21 (Card Game) solution

21.js

// a couple of FPish array enhancements

// gives a new array with value pushed on it
Array.prototype.$push = function(v){
  var a = this.slice();
  a.push(v);
  return a;
}

// gives the array without selected indexes
Array.prototype.$remove = function(inds){
  var a = [];
  for(var i = 0; i < this.length; i++){
    var keep = true;
    for(var k in inds){
      if(inds[k] == i){
        keep = false;
        break;
      }
    }
    keep && a.push(this[i]);
  }
  return a;
}

var operators = '+ - / *'.split(' ');

// All the ways that the cards can be combined
solutions = function(cards, cb){
  if(cards.length == 1){
    cb(cards[0]);
  }

  for(var i = 0; i < cards.length; i++){
    for(var j = i + 1; j < cards.length; j++){
      var rest = cards.$remove([i,j]);
      for(var o = 0; o < operators.length; o++){
        var applied = ['(', cards[i], operators[o], cards[j], ')'].join(' ');
        solutions(rest.$push(applied), cb);
      }

    }
  }
}

var select_ids = ['a','b','c','d'];

var check = function(){
    var cards = [];
    for (var i=0; i < select_ids.length; i++) {
        cards[i] = parseInt(document.getElementById(select_ids[i]).value,0);
    }

  document.getElementById('results').innerHTML = '';
  solutions(cards, function(solution){
    //check if the solution reaches 21
    if(eval(solution) === 21) {
      document.getElementById('results').innerHTML += ('<li>' + solution + '</li>');
    }
  });

  if(!document.getElementById('results').innerHTML.length){
    document.getElementById('results').innerHTML = '<li>No solutions found</li>';
  }
}

for (var i=0; i < select_ids.length; i++) {
    document.getElementById(select_ids[i]).onchange = check;
};

check();