// The famous sequence of input's representing the KONAMI cheat code.
var KONAMI_CODE = ['UP', 'UP', 'DOWN', 'DOWN', 'LEFT', 'RIGHT', 'LEFT', 'RIGHT', 'B', 'A'];

/* 
  Takes a KeyBoardEvent and returns either a KONAMI_CODE key
  or the value null.
  
  It will return a KONAMI_CODE key when the KeyBoardEvent's key
  matches a value in the KONAMI_CODE.
  
  It will return null when the key is not part of the KONAMI_CODE.
*/
function eventToKonamiCode(event) {
  switch (event.which) {
    case 38: return 'UP';
    case 40: return 'DOWN';
    case 37: return 'LEFT';
    case 39: return 'RIGHT';
    case 66: return 'B';
    case 65: return 'A';
  }
  
  return null;
}

/*
  Checks if the codes paramater, which is an array, matches the KONAMI_CODE exactly.
*/
function isKonamiCode(codes) {
  return _.isEqual(codes, KONAMI_CODE);
}

/*
  Appends the 'code' parameter to the 'codes' parameter, if the
  code is the next part of the KONAMI_CODE sequence.
  
  If the code is not the next sequence of the KONAMI_CODE an
  empty array is returned.
*/
function codeAccumulator(codes, code) {
  // Not a normal KONAMI_CODE key so return an empty array.
  if (code === null) {
    return [];
  }
   
  codes.push(code);

  // Check if 'codes' still matches the KONAMI_CODE.
  var subKonamiCode = _.take(KONAMI_CODE, codes.length);
  if (_.isEqual(codes, subKonamiCode)) {
    return codes;
  } else {
    return []; // No match back to square one.
  }
}

var konamiStream = Rx.Observable.fromEvent(document, 'keydown')
  .map(eventToKonamiCode)
  .scan(codeAccumulator, [])
  .filter(isKonamiCode);

var livesSpan = document.querySelector('span');

konamiStream.subscribe(function() {
  alert('You entered the Konami code!');
  livesSpan.textContent = '99';
});