/* Copyright (c) 2006-2011 by OpenLayers Contributors (see authors.txt for 
 * full list of contributors). Published under the Clear BSD license.  
 * See http://svn.openlayers.org/trunk/openlayers/license.txt for the
 * full text of the license. */


/**
 * @requires OpenLayers/Layer/HTTPRequest.js
 * @requires OpenLayers/Console.js
 */

/**
 * Class: OpenLayers.Layer.Grid
 * Base class for layers that use a lattice of tiles.  Create a new grid
 * layer with the <OpenLayers.Layer.Grid> constructor.
 *
 * Inherits from:
 *  - <OpenLayers.Layer.HTTPRequest>
 */
OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
    
    /**
     * APIProperty: tileSize
     * {<OpenLayers.Size>}
     */
    tileSize: null,

    /**
     * Property: tileOriginCorner
     * {String} If the <tileOrigin> property is not provided, the tile origin 
     *     will be derived from the layer's <maxExtent>.  The corner of the 
     *     <maxExtent> used is determined by this property.  Acceptable values
     *     are "tl" (top left), "tr" (top right), "bl" (bottom left), and "br"
     *     (bottom right).  Default is "bl".
     */
    tileOriginCorner: "bl",
    
    /**
     * APIProperty: tileOrigin
     * {<OpenLayers.LonLat>} Optional origin for aligning the grid of tiles.
     *     If provided, requests for tiles at all resolutions will be aligned
     *     with this location (no tiles shall overlap this location).  If
     *     not provided, the grid of tiles will be aligned with the layer's
     *     <maxExtent>.  Default is ``null``.
     */
    tileOrigin: null,
    
    /** APIProperty: tileOptions
     *  {Object} optional configuration options for <OpenLayers.Tile> instances
     *  created by this Layer, if supported by the tile class.
     */
    tileOptions: null,
    
    /**
     * Property: grid
     * {Array(Array(<OpenLayers.Tile>))} This is an array of rows, each row is 
     *     an array of tiles.
     */
    grid: null,

    /**
     * APIProperty: singleTile
     * {Boolean} Moves the layer into single-tile mode, meaning that one tile 
     *     will be loaded. The tile's size will be determined by the 'ratio'
     *     property. When the tile is dragged such that it does not cover the 
     *     entire viewport, it is reloaded.
     */
    singleTile: false,

    /** APIProperty: ratio
     *  {Float} Used only when in single-tile mode, this specifies the 
     *          ratio of the size of the single tile to the size of the map.
     */
    ratio: 1.5,

    /**
     * APIProperty: buffer
     * {Integer} Used only when in gridded mode, this specifies the number of 
     *           extra rows and colums of tiles on each side which will
     *           surround the minimum grid tiles to cover the map.
     *           For very slow loading layers, a larger value may increase
     *           performance somewhat when dragging, but will increase bandwidth
     *           use significantly. 
     */
    buffer: 0,

    /**
     * APIProperty: numLoadingTiles
     * {Integer} How many tiles are still loading?
     */
    numLoadingTiles: 0,

    /**
     * APIProperty: tileLoadingDelay
     * {Integer} - Number of milliseconds before we shift and load
     *     tiles. Default is 100.
     */
    tileLoadingDelay: 100,

    /**
     * Property: timerId
     * {Number} - The id of the tileLoadingDelay timer.
     */
    timerId: null,

    /**
     * Constructor: OpenLayers.Layer.Grid
     * Create a new grid layer
     *
     * Parameters:
     * name - {String}
     * url - {String}
     * params - {Object}
     * options - {Object} Hashtable of extra options to tag onto the layer
     */
    initialize: function(name, url, params, options) {
        OpenLayers.Layer.HTTPRequest.prototype.initialize.apply(this, 
                                                                arguments);
        
        //grid layers will trigger 'tileloaded' when each new tile is 
        // loaded, as a means of progress update to listeners.
        // listeners can access 'numLoadingTiles' if they wish to keep track
        // of the loading progress
        //
        this.events.addEventType("tileloaded");

        this.grid = [];
        
        this._moveGriddedTiles = OpenLayers.Function.bind(
            this.moveGriddedTiles, this
        );
    },

    /**
     * Method: removeMap
     * Called when the layer is removed from the map.
     *
     * Parameters:
     * map - {<OpenLayers.Map>} The map.
     */
    removeMap: function(map) {
        if(this.timerId != null) {
            window.clearTimeout(this.timerId);
            this.timerId = null;
        }
    },

    /**
     * APIMethod: destroy
     * Deconstruct the layer and clear the grid.
     */
    destroy: function() {
        this.clearGrid();
        this.grid = null;
        this.tileSize = null;
        OpenLayers.Layer.HTTPRequest.prototype.destroy.apply(this, arguments); 
    },

    /**
     * Method: clearGrid
     * Go through and remove all tiles from the grid, calling
     *    destroy() on each of them to kill circular references
     */
    clearGrid:function() {
        if (this.grid) {
            for(var iRow=0, len=this.grid.length; iRow<len; iRow++) {
                var row = this.grid[iRow];
                for(var iCol=0, clen=row.length; iCol<clen; iCol++) {
                    var tile = row[iCol];
                    this.removeTileMonitoringHooks(tile);
                    tile.destroy();
                }
            }
            this.grid = [];
        }
    },

    /**
     * APIMethod: clone
     * Create a clone of this layer
     *
     * Parameters:
     * obj - {Object} Is this ever used?
     * 
     * Returns:
     * {<OpenLayers.Layer.Grid>} An exact clone of this OpenLayers.Layer.Grid
     */
    clone: function (obj) {
        
        if (obj == null) {
            obj = new OpenLayers.Layer.Grid(this.name,
                                            this.url,
                                            this.params,
                                            this.getOptions());
        }

        //get all additions from superclasses
        obj = OpenLayers.Layer.HTTPRequest.prototype.clone.apply(this, [obj]);

        // copy/set any non-init, non-simple values here
        if (this.tileSize != null) {
            obj.tileSize = this.tileSize.clone();
        }
        
        // we do not want to copy reference to grid, so we make a new array
        obj.grid = [];

        return obj;
    },    

    /**
     * Method: moveTo
     * This function is called whenever the map is moved. All the moving
     * of actual 'tiles' is done by the map, but moveTo's role is to accept
     * a bounds and make sure the data that that bounds requires is pre-loaded.
     *
     * Parameters:
     * bounds - {<OpenLayers.Bounds>}
     * zoomChanged - {Boolean}
     * dragging - {Boolean}
     */
    moveTo:function(bounds, zoomChanged, dragging) {
        OpenLayers.Layer.HTTPRequest.prototype.moveTo.apply(this, arguments);
        
        bounds = bounds || this.map.getExtent();

        if (bounds != null) {
             
            // if grid is empty or zoom has changed, we *must* re-tile
            var forceReTile = !this.grid.length || zoomChanged;

            // total bounds of the tiles
            var tilesBounds = this.getTilesBounds();            
      
            if (this.singleTile) {
                
                // We want to redraw whenever even the slightest part of the 
                //  current bounds is not contained by our tile.
                //  (thus, we do not specify partial -- its default is false)
                if ( forceReTile || 
                     (!dragging && !tilesBounds.containsBounds(bounds))) {
                    this.initSingleTile(bounds);
                }
            } else {
             
                // if the bounds have changed such that they are not even 
                //  *partially* contained by our tiles (IE user has 
                //  programmatically panned to the other side of the earth) 
                //  then we want to reTile (thus, partial true).  
                //
                if (forceReTile || !tilesBounds.containsBounds(bounds, true)) {
                    this.initGriddedTiles(bounds);
                } else {
                    this.scheduleMoveGriddedTiles();
                }
            }
        }
    },

    /**
     * Method: moveByPx
     * Move the layer based on pixel vector.
     *
     * Parameters:
     * dx - {Number}
     * dy - {Number}
     */
    moveByPx: function(dx, dy) {
        if (!this.singleTile) {
            this.scheduleMoveGriddedTiles();
        }
    },

    /**
     * Method: scheduleMoveGriddedTiles
     * Schedule the move of tiles.
     */
    scheduleMoveGriddedTiles: function() {
        if (this.timerId != null) {
            window.clearTimeout(this.timerId);
        }
        this.timerId = window.setTimeout(
            this._moveGriddedTiles,
            this.tileLoadingDelay
        );
    },
    
    /**
     * APIMethod: setTileSize
     * Check if we are in singleTile mode and if so, set the size as a ratio
     *     of the map size (as specified by the layer's 'ratio' property).
     * 
     * Parameters:
     * size - {<OpenLayers.Size>}
     */
    setTileSize: function(size) { 
        if (this.singleTile) {
            size = this.map.getSize();
            size.h = parseInt(size.h * this.ratio);
            size.w = parseInt(size.w * this.ratio);
        } 
        OpenLayers.Layer.HTTPRequest.prototype.setTileSize.apply(this, [size]);
    },
        
    /**
     * Method: getGridBounds
     * Deprecated. This function will be removed in 3.0. Please use 
     *     getTilesBounds() instead.
     * 
     * Returns:
     * {<OpenLayers.Bounds>} A Bounds object representing the bounds of all the
     * currently loaded tiles (including those partially or not at all seen 
     * onscreen)
     */
    getGridBounds: function() {
        var msg = "The getGridBounds() function is deprecated. It will be " +
                  "removed in 3.0. Please use getTilesBounds() instead.";
        OpenLayers.Console.warn(msg);
        return this.getTilesBounds();
    },

    /**
     * APIMethod: getTilesBounds
     * Return the bounds of the tile grid.
     *
     * Returns:
     * {<OpenLayers.Bounds>} A Bounds object representing the bounds of all the
     *     currently loaded tiles (including those partially or not at all seen 
     *     onscreen).
     */
    getTilesBounds: function() {    
        var bounds = null; 
        
        if (this.grid.length) {
            var bottom = this.grid.length - 1;
            var bottomLeftTile = this.grid[bottom][0];
    
            var right = this.grid[0].length - 1; 
            var topRightTile = this.grid[0][right];
    
            bounds = new OpenLayers.Bounds(bottomLeftTile.bounds.left, 
                                           bottomLeftTile.bounds.bottom,
                                           topRightTile.bounds.right, 
                                           topRightTile.bounds.top);
            
        }   
        return bounds;
    },

    /**
     * Method: initSingleTile
     * 
     * Parameters: 
     * bounds - {<OpenLayers.Bounds>}
     */
    initSingleTile: function(bounds) {

        //determine new tile bounds
        var center = bounds.getCenterLonLat();
        var tileWidth = bounds.getWidth() * this.ratio;
        var tileHeight = bounds.getHeight() * this.ratio;
                                       
        var tileBounds = 
            new OpenLayers.Bounds(center.lon - (tileWidth/2),
                                  center.lat - (tileHeight/2),
                                  center.lon + (tileWidth/2),
                                  center.lat + (tileHeight/2));
  
        var ul = new OpenLayers.LonLat(tileBounds.left, tileBounds.top);
        var px = this.map.getLayerPxFromLonLat(ul);

        if (!this.grid.length) {
            this.grid[0] = [];
        }

        var tile = this.grid[0][0];
        if (!tile) {
            tile = this.addTile(tileBounds, px);
            
            this.addTileMonitoringHooks(tile);
            tile.draw();
            this.grid[0][0] = tile;
        } else {
            tile.moveTo(tileBounds, px);
        }           
        
        //remove all but our single tile
        this.removeExcessTiles(1,1);
    },

    /** 
     * Method: calculateGridLayout
     * Generate parameters for the grid layout.
     *
     * Parameters:
     * bounds - {<OpenLayers.Bound>}
     * origin - {<OpenLayers.LonLat>}
     * resolution - {Number}
     *
     * Returns:
     * Object containing properties tilelon, tilelat, tileoffsetlat,
     * tileoffsetlat, tileoffsetx, tileoffsety
     */
    calculateGridLayout: function(bounds, origin, resolution) {
        var tilelon = resolution * this.tileSize.w;
        var tilelat = resolution * this.tileSize.h;
        
        var offsetlon = bounds.left - origin.lon;
        var tilecol = Math.floor(offsetlon/tilelon) - this.buffer;
        var tilecolremain = offsetlon/tilelon - tilecol;
        var tileoffsetx = -tilecolremain * this.tileSize.w;
        var tileoffsetlon = origin.lon + tilecol * tilelon;
        
        var offsetlat = bounds.top - (origin.lat + tilelat);  
        var tilerow = Math.ceil(offsetlat/tilelat) + this.buffer;
        var tilerowremain = tilerow - offsetlat/tilelat;
        var tileoffsety = -tilerowremain * this.tileSize.h;
        var tileoffsetlat = origin.lat + tilerow * tilelat;
        
        return { 
          tilelon: tilelon, tilelat: tilelat,
          tileoffsetlon: tileoffsetlon, tileoffsetlat: tileoffsetlat,
          tileoffsetx: tileoffsetx, tileoffsety: tileoffsety
        };

    },
    
    /**
     * Method: getTileOrigin
     * Determine the origin for aligning the grid of tiles.  If a <tileOrigin>
     *     property is supplied, that will be returned.  Otherwise, the origin
     *     will be derived from the layer's <maxExtent> property.  In this case,
     *     the tile origin will be the corner of the <maxExtent> given by the 
     *     <tileOriginCorner> property.
     *
     * Returns:
     * {<OpenLayers.LonLat>} The tile origin.
     */
    getTileOrigin: function() {
        var origin = this.tileOrigin;
        if (!origin) {
            var extent = this.getMaxExtent();
            var edges = ({
                "tl": ["left", "top"],
                "tr": ["right", "top"],
                "bl": ["left", "bottom"],
                "br": ["right", "bottom"]
            })[this.tileOriginCorner];
            origin = new OpenLayers.LonLat(extent[edges[0]], extent[edges[1]]);
        }
        return origin;
    },

    /**
     * Method: initGriddedTiles
     * 
     * Parameters:
     * bounds - {<OpenLayers.Bounds>}
     */
    initGriddedTiles:function(bounds) {
        
        // work out mininum number of rows and columns; this is the number of
        // tiles required to cover the viewport plus at least one for panning

        var viewSize = this.map.getSize();
        var minRows = Math.ceil(viewSize.h/this.tileSize.h) + 
                      Math.max(1, 2 * this.buffer);
        var minCols = Math.ceil(viewSize.w/this.tileSize.w) +
                      Math.max(1, 2 * this.buffer);
        
        var origin = this.getTileOrigin();
        var resolution = this.map.getResolution();
        
        var tileLayout = this.calculateGridLayout(bounds, origin, resolution);

        var tileoffsetx = Math.round(tileLayout.tileoffsetx); // heaven help us
        var tileoffsety = Math.round(tileLayout.tileoffsety);

        var tileoffsetlon = tileLayout.tileoffsetlon;
        var tileoffsetlat = tileLayout.tileoffsetlat;
        
        var tilelon = tileLayout.tilelon;
        var tilelat = tileLayout.tilelat;

        this.origin = new OpenLayers.Pixel(tileoffsetx, tileoffsety);

        var startX = tileoffsetx; 
        var startLon = tileoffsetlon;

        var rowidx = 0;
        
        var layerContainerDivLeft = parseInt(this.map.layerContainerDiv.style.left);
        var layerContainerDivTop = parseInt(this.map.layerContainerDiv.style.top);
        
    
        do {
            var row = this.grid[rowidx++];
            if (!row) {
                row = [];
                this.grid.push(row);
            }

            tileoffsetlon = startLon;
            tileoffsetx = startX;
            var colidx = 0;
 
            do {
                var tileBounds = 
                    new OpenLayers.Bounds(tileoffsetlon, 
                                          tileoffsetlat, 
                                          tileoffsetlon + tilelon,
                                          tileoffsetlat + tilelat);

                var x = tileoffsetx;
                x -= layerContainerDivLeft;

                var y = tileoffsety;
                y -= layerContainerDivTop;

                var px = new OpenLayers.Pixel(x, y);
                var tile = row[colidx++];
                if (!tile) {
                    tile = this.addTile(tileBounds, px);
                    this.addTileMonitoringHooks(tile);
                    row.push(tile);
                } else {
                    tile.moveTo(tileBounds, px, false);
                }
     
                tileoffsetlon += tilelon;       
                tileoffsetx += this.tileSize.w;
            } while ((tileoffsetlon <= bounds.right + tilelon * this.buffer)
                     || colidx < minCols);
             
            tileoffsetlat -= tilelat;
            tileoffsety += this.tileSize.h;
        } while((tileoffsetlat >= bounds.bottom - tilelat * this.buffer)
                || rowidx < minRows);
        
        //shave off exceess rows and colums
        this.removeExcessTiles(rowidx, colidx);

        //now actually draw the tiles
        this.spiralTileLoad();
    },

    /**
     * Method: getMaxExtent
     * Get this layer's maximum extent. (Implemented as a getter for
     *     potential specific implementations in sub-classes.)
     *
     * Returns:
     * {OpenLayers.Bounds}
     */
    getMaxExtent: function() {
        return this.maxExtent;
    },
    
    /**
     * Method: spiralTileLoad
     *   Starts at the top right corner of the grid and proceeds in a spiral 
     *    towards the center, adding tiles one at a time to the beginning of a 
     *    queue. 
     * 
     *   Once all the grid's tiles have been added to the queue, we go back 
     *    and iterate through the queue (thus reversing the spiral order from 
     *    outside-in to inside-out), calling draw() on each tile. 
     */
    spiralTileLoad: function() {
        var tileQueue = [];
 
        var directions = ["right", "down", "left", "up"];

        var iRow = 0;
        var iCell = -1;
        var direction = OpenLayers.Util.indexOf(directions, "right");
        var directionsTried = 0;
        
        while( directionsTried < directions.length) {

            var testRow = iRow;
            var testCell = iCell;

            switch (directions[direction]) {
                case "right":
                    testCell++;
                    break;
                case "down":
                    testRow++;
                    break;
                case "left":
                    testCell--;
                    break;
                case "up":
                    testRow--;
                    break;
            } 
    
            // if the test grid coordinates are within the bounds of the 
            //  grid, get a reference to the tile.
            var tile = null;
            if ((testRow < this.grid.length) && (testRow >= 0) &&
                (testCell < this.grid[0].length) && (testCell >= 0)) {
                tile = this.grid[testRow][testCell];
            }
            
            if ((tile != null) && (!tile.queued)) {
                //add tile to beginning of queue, mark it as queued.
                tileQueue.unshift(tile);
                tile.queued = true;
                
                //restart the directions counter and take on the new coords
                directionsTried = 0;
                iRow = testRow;
                iCell = testCell;
            } else {
                //need to try to load a tile in a different direction
                direction = (direction + 1) % 4;
                directionsTried++;
            }
        } 
        
        // now we go through and draw the tiles in forward order
        for(var i=0, len=tileQueue.length; i<len; i++) {
            var tile = tileQueue[i];
            tile.draw();
            //mark tile as unqueued for the next time (since tiles are reused)
            tile.queued = false;       
        }
    },

    /**
     * APIMethod: addTile
     * Create a tile, initialize it, and add it to the layer div. 
     *
     * Parameters
     * bounds - {<OpenLayers.Bounds>}
     * position - {<OpenLayers.Pixel>}
     *
     * Returns:
     * {<OpenLayers.Tile>} The added OpenLayers.Tile
     */
    addTile:function(bounds, position) {
        return new OpenLayers.Tile.Image(this, position, bounds, null, 
                                         this.tileSize, this.tileOptions);
    },
    
    /** 
     * Method: addTileMonitoringHooks
     * This function takes a tile as input and adds the appropriate hooks to 
     *     the tile so that the layer can keep track of the loading tiles.
     * 
     * Parameters: 
     * tile - {<OpenLayers.Tile>}
     */
    addTileMonitoringHooks: function(tile) {
        
        tile.onLoadStart = function() {
            //if that was first tile then trigger a 'loadstart' on the layer
            if (this.numLoadingTiles == 0) {
                this.events.triggerEvent("loadstart");
            }
            this.numLoadingTiles++;
        };
        tile.events.register("loadstart", this, tile.onLoadStart);
      
        tile.onLoadEnd = function() {
            this.numLoadingTiles--;
            this.events.triggerEvent("tileloaded");
            //if that was the last tile, then trigger a 'loadend' on the layer
            if (this.numLoadingTiles == 0) {
                this.events.triggerEvent("loadend");
            }
        };
        tile.events.register("loadend", this, tile.onLoadEnd);
        tile.events.register("unload", this, tile.onLoadEnd);
    },

    /** 
     * Method: removeTileMonitoringHooks
     * This function takes a tile as input and removes the tile hooks 
     *     that were added in addTileMonitoringHooks()
     * 
     * Parameters: 
     * tile - {<OpenLayers.Tile>}
     */
    removeTileMonitoringHooks: function(tile) {
        tile.unload();
        tile.events.un({
            "loadstart": tile.onLoadStart,
            "loadend": tile.onLoadEnd,
            "unload": tile.onLoadEnd,
            scope: this
        });
    },
    
    /**
     * Method: moveGriddedTiles
     */
    moveGriddedTiles: function() {
        var shifted = true;
        var buffer = this.buffer || 1;
        var tlLayer = this.grid[0][0].position;
        var offsetX = parseInt(this.map.layerContainerDiv.style.left);
        var offsetY = parseInt(this.map.layerContainerDiv.style.top);
        var tlViewPort = tlLayer.add(offsetX, offsetY);
        if (tlViewPort.x > -this.tileSize.w * (buffer - 1)) {
            this.shiftColumn(true);
        } else if (tlViewPort.x < -this.tileSize.w * buffer) {
            this.shiftColumn(false);
        } else if (tlViewPort.y > -this.tileSize.h * (buffer - 1)) {
            this.shiftRow(true);
        } else if (tlViewPort.y < -this.tileSize.h * buffer) {
            this.shiftRow(false);
        } else {
            shifted = false;
        }
        if (shifted) {
            // we may have other row or columns to shift, schedule it
            // with a setTimeout, to give the user a chance to sneak
            // in moveTo's
            this.timerId = window.setTimeout(this._moveGriddedTiles, 0);
        }
    },

    /**
     * Method: shiftRow
     * Shifty grid work
     *
     * Parameters:
     * prepend - {Boolean} if true, prepend to beginning.
     *                          if false, then append to end
     */
    shiftRow:function(prepend) {
        var modelRowIndex = (prepend) ? 0 : (this.grid.length - 1);
        var grid = this.grid;
        var modelRow = grid[modelRowIndex];

        var resolution = this.map.getResolution();
        var deltaY = (prepend) ? -this.tileSize.h : this.tileSize.h;
        var deltaLat = resolution * -deltaY;

        var row = (prepend) ? grid.pop() : grid.shift();

        for (var i=0, len=modelRow.length; i<len; i++) {
            var modelTile = modelRow[i];
            var bounds = modelTile.bounds.clone();
            var position = modelTile.position.clone();
            bounds.bottom = bounds.bottom + deltaLat;
            bounds.top = bounds.top + deltaLat;
            position.y = position.y + deltaY;
            row[i].moveTo(bounds, position);
        }

        if (prepend) {
            grid.unshift(row);
        } else {
            grid.push(row);
        }
    },

    /**
     * Method: shiftColumn
     * Shift grid work in the other dimension
     *
     * Parameters:
     * prepend - {Boolean} if true, prepend to beginning.
     *                          if false, then append to end
     */
    shiftColumn: function(prepend) {
        var deltaX = (prepend) ? -this.tileSize.w : this.tileSize.w;
        var resolution = this.map.getResolution();
        var deltaLon = resolution * deltaX;

        for (var i=0, len=this.grid.length; i<len; i++) {
            var row = this.grid[i];
            var modelTileIndex = (prepend) ? 0 : (row.length - 1);
            var modelTile = row[modelTileIndex];
            
            var bounds = modelTile.bounds.clone();
            var position = modelTile.position.clone();
            bounds.left = bounds.left + deltaLon;
            bounds.right = bounds.right + deltaLon;
            position.x = position.x + deltaX;

            var tile = prepend ? this.grid[i].pop() : this.grid[i].shift();
            tile.moveTo(bounds, position);
            if (prepend) {
                row.unshift(tile);
            } else {
                row.push(tile);
            }
        }
    },
    
    /**
     * Method: removeExcessTiles
     * When the size of the map or the buffer changes, we may need to
     *     remove some excess rows and columns.
     * 
     * Parameters:
     * rows - {Integer} Maximum number of rows we want our grid to have.
     * columns - {Integer} Maximum number of columns we want our grid to have.
     */
    removeExcessTiles: function(rows, columns) {
        
        // remove extra rows
        while (this.grid.length > rows) {
            var row = this.grid.pop();
            for (var i=0, l=row.length; i<l; i++) {
                var tile = row[i];
                this.removeTileMonitoringHooks(tile);
                tile.destroy();
            }
        }
        
        // remove extra columns
        while (this.grid[0].length > columns) {
            for (var i=0, l=this.grid.length; i<l; i++) {
                var row = this.grid[i];
                var tile = row.pop();
                this.removeTileMonitoringHooks(tile);
                tile.destroy();
            }
        }
    },

    /**
     * Method: onMapResize
     * For singleTile layers, this will set a new tile size according to the
     * dimensions of the map pane.
     */
    onMapResize: function() {
        if (this.singleTile) {
            this.clearGrid();
            this.setTileSize();
        }
    },
    
    /**
     * APIMethod: getTileBounds
     * Returns The tile bounds for a layer given a pixel location.
     *
     * Parameters:
     * viewPortPx - {<OpenLayers.Pixel>} The location in the viewport.
     *
     * Returns:
     * {<OpenLayers.Bounds>} Bounds of the tile at the given pixel location.
     */
    getTileBounds: function(viewPortPx) {
        var maxExtent = this.maxExtent;
        var resolution = this.getResolution();
        var tileMapWidth = resolution * this.tileSize.w;
        var tileMapHeight = resolution * this.tileSize.h;
        var mapPoint = this.getLonLatFromViewPortPx(viewPortPx);
        var tileLeft = maxExtent.left + (tileMapWidth *
                                         Math.floor((mapPoint.lon -
                                                     maxExtent.left) /
                                                    tileMapWidth));
        var tileBottom = maxExtent.bottom + (tileMapHeight *
                                             Math.floor((mapPoint.lat -
                                                         maxExtent.bottom) /
                                                        tileMapHeight));
        return new OpenLayers.Bounds(tileLeft, tileBottom,
                                     tileLeft + tileMapWidth,
                                     tileBottom + tileMapHeight);
    },
    
    CLASS_NAME: "OpenLayers.Layer.Grid"
});