define([
        '../ThirdParty/when',
        './Credit',
        './defaultValue',
        './defined',
        './defineProperties',
        './deprecationWarning',
        './DeveloperError',
        './Ellipsoid',
        './Event',
        './GeographicTilingScheme',
        './getImagePixels',
        './HeightmapTerrainData',
        './loadImage',
        './loadXML',
        './Math',
        './Rectangle',
        './Request',
        './RequestType',
        './TerrainProvider',
        './TileProviderError'
    ], function(
        when,
        Credit,
        defaultValue,
        defined,
        defineProperties,
        deprecationWarning,
        DeveloperError,
        Ellipsoid,
        Event,
        GeographicTilingScheme,
        getImagePixels,
        HeightmapTerrainData,
        loadImage,
        loadXML,
        CesiumMath,
        Rectangle,
        Request,
        RequestType,
        TerrainProvider,
        TileProviderError) {
    'use strict';

    function DataRectangle(rectangle, maxLevel) {
        this.rectangle = rectangle;
        this.maxLevel = maxLevel;
    }

    /**
     * A {@link TerrainProvider} that produces terrain geometry by tessellating height maps
     * retrieved from a {@link http://vr-theworld.com/|VT MÄK VR-TheWorld server}.
     *
     * @alias VRTheWorldTerrainProvider
     * @constructor
     *
     * @param {Object} options Object with the following properties:
     * @param {String} options.url The URL of the VR-TheWorld TileMap.
     * @param {Object} [options.proxy] A proxy to use for requests. This object is expected to have a getURL function which returns the proxied URL, if needed.
     * @param {Ellipsoid} [options.ellipsoid=Ellipsoid.WGS84] The ellipsoid.  If this parameter is not
     *                    specified, the WGS84 ellipsoid is used.
     * @param {Credit|String} [options.credit] A credit for the data source, which is displayed on the canvas.
     *
     *
     * @example
     * var terrainProvider = new Cesium.VRTheWorldTerrainProvider({
     *   url : 'https://www.vr-theworld.com/vr-theworld/tiles1.0.0/73/'
     * });
     * viewer.terrainProvider = terrainProvider;
     *
     * @see TerrainProvider
     */
    function VRTheWorldTerrainProvider(options) {
        options = defaultValue(options, defaultValue.EMPTY_OBJECT);
        //>>includeStart('debug', pragmas.debug);
        if (!defined(options.url)) {
            throw new DeveloperError('options.url is required.');
        }
        //>>includeEnd('debug');

        this._url = options.url;
        if (this._url.length > 0 && this._url[this._url.length - 1] !== '/') {
            this._url += '/';
        }

        this._errorEvent = new Event();
        this._ready = false;
        this._readyPromise = when.defer();

        this._proxy = options.proxy;

        this._terrainDataStructure = {
                heightScale : 1.0 / 1000.0,
                heightOffset : -1000.0,
                elementsPerHeight : 3,
                stride : 4,
                elementMultiplier : 256.0,
                isBigEndian : true,
                lowestEncodedHeight : 0,
                highestEncodedHeight : 256 * 256 * 256 - 1
            };

        var credit = options.credit;
        if (typeof credit === 'string') {
            credit = new Credit(credit);
        }
        this._credit = credit;

        this._tilingScheme = undefined;
        this._rectangles = [];

        var that = this;
        var metadataError;
        var ellipsoid = defaultValue(options.ellipsoid, Ellipsoid.WGS84);

        function metadataSuccess(xml) {
            var srs = xml.getElementsByTagName('SRS')[0].textContent;
            if (srs === 'EPSG:4326') {
                that._tilingScheme = new GeographicTilingScheme({ ellipsoid : ellipsoid });
            } else {
                metadataFailure('SRS ' + srs + ' is not supported.');
                return;
            }

            var tileFormat = xml.getElementsByTagName('TileFormat')[0];
            that._heightmapWidth = parseInt(tileFormat.getAttribute('width'), 10);
            that._heightmapHeight = parseInt(tileFormat.getAttribute('height'), 10);
            that._levelZeroMaximumGeometricError = TerrainProvider.getEstimatedLevelZeroGeometricErrorForAHeightmap(ellipsoid, Math.min(that._heightmapWidth, that._heightmapHeight), that._tilingScheme.getNumberOfXTilesAtLevel(0));

            var dataRectangles = xml.getElementsByTagName('DataExtent');

            for (var i = 0; i < dataRectangles.length; ++i) {
                var dataRectangle = dataRectangles[i];

                var west = CesiumMath.toRadians(parseFloat(dataRectangle.getAttribute('minx')));
                var south = CesiumMath.toRadians(parseFloat(dataRectangle.getAttribute('miny')));
                var east = CesiumMath.toRadians(parseFloat(dataRectangle.getAttribute('maxx')));
                var north = CesiumMath.toRadians(parseFloat(dataRectangle.getAttribute('maxy')));
                var maxLevel = parseInt(dataRectangle.getAttribute('maxlevel'), 10);

                that._rectangles.push(new DataRectangle(new Rectangle(west, south, east, north), maxLevel));
            }

            that._ready = true;
            that._readyPromise.resolve(true);
        }

        function metadataFailure(e) {
            var message = defaultValue(e, 'An error occurred while accessing ' + that._url + '.');
            metadataError = TileProviderError.handleError(metadataError, that, that._errorEvent, message, undefined, undefined, undefined, requestMetadata);
        }

        function requestMetadata() {
            when(loadXML(that._url), metadataSuccess, metadataFailure);
        }

        requestMetadata();
    }

    defineProperties(VRTheWorldTerrainProvider.prototype, {
        /**
         * Gets an event that is raised when the terrain provider encounters an asynchronous error.  By subscribing
         * to the event, you will be notified of the error and can potentially recover from it.  Event listeners
         * are passed an instance of {@link TileProviderError}.
         * @memberof VRTheWorldTerrainProvider.prototype
         * @type {Event}
         */
        errorEvent : {
            get : function() {
                return this._errorEvent;
            }
        },

        /**
         * Gets the credit to display when this terrain provider is active.  Typically this is used to credit
         * the source of the terrain.  This function should not be called before {@link VRTheWorldTerrainProvider#ready} returns true.
         * @memberof VRTheWorldTerrainProvider.prototype
         * @type {Credit}
         */
        credit : {
            get : function() {
                return this._credit;
            }
        },

        /**
         * Gets the tiling scheme used by this provider.  This function should
         * not be called before {@link VRTheWorldTerrainProvider#ready} returns true.
         * @memberof VRTheWorldTerrainProvider.prototype
         * @type {GeographicTilingScheme}
         */
        tilingScheme : {
            get : function() {
                //>>includeStart('debug', pragmas.debug);
                if (!this.ready) {
                    throw new DeveloperError('requestTileGeometry must not be called before ready returns true.');
                }
                //>>includeEnd('debug');

                return this._tilingScheme;
            }
        },

        /**
         * Gets a value indicating whether or not the provider is ready for use.
         * @memberof VRTheWorldTerrainProvider.prototype
         * @type {Boolean}
         */
        ready : {
            get : function() {
                return this._ready;
            }
        },

        /**
         * Gets a promise that resolves to true when the provider is ready for use.
         * @memberof VRTheWorldTerrainProvider.prototype
         * @type {Promise.<Boolean>}
         * @readonly
         */
        readyPromise : {
            get : function() {
                return this._readyPromise.promise;
            }
        },

        /**
         * Gets a value indicating whether or not the provider includes a water mask.  The water mask
         * indicates which areas of the globe are water rather than land, so they can be rendered
         * as a reflective surface with animated waves.  This function should not be
         * called before {@link VRTheWorldTerrainProvider#ready} returns true.
         * @memberof VRTheWorldTerrainProvider.prototype
         * @type {Boolean}
         */
        hasWaterMask : {
            get : function() {
                return false;
            }
        },

        /**
         * Gets a value indicating whether or not the requested tiles include vertex normals.
         * This function should not be called before {@link VRTheWorldTerrainProvider#ready} returns true.
         * @memberof VRTheWorldTerrainProvider.prototype
         * @type {Boolean}
         */
        hasVertexNormals : {
            get : function() {
                return false;
            }
        }
    });

    /**
     * Requests the geometry for a given tile.  This function should not be called before
     * {@link VRTheWorldTerrainProvider#ready} returns true.  The result includes terrain
     * data and indicates that all child tiles are available.
     *
     * @param {Number} x The X coordinate of the tile for which to request geometry.
     * @param {Number} y The Y coordinate of the tile for which to request geometry.
     * @param {Number} level The level of the tile for which to request geometry.
     * @param {Request} [request] The request object. Intended for internal use only.
     * @returns {Promise.<TerrainData>|undefined} A promise for the requested geometry.  If this method
     *          returns undefined instead of a promise, it is an indication that too many requests are already
     *          pending and the request will be retried later.
     */
    VRTheWorldTerrainProvider.prototype.requestTileGeometry = function(x, y, level, request) {
        //>>includeStart('debug', pragmas.debug);
        if (!this.ready) {
            throw new DeveloperError('requestTileGeometry must not be called before ready returns true.');
        }
        //>>includeEnd('debug');

        var yTiles = this._tilingScheme.getNumberOfYTilesAtLevel(level);
        var url = this._url + level + '/' + x + '/' + (yTiles - y - 1) + '.tif?cesium=true';

        var proxy = this._proxy;
        if (defined(proxy)) {
            url = proxy.getURL(url);
        }

        if (typeof request === 'boolean') {
            deprecationWarning('throttleRequests', 'The throttleRequest parameter for requestTileGeometry was deprecated in Cesium 1.35.  It will be removed in 1.37.');
            request = new Request({
                throttle : request,
                throttleByServer : request,
                type : RequestType.TERRAIN
            });
        }

        var promise = loadImage(url, undefined, request);
        if (!defined(promise)) {
            return undefined;
        }

        var that = this;
        return when(promise, function(image) {
            return new HeightmapTerrainData({
                buffer : getImagePixels(image),
                width : that._heightmapWidth,
                height : that._heightmapHeight,
                childTileMask : getChildMask(that, x, y, level),
                structure : that._terrainDataStructure
            });
        });
    };

    /**
     * Gets the maximum geometric error allowed in a tile at a given level.
     *
     * @param {Number} level The tile level for which to get the maximum geometric error.
     * @returns {Number} The maximum geometric error.
     */
    VRTheWorldTerrainProvider.prototype.getLevelMaximumGeometricError = function(level) {
        //>>includeStart('debug', pragmas.debug);
        if (!this.ready) {
            throw new DeveloperError('requestTileGeometry must not be called before ready returns true.');
        }
        //>>includeEnd('debug');
        return this._levelZeroMaximumGeometricError / (1 << level);
    };

    var rectangleScratch = new Rectangle();

    function getChildMask(provider, x, y, level) {
        var tilingScheme = provider._tilingScheme;
        var rectangles = provider._rectangles;
        var parentRectangle = tilingScheme.tileXYToRectangle(x, y, level);

        var childMask = 0;

        for (var i = 0; i < rectangles.length && childMask !== 15; ++i) {
            var rectangle = rectangles[i];
            if (rectangle.maxLevel <= level) {
                continue;
            }

            var testRectangle = rectangle.rectangle;

            var intersection = Rectangle.intersection(testRectangle, parentRectangle, rectangleScratch);
            if (defined(intersection)) {
                // Parent tile is inside this rectangle, so at least one child is, too.
                if (isTileInRectangle(tilingScheme, testRectangle, x * 2, y * 2, level + 1)) {
                    childMask |= 4; // northwest
                }
                if (isTileInRectangle(tilingScheme, testRectangle, x * 2 + 1, y * 2, level + 1)) {
                    childMask |= 8; // northeast
                }
                if (isTileInRectangle(tilingScheme, testRectangle, x * 2, y * 2 + 1, level + 1)) {
                    childMask |= 1; // southwest
                }
                if (isTileInRectangle(tilingScheme, testRectangle, x * 2 + 1, y * 2 + 1, level + 1)) {
                    childMask |= 2; // southeast
                }
            }
        }

        return childMask;
    }

    function isTileInRectangle(tilingScheme, rectangle, x, y, level) {
        var tileRectangle = tilingScheme.tileXYToRectangle(x, y, level);
        return defined(Rectangle.intersection(tileRectangle, rectangle, rectangleScratch));
    }

    /**
     * Determines whether data for a tile is available to be loaded.
     *
     * @param {Number} x The X coordinate of the tile for which to request geometry.
     * @param {Number} y The Y coordinate of the tile for which to request geometry.
     * @param {Number} level The level of the tile for which to request geometry.
     * @returns {Boolean} Undefined if not supported, otherwise true or false.
     */
    VRTheWorldTerrainProvider.prototype.getTileDataAvailable = function(x, y, level) {
        return undefined;
    };

    return VRTheWorldTerrainProvider;
});
