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.

• Ugh. You have no idea how much I wish it were that simple. Dec 10 '13 at 21:13
• Are the cards guaranteed to be maintained as sorted? Dec 10 '13 at 21:15
• Indeed the straight seems to be one of the hardest things about this week's challenge. I like your approach but remember that Ace can be used as both 1 and 14 in a straight (Ace - 5 and 10 - Ace). Also, why are you starting the loop at i = 1 when arrays indexes tends to be 0-based? Dec 10 '13 at 21:17
• Although I have to add: If you don't want to handle both the low & high ace case, that's OK for now. Dec 10 '13 at 21:26
• Wait a minute, a straight is 5 cards, even if your hand has 7 cards ? Dec 10 '13 at 21:27

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.