define([
    "dojo/query",
    "dojo/request",
    "dojo/_base/declare",
    "tiles/base64utils",
    "tiles/TilesStore",
    "tiles/tilingScheme",
    "tiles/FileSaver"
    ], function(query, request, declare,Base64Utils,TilesStore,TilingScheme,FileSaver)
    {
    "use strict";
    return declare([],{
            /**
             * Utility method to get the basemap layer reference
             * @param map
             * @returns {Number} layerId
             */
            getBasemapLayer: function(map)
            {
                var layerId = map.layerIds[0];
                return map.getLayer(layerId);
            },

            /**
             * Method that extends a layer object with the offline capability.
             * After extending one layer, you can call layer.goOffline() or layer.goOnline()
             * @param layer
             * @param callback
             * @returns {callback} callback(boolean, string)
             */
            extend: function(layer,callback)
            {
                console.log("extending layer", layer.url);

                layer._lastTileUrl = "";                

                /* we add some methods to the layer object */
                /* we don't want to extend the tiled layer class, as it is a capability that we want to add only to one instance */
                /* we also add some additional attributes inside an "offline" object */

                layer._getTileUrl = layer.getTileUrl;
                layer.offline = {
                    online: true,
                    store: new TilesStore(),
                    proxyPath: "../lib/resource-proxy/proxy.php"
                };

                if( /*false &&*/ layer.offline.store.isSupported() )
                {
                    layer.offline.store.init(callback);                    
                }
                else
                {
                    return callback(false, "indexedDB not supported");                    
                }

                layer.resampling = false;

                /**
                 * Internal method that overrides the getTileUrl() method.
                 * If application is offline then tiles are written to local storage.
                 * Retrieves tiles as requested by the ArcGIS API for JavaScript.
                 * If a tile is in cache it is returned.
                 * If it is not in cache then one is retrieved over the internet.
                 * @param level
                 * @param row
                 * @param col
                 * @returns {String} URL
                 */
                layer.getTileUrl = function(level,row,col)
                {
                    console.assert(!isNaN(level) && !isNaN(row) && !isNaN(col), "bad tile requested");

                    console.log("looking for tile",level,row,col);
                    var url = this._getTileUrl(level,row,col);
console.log("LIBRARY ONLINE " + this.offline.online)
                    if( this.offline.online )
                    {
                        console.log("fetching url online: ", url);
                        layer._lastTileUrl = url;
                        return url;
                    }

                    url = url.split("?")[0];

                    /* temporary URL returned immediately, as we haven't retrieved the image from the indexeddb yet */
                    var tileid = "void:/"+level+"/"+row+"/"+col;

                    this.offline.store.retrieve(url, function(success, offlineTile)
                    {
                        /* when the .get() callback is called we replace the temporary URL originally returned by the data:image url */
                        // search for the img with src="void:"+level+"-"+row+"-"+col and replace with actual url
                        var img = query("img[src="+tileid+"]")[0];
                        var imgURL;

                        console.assert(img !== "undefined", "undefined image detected");

                        if( success )
                        {
                            img.style.borderColor = "blue";
                            console.log("found tile offline", url);
                            imgURL = "data:image;base64," + offlineTile.img;
                        }
                        else
                        {
                            img.style.borderColor = "green";
                            console.log("tile is not in the offline store", url);
                            imgURL = "data:image/png;base64,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";
                        }
                        // when we return a nonexistent url to the image, the TiledMapServiceLayer::_tileErrorHandler() method
                        // sets img visibility to 'hidden', so we need to show the image back once we have put the data:image
                        img.style.visibility = "visible";
                        img.src = imgURL;
                        return "";  /* this result goes nowhere, seriously */
                    });

                    return tileid;
                };

                /**
                 * Returns an object that contains the number of tiles that would need to be downloaded
                 * for the specified extent and zoom level, and the estimated byte size of such tiles.
                 * This method is useful to give the user an indication of the required time and space
                 * before launching the actual download operation. The byte size estimation is very rough.
                 * @param extent
                 * @param level
                 * @param tileSize
                 * @returns {{level: *, tileCount: Number, sizeBytes: number}}
                 */
                layer.getLevelEstimation = function(extent, level, tileSize)
                {
                    var tilingScheme = new TilingScheme(this);
                    var cellIds = tilingScheme.getAllCellIdsInExtent(extent,level);

                    var levelEstimation = {
                        level: level,
                        tileCount: cellIds.length,
                        sizeBytes: cellIds.length * tileSize
                    };

                    return levelEstimation;
                };

                /**
                 * Retrieves tiles and stores them in the local cache.
                 * @param minLevel
                 * @param maxLevel
                 * @param extent
                 * @param reportProgress
                 */
                layer.prepareForOffline = function(minLevel, maxLevel, extent, reportProgress)
                {
                    /* create list of tiles to store */
                    var tilingScheme = new TilingScheme(this);
                    var cells = [];
                    var level;

                    for(level=minLevel; level<=maxLevel; level++)
                    {
                        var level_cell_ids = tilingScheme.getAllCellIdsInExtent(extent,level);

                        level_cell_ids.forEach(function(cell_id)
                        {
                            cells.push({ level: level, row: cell_id[1], col: cell_id[0]});
                        });

                        // if the number of requested tiles is excessive, we just stop
                        if( cells.length > 5000 && level !== maxLevel)
                        {
                            console.log("enough is enough!");
                            break;
                        }
                    }

                    /* launch tile download */
                    this._doNextTile(0, cells, reportProgress);
                };

                /**
                 * This method puts the layer in offline mode. When in offline mode,
                 * the layer will not fetch any tile from the remote server. It
                 * will look up the tiles in the indexed db database and display them in the layer.
                 * If the tile can't be found in the local database it will show up blank
                 * (even if there is actual connectivity). The pair of methods goOffline() and
                 * goOnline()allows the developer to manually control the behaviour of the layer.
                 * Used in conjunction with the offline dectection library, you can put the layer in
                 * the appropriate mode when the offline condition changes.
                 */
                layer.goOffline = function()
                {
                    this.offline.online = false;
                };

                /**
                 * This method puts the layer in online mode. When in online mode, the layer will
                 * behave as regular layers, fetching all tiles from the remote server.
                 * If there is no internet connectivity the tiles may appear thanks to the browsers cache,
                 * but no attempt will be made to look up tiles in the local database.
                 */
                layer.goOnline = function()
                {
                    this.offline.online = true;
                    this.refresh();
                };

                /**
                 * Determines if application is online or offline
                 * @returns {boolean}
                 */
                layer.isOnline = function()
                {
                    return this.offline.online;
                };

                /**
                 * Clears the local cache of tiles.
                 * @param callback callback(boolean, errors)
                 */
                layer.deleteAllTiles = function(callback) // callback(success) or callback(false, error)
                {
                    var store = this.offline.store;
                    store.deleteAll(callback);
                };

                /**
                 * Gets the size in bytes of the local tile cache.
                 * @param callback  callback(size, error)
                 */
                layer.getOfflineUsage = function(callback) // callback({size: <>, tileCount: <>}) or callback(null,error)
                {
                    var store = this.offline.store;
                    store.usedSpace(callback);
                };

                /**
                 * Gets polygons representing all cached cell ids within a particular
                 * zoom level and bounded by an extent.
                 * @param callback callback(polygon, error)
                 */
                layer.getTilePolygons = function(callback)	// callback(Polygon polygon) or callback(null, error)
                {
                    var store = this.offline.store;
                    var tilingScheme = new TilingScheme(this);
                    store.getAllTiles(function(url,img,err)
                    {
                        if(url)
                        {
                            var components = url.split("/");
                            var level = parseInt(components[ components.length - 3],10);
                            var col = parseInt(components[ components.length - 2],10);
                            var row = parseInt(components[ components.length - 1],10);
                            var cellId = [row,col];
                            var polygon = tilingScheme.getCellPolygonFromCellId(cellId, level);
                            //if( level == 15)
                                callback(polygon);
                        }
                        else
                        {
                            callback(null,err);
                        }
                    });
                };

                /**
                 * Saves tile cache into a portable csv format.
                 * @param fileName
                 * @param callback callback( boolean, error)
                 */
                layer.saveToFile = function(fileName, callback) // callback(success, msg)
                {
                    var store = this.offline.store;
                    var csv = [];

                    csv.push("url,img");
                    store.getAllTiles(function(url,img,evt)
                    {
                        if(evt==="end")
                        {
                            var blob = new Blob([ csv.join("\r\n") ], {type:"text/plain;charset=utf-8"});
                            var saver = FileSaver.saveAs(blob, fileName);

                            if( saver.readyState === saver.DONE )
                            {
                                if( saver.error )
                                {
                                    return callback(false,"Error saving file " + fileName);
                                }
                                return callback(true, "Saved " + (csv.length-1) + " tiles (" + Math.floor(blob.size / 1024 / 1024 * 100) / 100 + " Mb) into " + fileName);
                            }
                            saver.onerror = function() {
                                callback(false,"Error saving file " + fileName);
                            };
                            saver.onwriteend = function()
                            {
                                callback(true, "Saved " + (csv.length-1) + " tiles (" + Math.floor(blob.size / 1024 / 1024 * 100) / 100 + " Mb) into " + fileName);
                            };
                        }
                        else
                        {
                            csv.push(url+","+img);
                        }
                    });
                };

                /**
                 * Reads a csv file into local tile cache.
                 * @param file
                 * @param callback callback( boolean, error)
                 */
                layer.loadFromFile = function(file, callback) // callback(success,msg)
                {
                    console.log("reading",file);

                    var store = this.offline.store;
                    var i;

                    if (window.File && window.FileReader && window.FileList && window.Blob)
                    {
                        // Great success! All the File APIs are supported.
                        var reader = new FileReader();
                        reader.onload = function(evt)
                        {
                            var csvContent = evt.target.result;
                            var tiles = csvContent.split("\r\n");
                            var tileCount = 0;
                            var pair, tile;

                            if(tiles[0] !== "url,img")
                            {                                
                                return callback(false, "File " + file.name + " doesn't contain tiles that can be loaded");
                            }

                            for(i=1; i<tiles.length; i++)
                            {
                                pair = tiles[i].split(",");
                                tile = {
                                    url: pair[0],
                                    img: pair[1]
                                };
                                console.log("read",tile.url);
                                store.store(tile,function(success)
                                {
                                    console.log(".");

                                    if( success )
                                    {
                                        tileCount += 1;
                                    }

                                    if( tileCount === tiles.length-1)
                                    {
                                        console.log("finished!");
                                        window.setTimeout(function() { /* refresh layer by zooming in and out, or some way that really refreshes the layer */ }, 1000);
                                        callback(true, tileCount + " tiles loaded from " + file.name);
                                    }
                                });
                            }
                        };
                        reader.readAsText(file);
                    }
                    else
                    {
                        callback(false, "The File APIs are not fully supported in this browser.");
                    }
                };

                /* internal methods */

                /**
                 * Makes a request to a tile url and uses that as a basis for the
                 * the average tile size.
                 * Future Iterations could call multiple tiles and do an actual average.
                 * @param callback
                 * @returns {Number} Returns NaN if there was a problem retrieving the tile
                 */
                layer.estimateTileSize = function(callback)
                {
                    if(layer._lastTileUrl)
                    {
                        var url = this.offline.proxyPath? this.offline.proxyPath + "?" +  layer._lastTileUrl : layer._lastTileUrl;
                        request.get(url,{
                            handleAs: "text/plain; charset=x-user-defined",
                            headers: {
                                "X-Requested-With": "" //bypasses a dojo xhr bug
                            },
                            timeout: 2000
                        }).then(function(response){
                                var img = Base64Utils.wordToBase64(Base64Utils.stringToWord(response));
                                callback(img.length + url.length,null);
                            },
                        function(err){
                            callback(null,err);
                        });
                    }
                    else{
                        callback(NaN);
                    }
                };

                /**
                 * Helper method that returns a new extent buffered by a given measurement that's based on map units.
                 * E.g. If you are using mercator then buffer would be in meters
                 * @param buffer
                 * @returns {Extent}
                 */
                layer.getExtentBuffer = function(/* int */ buffer, /* Extent */ extent){
                    extent.xmin -= buffer; extent.ymin -= buffer;
                    extent.xmax += buffer; extent.ymax += buffer;
                    return extent;
                };

                /**
                 * Helper method that returns an array of tile urls within a given extent and level
                 * @returns Array
                 */
                layer.getTileUrlsByExtent = function(extent,level){
                    var tilingScheme = new TilingScheme(layer);
                    var level_cell_ids = tilingScheme.getAllCellIdsInExtent(extent,level);
                    var cells = [];

                    level_cell_ids.forEach(function(cell_id)
                    {
                        cells.push(layer.url + "/" + level + "/" + cell_id[1] + "/" + cell_id[0]);
                    }.bind(this));

                    return cells;
                };

                layer._doNextTile = function(i, cells, reportProgress)
                {
                    var cell = cells[i];

                    this._storeTile(cell.level,cell.row,cell.col, function(success, error)
                    {
                        if(!success)
                        {
                            console.log("error storing tile", cell, error);
                            error = { cell:cell, msg:error};
                        }

                        var cancelRequested = reportProgress({countNow:i, countMax:cells.length, cell: cell, error: error, finishedDownloading:false});

                        if( cancelRequested || i === cells.length-1 )
                        {
                            reportProgress({ finishedDownloading: true, cancelRequested: cancelRequested});
                        }
                        else
                        {                            
                            this._doNextTile(i+1, cells, reportProgress);
                        }

                    }.bind(this));
                };

                layer._storeTile = function(level,row,col,callback) // callback(success, msg)
                {
                    var store = this.offline.store;
                    var url = this._getTileUrl(level,row,col);
                    url = url.split("?")[0];

                    /* download the tile */
                    var imgurl = this.offline.proxyPath? this.offline.proxyPath + "?" + url : url; 
                    var req = new XMLHttpRequest();
                    req.open("GET", imgurl, true);
                    req.overrideMimeType("text/plain; charset=x-user-defined"); // https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest?redirectlocale=en-US&redirectslug=DOM%2FXMLHttpRequest%2FUsing_XMLHttpRequest#Handling_binary_data

                    req.onload = function()
                    {
                        if( req.status === 200 && req.responseText !== "")
                        {
                            var img = Base64Utils.wordToBase64(Base64Utils.stringToWord(this.responseText));

                            var tile = {
                                url: url,
                                img: img
                            };

                            store.store(tile,callback);
                        }
                        else
                        {
                            console.log("xhr failed for", imgurl);
                            callback(false, req.status + " " + req.statusText + ": " + req.response + " when downloading " + imgurl);
                        }
                    };
                    req.onerror = function(e)
                    {
                        console.log("xhr failed for", imgurl);
                        callback(false, e);
                    };                    
                    req.send(null);
                };
            }
    }); // declare
}); // define