var lon = "3.82";
var lat = "50.71";
var zoom = "12";
var PI = 3.14159265358979323846;
lon = parseFloat(lon) * 20037508.34 / 180;
lat = Math.log (Math.tan ((90 + parseFloat(lat)) * PI / 360)) / (PI / 180);
lat = lat * 20037508.34 / 180;
var map;

function get_osm_url (bounds) {
    var res = this.map.getResolution();
    var x = Math.round ((bounds.left - this.maxExtent.left) / (res * this.tileSize.w));
    var y = Math.round ((this.maxExtent.top - bounds.top) / (res * this.tileSize.h));
    var z = this.map.getZoom();
    var path = z + "/" + x + "/" + y + "." + this.type;
    var url = this.url;
        if (url instanceof Array) {
            url = this.selectUrl(path, url);
        }
        return url + path;
    }

function getTileURL(bounds) {
    var res = this.map.getResolution();
    var x = Math.round((bounds.left - this.maxExtent.left) / (res * this.tileSize.w));
    var y = Math.round((this.maxExtent.top - bounds.top) / (res * this.tileSize.h));
    var z = this.map.getZoom();
    var limit = Math.pow(2, z);
    if (y < 0 || y >= limit){
        return null;
        }
    else {
        x = ((x % limit) + limit) % limit;
        return this.url + z + "/" + x + "/" + y + "." + this.type;
        }
    }

function init() {
    map = new OpenLayers.Map ("map", {
        controls:[
            new OpenLayers.Control.Permalink(),
            new OpenLayers.Control.MouseDefaults(),
            new OpenLayers.Control.MousePosition(),
            new OpenLayers.Control.PanZoomBar()],
        maxExtent: new OpenLayers.Bounds(-20037508.34,-20037508.34,20037508.34,20037508.34),
        numZoomLevels:19, 
        maxResolution:156543,
        units:'meters', 
    });
    map.addLayer(new OpenLayers.Layer.TMS("Mapnik",
        ["http://tile.openstreetmap.org/"],
        { type: 'png', getURL: getTileURL, displayOutsideMaxExtent: true }));
    map.addLayer(new OpenLayers.Layer.TMS("OpenStreetMap",
        ["http://osm-tah-cache.firefishy.com/~ojw/Tiles/tile.php/"],
        {type:'png', getURL: get_osm_url} ));
    map.addLayer(new OpenLayers.Layer.TMS("Cycle", 
        ["http://thunderflames.org/tiles/cycle/"],
        { type: 'png', getURL: getTileURL, displayOutsideMaxExtent: true}));
    var res = this.map.getResolution();
        map.addControl(new OpenLayers.Control.LayerSwitcher());
        map.setCenter (new OpenLayers.LonLat(lon, lat), zoom);
    }