rock paper scissors for more than 2 players and extensible

rps.js

function RPS(noPlayers, proto) {
	var obj = Object.create(proto || RPS.prototype)
	obj.players = noPlayers
	obj.score = {}
	obj.moves = {}
	for (var i = 0; i < noPlayers; i++) {
		obj.score[i] = 0
		obj.moves[i] = [] }
	return obj }

RPS.prototype.beats =
  { "rock": "scissors"
  , "paper": "rock"
  , "scissors": "paper" }

RPS.prototype.wins = function(p1, p2) {
	return (this.beats[p1] == p2) }

// found on stackoverflow
Array.prototype.unique = function(a){
	return function(){return this.filter(a)}}
(function(a,b,c){return c.indexOf(a,b+1)<0
});

RPS.prototype.calcscore = function(winners, move) {
	return (winners.indexOf(move) != -1) ? 1 : 0 }

// if the weapons don't form a cycle, the upper one wins
// otherwise, it's a draw
RPS.prototype.winningweapons = function(moves) {
	var uniquemoves = moves.unique()
	var wins = this.wins.bind(this)
	return uniquemoves.filter(function(weapon) {
		// every move is beaten by this one, or it is this one
		// but it needs to beat at least one move
		var others = uniquemoves.filter(function(move) {
			return move != weapon })
		return others.length > 0 &&
			   others.every(wins.bind(0, weapon)) })}

RPS.prototype.move = function(moves) {
	for (var i = 0; i < this.players; i++)
		this.moves[i].push(moves[i])
	var winners = this.winningweapons(moves)
	var scores = moves.map(this.calcscore.bind(this, winners))
	for (var i = 0; i < this.players; i++)
		this.score[i] += scores[i]
	// return all the winning players
	var winning_players = moves.reduce(function(p, c, i) {
		return winners.indexOf(c) != -1 ? p.concat([i]) : p
	}, [])
	return winning_players }

function randfromArr(a) {
	return a[(a.length * Math.random()) | 0] }

RPS.prototype.AI = function() {
	return randfromArr(Object.keys(this.beats)) }

if (module && module.exports) {
	module.exports = RPS }

// example usage:
// game = RPS(2) // 2 is the number of players
// game.move(["scissors", game.AI()])
// the return object is an array of the winning players
// score is inside game.score