isStraight evaluation function

The context is Weekend Challenge #2 (Poker hand evaluation).

Hand.prototype.isStraight = function()
{
  for( var i = 1 ; i < this.cards.length ; i++ )
    if( this.cards[i].value + 1 != this.cards[i-1].value )
      return false;
  return true;
}

cards is an array with card objects { suit : "x" , value : "0->12" }. The cards are sorted during the creation of the hand.

My approach to detect straightness seems like the hard way. Can anyone suggest something else?

Okay, Poker is tough. I will revise this question once I have my new isStraight.

It's not that hard.

Instead of sorting the cards, looping over them and skipping double results, let's convert them to a more useful format:

var bitmap = 0;

for(var i = 0; i < cards.length; i++)
{
    var value = cards[i].value;

    // set i+1 bit in the bitmap
    bitmap |= 1 << (value + 1);

    // if it's an ace, also set the low bit
    if(value === 12)
        bitmap |= 1;
}

Now, if there's a card of value i in your hand, the i+1 bit will be set in the bitmap. An Ace is treated as two cards with values 13 and 0.
Next, we scan the bitmap for 5 consecutive bits set, which is equivalent to 31 (1 | 2 | 4 | 8 | 16). We start with the highest straight, which is 9.
The lowest straight is 0, if there is no straight i is -1.

for(var i = 10; i--; )
{
    if((bitmap & 31 << i) === (31 << i))
    {
        break;
    }
}

The method works for games with more than 5 cards. Note that in any variant of Poker, 5 cards make a hand, so even in Omaha with 9 cards, 5 cards give you a straight.

Note: The method is roughly equivalent to creating a set (and checking for 5 consecutive elements). As you can see, with bit-wise operators it is much simpler to check for 5 consecutive cards.

Another note: There's a harder-to-understand but slightly faster method for testing a straight given the bitmap. I might add it later, but for now I think this method hard enough to understand for beginners.

StackExchange Code Review Q#37077, answer score: 8