source.url = function()

in modules/renderer/background_source.js [112:227]
95 lines of code
26 McCabe index (conditional complexity)

    source.url = function(coord) {
        var result = _template;
        if (result === '') return result;   // source 'none'


        // Guess a type based on the tokens present in the template
        // (This is for 'custom' source, where we don't know)
        if (!source.type) {
            if (/SERVICE=WMS|\{(proj|wkid|bbox)\}/.test(_template)) {
                source.type = 'wms';
                source.projection = 'EPSG:3857';  // guess
            } else if (/\{(x|y)\}/.test(_template)) {
                source.type = 'tms';
            } else if (/\{u\}/.test(_template)) {
                source.type = 'bing';
            }
        }


        if (source.type === 'wms') {
            var tileToProjectedCoords = (function(x, y, z) {
                //polyfill for IE11, PhantomJS
                var sinh = Math.sinh || function(x) {
                    var y = Math.exp(x);
                    return (y - 1 / y) / 2;
                };

                var zoomSize = Math.pow(2, z);
                var lon = x / zoomSize * Math.PI * 2 - Math.PI;
                var lat = Math.atan(sinh(Math.PI * (1 - 2 * y / zoomSize)));

                switch (source.projection) {
                    case 'EPSG:4326':
                        return {
                            x: lon * 180 / Math.PI,
                            y: lat * 180 / Math.PI
                        };
                    default: // EPSG:3857 and synonyms
                        var mercCoords = d3_geoMercatorRaw(lon, lat);
                        return {
                            x: 20037508.34 / Math.PI * mercCoords[0],
                            y: 20037508.34 / Math.PI * mercCoords[1]
                        };
                }
            });

            var tileSize = source.tileSize;
            var projection = source.projection;
            var minXmaxY = tileToProjectedCoords(coord[0], coord[1], coord[2]);
            var maxXminY = tileToProjectedCoords(coord[0]+1, coord[1]+1, coord[2]);

            result = result.replace(/\{(\w+)\}/g, function (token, key) {
              switch (key) {
                case 'width':
                case 'height':
                    return tileSize;
                case 'proj':
                    return projection;
                case 'wkid':
                    return projection.replace(/^EPSG:/, '');
                case 'bbox':
                    // WMS 1.3 flips x/y for some coordinate systems including EPSG:4326 - #7557
                    if (projection === 'EPSG:4326' &&
                        // The CRS parameter implies version 1.3 (prior versions use SRS)
                        /VERSION=1.3|CRS={proj}/.test(source.template().toUpperCase())) {
                        return maxXminY.y + ',' + minXmaxY.x + ',' + minXmaxY.y + ',' + maxXminY.x;
                    } else {
                        return minXmaxY.x + ',' + maxXminY.y + ',' + maxXminY.x + ',' + minXmaxY.y;
                    }
                case 'w':
                    return minXmaxY.x;
                case 's':
                    return maxXminY.y;
                case 'n':
                    return maxXminY.x;
                case 'e':
                    return minXmaxY.y;
                default:
                    return token;
              }
            });

        } else if (source.type === 'tms') {
            result = result
                .replace('{x}', coord[0])
                .replace('{y}', coord[1])
                // TMS-flipped y coordinate
                .replace(/\{[t-]y\}/, Math.pow(2, coord[2]) - coord[1] - 1)
                .replace(/\{z(oom)?\}/, coord[2])
                // only fetch retina tiles for retina screens
                .replace(/\{@2x\}|\{r\}/, isRetina ? '@2x' : '');

        } else if (source.type === 'bing') {
            result = result
                .replace('{u}', function() {
                    var u = '';
                    for (var zoom = coord[2]; zoom > 0; zoom--) {
                        var b = 0;
                        var mask = 1 << (zoom - 1);
                        if ((coord[0] & mask) !== 0) b++;
                        if ((coord[1] & mask) !== 0) b += 2;
                        u += b.toString();
                    }
                    return u;
                });
        }

        // these apply to any type..
        result = result.replace(/\{switch:([^}]+)\}/, function(s, r) {
            var subdomains = r.split(',');
            return subdomains[(coord[0] + coord[1]) % subdomains.length];
        });


        return result;
    };