define(["esri/graphic"], function(Graphic)
{
    "use strict";
	/* private consts */
	var EDITS_QUEUE_KEY = "esriEditsQueue";
	var SEPARATOR = "|@|";

	return {

		//
		// public interface
		//

		// enum
		
		ADD: "add",
		UPDATE: "update",
		DELETE:"delete",

		// ERROR_DUPLICATE_EDIT: "Attempt to insert duplicated edit",
		ERROR_LOCALSTORAGE_FULL: "LocalStorage capacity exceeded",

		isSupported: function() 
		{
			// http://stackoverflow.com/questions/11214404/how-to-detect-if-browser-supports-html5-local-storage
            var mod = "esriLocalStorageTest";
			try {
				window.localStorage.setItem(mod, mod);
				window.localStorage.removeItem(mod);
				return true;
			} catch(e) {
				return false;
            }
        },

		pushEdit: function(operation,layer,graphic)
		{
			var edit = {
				operation: operation,
				layer: layer,
				graphic: this._serialize(graphic)
			};

			var edits = this._retrieveEditsQueue();
			edits.push(edit);
			var success = this._storeEditsQueue(edits);
			return { success: success, error: success? undefined : {code: 1000, description:this.ERROR_LOCALSTORAGE_FULL} };
		},

		peekFirstEdit: function()
		{
			var edits = this._retrieveEditsQueue();
			var firstEdit;

			if( edits )
			{
				firstEdit = edits[0];
				firstEdit.graphic = this._deserialize(firstEdit.graphic);
				return firstEdit;	
			}
			return null;			
		},

		popFirstEdit: function()
		{
			var edits = this._retrieveEditsQueue();
			var firstEdit;

			if( edits )
			{
				firstEdit = edits.shift();
				this._storeEditsQueue(edits);
				firstEdit.graphic = this._deserialize(firstEdit.graphic);
				return firstEdit;			
			}
			return null;			
		},

		hasPendingEdits: function()
		{
			var storedValue = window.localStorage.getItem(EDITS_QUEUE_KEY) || "";
			return ( storedValue !== "" );
		},

		pendingEditsCount: function()
		{
			var storedValue = window.localStorage.getItem(EDITS_QUEUE_KEY) || "";

			if( storedValue === "" )
			{
				return 0;	// fast easy case				
			}

			var editsArray = this._unpackArrayOfEdits(storedValue);
			return editsArray.length;
		},

		resetEditsQueue: function()
		{
			window.localStorage.setItem(EDITS_QUEUE_KEY, "");
		},

		getEditsStoreSizeBytes: function()
		{
			var editsQueueValue = window.localStorage.getItem(EDITS_QUEUE_KEY);

			return (editsQueueValue? EDITS_QUEUE_KEY.length + editsQueueValue.length : 0); 
		},

		getLocalStorageSizeBytes: function()
		{
			var bytes = 0,
				key, value;

			for(key in window.localStorage )
			{
				if( window.localStorage.hasOwnProperty(key))
				{
					value = window.localStorage.getItem(key);
					bytes += key.length + value.length;					
				}
			}
			return bytes;
		},

		//
		// internal methods
		//

		//
		// graphic serialization/deserialization
		//
		_serialize: function(graphic)
		{
			// keep only attributes and geometry, that are the values that get sent to the server by applyEdits() 
			// see http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#/Apply_Edits_Feature_Service_Layer/02r3000000r6000000/
			// use graphic's built-in serializing method
			var json = graphic.toJson();
			var jsonClean = 
			{
				attributes: json.attributes,
				geometry: json.geometry
			};
			return JSON.stringify(jsonClean);
		},

		_deserialize: function(json)
		{	
			var graphic = new Graphic(JSON.parse(json));
			return graphic;
		},

		_retrieveEditsQueue: function()
		{
			var storedValue = window.localStorage.getItem(EDITS_QUEUE_KEY) || "";
			return this._unpackArrayOfEdits(storedValue);
		},

		_storeEditsQueue: function(edits)
		{
			try 
			{
				var serializedEdits = this._packArrayOfEdits(edits);
				window.localStorage.setItem(EDITS_QUEUE_KEY, serializedEdits);
				return true;
			}
			catch(err)
			{
				return false;
			}
		},

		_packArrayOfEdits: function(edits)
		{
			var serializedEdits = [];
			edits.forEach(function(edit)
			{
				serializedEdits.push( JSON.stringify(edit) );
			});
			return serializedEdits.join(SEPARATOR);
		},

		_unpackArrayOfEdits: function(serializedEdits)
		{
			if( !serializedEdits )
			{
				return [];				
			}

			var edits = [];
			serializedEdits.split(SEPARATOR).forEach( function(serializedEdit)
			{
				edits.push( JSON.parse(serializedEdit) );
			});

			return edits;
		},

		_isEditDuplicated: function(newEdit,edits)
		{
			var i,
				edit;

			for(i=0; i<edits.length; i++)
			{	
				edit = edits[i];
				if( edit.operation === newEdit.operation &&
					edit.layer     === newEdit.layer     &&
					edit.graphic   === newEdit.graphic )
				{
					return true;
				}
			}
			return false;
		}
	};
});