battle.js

app.get('/battle/:winner/:loser', function(req, res){  
        if (req.params.winner && req.params.loser) {
                var winner = req.params.winner;
                var loser = req.params.loser;
                db.mget('image:' + winner + ':rating',
                        'image:' + loser + ':rating',
                        function (err, ratings) {
                                if (err) { throw err; }
                                ratings[0] = ratings[0] ? parseFloat(ratings[0]) : 1000;
                                ratings[1] = ratings[1] ? parseFloat(ratings[1]) : 1000;
                                var winnerExpected = 1 / (1 + (Math.pow(10,(ratings[1] - ratings[0]) / 400)));
                                var loserExpected = 1 / (1 + (Math.pow(10,(ratings[0] - ratings[1]) / 400)));
                                var k = 30;
                                var winnerAdjustment = Math.round(ratings[0] + (k * (1 - winnerExpected)));
                                var loserAdjustment = Math.round(ratings[1] + (k * (0 - loserExpected)));
                                db.mset('image:' + winner + ':rating', winnerAdjustment,
                                        'image:' + loser + ':rating', loserAdjustment,
                                        function (err, newRatings) {
                                                if (err) { throw err; }
                                                res.redirect('/');
                                });
                });
        } else { res.redirect('/'); }
});