+  "dependencies": {
+    "long": "^3.2.0"
+  }
 }

src/s2geometry.js

@@ -298,6 +298,58 @@ S2.S2Cell.prototype.getNeighbors = function() {
 
 };
 
+//
+// Functional Style
+//
+S2.FACE_BITS = 3;
+S2.MAX_LEVEL = 30;
+S2.POS_BITS = (2 * S2.MAX_LEVEL) + 1;
+
+S2.fromFacePosLevel = function (faceN, posS, levelN) {
+  var Long = require('long');
+
+  if (!levelN) {
+    levelN = posS.length;
+  }
+  if (posS.length > levelN) {
+    posS = posS.substr(0, levelN);
+  }
+
+  var posB = Long.fromString(posS, true, 4).toString(2);
+  while (posB.length < (2 * levelN)) {
+    posB = '0' + posB;
+  }
+  var bin = Long.fromString(faceN.toString(10), true, 10).toString(2);
+  bin += posB;
+  bin += '1';
+  while (bin.length < (S2.FACE_BITS + S2.POS_BITS)) {
+    bin += '0';
+  }
+
+  return Long.fromString(bin, true, 2).toString();
+};
+
+S2.toHilbertQuadkey = function (idS) {
+  var Long = require('long');
+  var bin = Long.fromString(idS, true, 10).toString(2);
+  var lsbIndex = bin.lastIndexOf('1');
+
+  // substr(start, len)
+  // substring(start, end)
+  var faceB = bin.substr(0, 3);
+  // posB will always be a multiple of 2 (or it's invalid)
+  var posB = bin.substring(4, lsbIndex);
+  var levelN = posB.length / 2;
+
+  var faceS = Long.fromString(faceB, true, 2).toString(10);
+  var posS = Long.fromString(posB, true, 2).toString(4);
+
+  while (posS.length < levelN) {
+    posS = '0' + posS;
+  }
+
+  return faceS + '/' + posS;
+};
 
 })('undefined' !== typeof window ? window : module.exports);
 

tests/ids.js

@@ -0,0 +1,9 @@
+'use strict';
+
+var S2 = require('../src/s2geometry').S2;
+
+var cellId = S2.fromFacePosLevel(4, '032212303102210');
+var hilbertQuadkey = S2.toHilbertQuadkey('9749618446378729472');
+
+console.log(cellId, '9749618446378729472' === cellId);
+console.log(hilbertQuadkey, '4/032212303102210' === hilbertQuadkey);