Merge pull request #274 from tschaub/utfgrid

UTFGrid Tile, Layer, and Control.  This adds support for responsive handling of interactions with large numbers of features represented by UTFGrids.

lib/OpenLayers/Layer/Grid.js

@@ -419,6 +419,64 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
             }
         }
     },
+
+    /**
+     * Method: getTileData
+     * Given a map location, retrieve a tile and the pixel offset within that
+     *     tile corresponding to the location.  If there is not an existing 
+     *     tile in the grid that covers the given location, null will be 
+     *     returned.
+     *
+     * Parameters:
+     * loc - {<OpenLayers.LonLat>} map location
+     *
+     * Returns:
+     * {Object} Object with the following properties: tile ({<OpenLayers.Tile>}),
+     *     i ({Number} x-pixel offset from top left), and j ({Integer} y-pixel
+     *     offset from top left).
+     */
+    getTileData: function(loc) {
+        var data = null,
+            x = loc.lon,
+            y = loc.lat,
+            numRows = this.grid.length;
+
+        if (this.map && numRows) {
+            var res = this.map.getResolution(),
+                tileWidth = this.tileSize.w,
+                tileHeight = this.tileSize.h,
+                bounds = this.grid[0][0].bounds,
+                left = bounds.left,
+                top = bounds.top;
+
+            if (x < left) {
+                // deal with multiple worlds
+                if (this.map.baseLayer.wrapDateLine) {
+                    var worldWidth = this.map.getMaxExtent().getWidth();
+                    var worldsAway = Math.ceil((left - x) / worldWidth);
+                    x += worldWidth * worldsAway;
+                }
+            }
+            // tile distance to location (fractional number of tiles);
+            var dtx = (x - left) / (res * tileWidth);
+            var dty = (top - y) / (res * tileHeight);
+            // index of tile in grid
+            var col = Math.floor(dtx);
+            var row = Math.floor(dty);
+            if (row >= 0 && row < numRows) {
+                var tile = this.grid[row][col];
+                if (tile) {
+                    data = {
+                        tile: tile,
+                        // pixel index within tile
+                        i: Math.floor((dtx - col) * tileWidth),
+                        j: Math.floor((dty - row) * tileHeight)
+                    };                    
+                }
+            }
+        }
+        return data;
+    },
     
     /**
      * Method: queueTileDraw