/*
	HTML5 Speedtest v4.6.1 MPOT - Server selector
	by Federico Dossena
	https://github.com/adolfintel/speedtest/
	GNU LGPLv3 License
*/

//pings the specified URL, then calls the function result. Result will receive a parameter which is either the time it took to ping the URL, or -1 if something went wrong.
//var PING_TIMEOUT=500; //disabled because it breaks IE11
function ping(url,result){
	var xhr=new XMLHttpRequest();
	var t=new Date().getTime();
	xhr.onload=function(){
		if(xhr.responseText.length==0){ //we expect an empty response
			var instspd=new Date().getTime()-t; //rough timing estimate
			try{
				//try to get more accurate timing using performance API
				var p=performance.getEntries();
				p=p[p.length-1];
				var d=p.responseStart-p.requestStart;
				if(d<=0) d=p.duration;
				if(d>0&&d<instspd) instspd=d;
			}catch(e){
			}
			result(instspd);
		}else result(-1);
	}.bind(this);
	xhr.onerror=function(){
		result(-1);
	}.bind(this);
	xhr.open("GET",url);
	/*try{ //disabled because it breaks IE11
		xhr.timeout=PING_TIMEOUT;
	}catch(e){}*/
	xhr.send();
}

//this function repeatedly pings a server to get a good estimate of the ping. When it's done, it calls the done function without parameters. At the end of the execution, the server will have a new parameter called pingT, which is either the best ping we got from the server or -1 if something went wrong.
var PINGS=3, //up to 3 pings are performed, unless the server is down...
	SLOW_THRESHOLD=500; //...or one of the pings is above this threshold
function checkServer(server,done){
	var i=0;
	server.pingT=-1;
	if(server.server.indexOf(location.protocol)==-1) done(); else{
		var nextPing=function(){
			if(i++==PINGS){done(); return;}
			ping(server.server+server.pingURL,function(t){
				if(t>=0){
					if(t<server.pingT||server.pingT==-1) server.pingT=t;
					if(t<SLOW_THRESHOLD) nextPing(); else done();
				}else done();
			}.bind(this));
		}.bind(this);
		nextPing();
	}
}

/*this function goes through a list of servers, each with this format:
{
	name: "User friendly name",
	server:"http://yourBackend.com/",     <---- make sure there's a / at the end!
	dlURL:"garbage.php"    <----- path to garbage.php or its replacement on the server
	ulURL:"empty.php"    <----- path to empty.php or its replacement on the server
	pingURL:"empty.php"    <----- path to empty.php or its replacement on the server. This is used to ping the server by this selector
	getIpURL:"getIP.php"    <----- path to getIP.php or its replacement on the server
}
For each server, the ping is measured, then the server with the function result is called with the best server, or null if all the servers were down.
*/
function selectServer(serverList,result){
	var i=0;
	var done=function(){
		var bestServer=null;
		for(var i=0;i<serverList.length;i++){
			if(serverList[i].pingT!=-1&&(bestServer==null||serverList[i].pingT<bestServer.pingT)) bestServer=serverList[i];
		}
		result(bestServer);
	}.bind(this);
	var nextServer=function(){
		if(i==serverList.length){done(); return;}
		checkServer(serverList[i++],nextServer);
	}.bind(this);
	nextServer();
}

/* 
this function is a faster version of selectServer that tests multiple servers concurrently. Useful for large server lists	
*/
var CONCURRENCY=4; //4 seems to be the safest value
function fastSelectServer(serverList,result){
	var serverLists=[];
	for(var i=0;i<CONCURRENCY;i++){
		serverLists[i]=[];
	}
	for(var i=0;i<serverList.length;i++){
		serverLists[i%CONCURRENCY].push(serverList[i]);
	}
	var completed=0;
	var bestServer=null;
	for(var i=0;i<CONCURRENCY;i++){
		selectServer(serverLists[i],function(server){
			if(server!=null){
				if(bestServer==null||server.pingT<bestServer.pingT) bestServer=server;
			}
			completed++;
			if(completed==CONCURRENCY) result(bestServer);
		}.bind(this));
	}
}