Poker Hand Evaluation in VB.NET

Posted on 2009-05-05
Last Modified: 2013-12-17
I'm writing a game of Texas Hold'Em in VB.NET and C#, and need to build some logic on calculating the resulting poker hand from a player's two cards and the cards on the table (totalling 7 in Texas Hold'Em).

For example:
Players cards: 7H, JS
Cards on table: TD, 2H, 8H, 7C, 9S

In this case the player has a Jack high straight - 7H, 8H, 9S, TD, JS. The algorithm also has to discern that even though the player also has a pair (7H, 7C) the real hand is the straight.

I tried writing my own logic, but it ended up being 250+ lines of code by the time it could detect a royal flush. I've seen some code around that is meant to do it, but most of it is either in C/C++ or for other versions of poker.

I can write my own logic to discern a winner from a list of hands (i.e. that 3 of a kind beats two pair, and that an ace high straight beats a ten high straight). However, what I'm looking for is a function (or class) written in C# or VB.NET to return a poker hand (e.g. "Two Pair - 3s and Jacks" or "Four Aces") from a set of cards.

I've attached the VB.NET code for my structures and enums, but I can easily convert from another result format.
Private Enum CardSuits As Integer

        Spade = 4

        Club = 3

        Diamond = 2

        Heart = 1

    End Enum

    Private Enum CardValues As Integer

        AceLow = 1

        Two = 2

        Three = 3

        Four = 4

        Five = 5

        Six = 6

        Seven = 7

        Eight = 8

        Nine = 9

        Ten = 10

        Jack = 11

        Queen = 12

        King = 13

        Ace = 14

    End Enum

    Private Enum HandTypes As Integer

        HighCard = 0

        Pair = 1

        TwoPair = 2

        ThreeOfKind = 3

        Straight = 4

        Flush = 5

        FullHouse = 6

        FourOfKind = 7

        StraightFlush = 8

    End Enum

    Private Structure Card

        Public Suit As CardSuits

        Public Value As CardValues

        Public Overrides Function ToString() As String

            Dim ret As String = ""

            If Value <= CardValues.Ten Then

                ret = Value


                Select Case Value

                    Case CardValues.Jack

                        ret = "Jack"

                    Case CardValues.Queen

                        ret = "Queen"

                    Case CardValues.King

                        ret = "King"

                    Case CardValues.Ace

                        ret = "Ace"

                    Case Else

                        ret = "Error"

                End Select

            End If

            Select Case Suit

                Case CardSuits.Club

                    ret &= " of Clubs"

                Case CardSuits.Diamond

                    ret &= " of Diamonds"

                Case CardSuits.Heart

                    ret &= " of Hearts"

                Case CardSuits.Spade

                    ret &= " of Spades"

                Case Else

                    ret &= " of Error"

            End Select

            Return ret

        End Function

    End Structure

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Question by:burningmace
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LVL 73

Expert Comment

Comment Utility
I would write a series of mini functions


for the 2 biggest hands...

Straight Flush - simply check if both straight and flush
Royal Flush - check if straight and flush and highcard=Ace

Then check in reverse order the first score that returns positive is the one you keep.

Yes, this could mean you could triple check for a straight, if that's a significant concern then save those check results  in a variable and return that value instead of re-evaluating.

Author Comment

Comment Utility
That doesn't really help, as that was what I was doing in the first place. The issue is that the code I wrote for checking hands became extremely complex and bulky within only a single check. For example, my code to check for a straight required the following:

1) Convert the list of 7 cards into a SortedList(Of Integer, Integer) in order to sort the values from lowest to highest. If Highest - Lowest > 4 then there's a possible straight.
2) Loop from highest to lowest, if Abs(c(n) - c(n-1)) = 1 then increment runCount, if not then runCount = 0
3) If runCount reaches 4 (e.g. in case of Q->J, J->10, 10->9, 9->8) then we have a straight.
4) If no straight was detected, replace all instances of Ace with AceLow and try again.
6) Return the list of cards that creates the highest score.

But we have a problem. What if there is a 2H,3H,4H,5H,6H straightf flush and a 3H,4H,5H,6H,7D straight? The algorithm will have completely missed the straight flush, which is the highest possilbe score for Hold'Em. This now means we have to re-engineer the algorithm:

1) Load the suits of the input cards into a frequency array.
2) Check if the frequency of a suit is 5 or more. Only one suit's frequency will ever be >4.
3) Discard any cards not matching that suit. Call the remaining cards FlushDeck
4) Check for straights within FlushDeck. If one exists, we have a straight flush. Return StraightFlush(HighCard).
5) If we don't have a straight, replace all Aces with AceLows and check for straights again. If one exists, we have a straight flush.  Return StraightFlush(HighCard).
6) If we don't have a straight flush, we still have a flush. Note tha we have a Flush(HighCard).
7) Go on to check for Four of a Kind and Full House (another algorithm to write).
8) If we didn't have a 4Ok or an FH then return StraightFlush(HighCard)

And that's the tip of the iceberg. I still would have to write logic for six other hand types. On top of that, I'd have to work on determining winners when the top two hands match (e.g. Twos and Fives, Threes and Fives or Sevens and Jacks with 8 kicker, Sevens and Jacks with 5 kicker). I also have to code logic for draws (e.g. both players have 3,4,5,6,7 straights).

What I need is a pre-existing algorithm that can do it for me. They definately exist, but they seem to have eluded me so far.
LVL 73

Expert Comment

Comment Utility
"The algorithm will have completely missed the straight flush, which is the highest possilbe score for Hold'Em."

No,  look again.  

"Then check in reverse order the first score that returns positive is the one you keep."

So, check for royal flush, if no, check for straight flush, if no, check for 4 of a kind, if no, check for flush, etc.  and stop the first time any check returns a positive result.

LVL 73

Expert Comment

Comment Utility
sorry, shouldn't have hit submit so soon

ranking them isn't so hard, write each individual rank routine so it returns the hand in rank order.

that is,  if you have 7c7sKhKd8c  as your best hand  it would return 2 outputs
KhKd7s7c8c (as a string or array or whatever you want)
to compare this hand to another 2 pair ranked hand you would simply walk through the list and compare each of the 5 cards until one hand beat the other.

if the hand does not match the rank, then the output would be null


Accepted Solution

burningmace earned 0 total points
Comment Utility
I know how to compare the hands, you seem to have misunderstood.

The issue is that I've had to re-write the order of things. First I have to look for flushes. If there's a flush, check if a straight can be made out of thel cards that match the suit that the flush is in. If so, then we have a straight flush.

All cards being checked for a straight must always be sorted into order, so that the highest possible straight can be determined. For example:

1) Qh, Js, 10d, 8d, Ks, 2s, 9c is a straight of 9, 10, J, Q, K. However, it also has a straight of 8, 9, 10, J, Q. In order to properly evaluate the hand we re-order the input to Ks, Qh, Js, 10d, 9c, 8d, 2c. It is then trivial to check that there is a sequence beginning with Ks that decrements 5 times. This means that the false Jack high straight is not returned.

2) 6h, 7h, 6d, 9c, 8d, 5h, 8s is a straight of 5, 6, 7, 8, 9. The issue here is that there is a duplicate 6 and a duplicate 8. So, when in order it would be viewed as 9c, 8s, 8d, 7h, 6d, 6h, 5h. There is no simple decrementing pattern here. So, what I had to do is alter my algorithm to ignore cases where number pairs are equal:
9->8 decrements, 8->8 ignored, 8->7 decrements, 7->6 decrements, 6->6 ignored, 6->5 decrements.
There are 4 decrements linking 9 to 5 without a broken link (i.e. 7->4 does not match b = a-1), therefore there is a 9 high straight here.

I have no proiblem understanding the logic behind it, I was asking if anyone knew where I could find a pre-written algorithm to save me the time and effort.

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