function TicTacToe() {
 var ended=false;
 var moves=0;
 var pos=[0,0,0,
 0,0,0,
 0,0,0];
 var nextSide=1;
 var aiSwitch=function(){
   nextSide=nextSide==1?2:1;
 }
 var prompts={
  ended:"Game ended",
  userWin:"You win!",
  draw:"Draw!",
  aiWin:"I win!",
  next:"Your move?",
  illegal:"Illegal move"
 };
 var checkWin=function(lastMove){
   // Use fast lookup table
   var wins=[[[1,2],[3,6],[4,8]],//0
   [[0,2],[4,7]],//1
   [[0,1],[5,8],[4,6]],//2
   [[4,5],[0,6]],//3
   [[3,5],[2,7],[0,8],[2,6]],//4
   [[2,8],[3,4]],//5
   [[7,8],[1,3],[2,4]],//6
   [[6,8],[1,4]],//7
   [[6,7],[2,5],[0,4]]//8
   ];
   return wins[lastMove].reduce(function(last,curr){
     return last||curr.reduce(function(all,mov){
       return all&&pos[mov]==pos[lastMove];
     },true);
   },false);
 };
 var aiMove=function(){
   var moveList=[4,0,2,6,8,1,3,5,7];
   return moveList.reduce(function(empty,next){
     return (empty>=0||pos[next])?empty:next;       
   },-1);
 };
 var renderPos=function(){
   var renderXO=function(i){return ["_","X","0"][i];};
   console.log(pos.slice(0,3).map(renderXO).join("|"));
   console.log(pos.slice(3,6).map(renderXO).join("|"));
   console.log(pos.slice(6,9).map(renderXO).join("|"));
 };
 this.move = function(field) {
    var realMove=field-1;
    if(ended){
      return [0,prompts.ended];
    }
    if(pos[realMove]){
      return [0,prompts.illegal];
    }
    if(moves==0&&!isFinite(realMove)){
      moves++;
      pos[4]=nextSide;
      renderPos();
      aiSwitch();
      return [5, prompts.next];
    }
    if(isFinite(realMove)){
      pos[realMove]=nextSide;
      aiSwitch();
      moves++;
      if(checkWin(realMove)){
        ended=true;
        renderPos();
        return [0, prompts.userWin];
      }
      if(moves>=9){
        ended=true;
        renderPos();
        return [0, prompts.draw];
      }
    }
    var next=aiMove();
    pos[next]=nextSide;
    moves++;
    if(checkWin(next)){
      ended=true;
      renderPos();
      return [next+1,prompts.aiWin];
    }
    if(moves>=9){
      ended=true;
      renderPos();
      return [next+1, prompts.draw];
    }
    renderPos();
    aiSwitch();
    return [next+1,prompts.next];
  };
}