server.js

var io = require('socket.io')(process.env.PORT || 3000);
var shortid = require('shortid');

console.log('server started');

var players = [];
var playerSpeed = 3;

io.on('connection', function(socket){
	var thisPlayerId = shortid.generate();

	var player = {
		id:thisPlayerId,
		destination: {
			x:0,y:0
		},
		lastPosition: {
			x:0,y:0
		},
		lastMoveTime: 0
	};

	players[thisPlayerId] = player;

	console.log('client connected, broadcasting spawn, id: ', thisPlayerId);

	socket.emit('register', {id:thisPlayerId});
	socket.broadcast.emit('spawn' , { id: thisPlayerId });
	socket.broadcast.emit('requestPosition');

	for(var playerId in players){

		if(playerId == thisPlayerId)
			continue;

		socket.emit('spawn', players[playerId]);
		console.log('sending spawn to new player for id', playerId);
	}

	socket.on('move', function(data){
		data.id = thisPlayerId;
		console.log('client moved', JSON.stringify(data));

		player.destination.x = data.d.x;
		player.destination.y = data.d.y;

		console.log("distance detween current and destination: ", lineDistance(data.c, data.d));

		var elapsedTime = Date.now() - player.lastMoveTime;

		var travelDistanceLimit = elapsedTime * playerSpeed / 1000;

		var requestedDistanceTraveled = lineDistance(player.lastPosition, data.c);

		console.log("travelDistanceLimit: ", travelDistanceLimit);
		console.log("requestedDistanceTraveled: ", requestedDistanceTraveled);

		if(requestedDistanceTraveled > travelDistanceLimit){
			//someone is cheating
		}

		player.lastMoveTime = Date.now();

		player.lastPosition = data.c;

		delete data.c;

		data.x = data.d.x;
		data.y = data.d.y;

		delete data.d;

		socket.broadcast.emit('move', data);
	});

	socket.on('follow', function(data){
		console.log("follow request: ", data);
		data.id = thisPlayerId;

		socket.broadcast.emit('follow', data);
	});

	socket.on('updatePosition', function(data){
		console.log("update position: ", data);

		data.id = thisPlayerId;

		socket.broadcast.emit('updatePosition', data);
	});

	socket.on('attack', function(data) {
		console.log("attack request: ", data);
		data.id = thisPlayerId;

		io.emit('attack', data);
	});

	socket.on('disconnect', function(data){
		console.log('client disconnected');

		delete players[thisPlayerId];

		socket.broadcast.emit('disconnected', {id : thisPlayerId});
	});
});

function lineDistance(vectorA, vectorB){
	var xs = 0;
	var ys = 0;

	xs = vectorB.x - vectorA.x;
	xs = xs * xs;

	ys = vectorB.y - vectorA.y;
	ys = ys * ys;

	return Math.sqrt(xs * ys);
}