Generating all 5 card poker hands

Question

This problem sounds simple at first glance, but turns out to be a lot more complicated than it seems. It\'s got me stumped for the moment.

There are 52c5 = 2,598,960

Answer 1

You could simply give all hands a canonical ordering of values (A to K), then assign abstract suit letters according to their order of first appearance in that order.

Example: JH 4C QD 9C 3D would convert to 3a 4b 9b Jc Qa.

Generation should work best as dynamic programming:

• start with a set of a single hand that is empty,
• make a new set:
  • for each hand in the old set, generate each possible hand by adding one of the remaining cards
  • canonicalize all new hands
  • remove duplicates

Answer 2

If you are just interested in hands that result in different hand rankings, there are actually only 7462 distinct hand classes that have to be considered (see Wikipedia).

By creating a table with an example for each class and their accompanying multiplicity you can check all relevant hands weighted with their probability quite fast. That is, assuming that no cards are known and therefore fixed beforehand already.

Answer 3

Initial input:

H 0 0 0 0 0 0 0 0 0 0 0 0 0
C 1 0 0 0 0 0 0 0 0 0 0 0 0
D 1 0 0 0 0 0 0 0 0 0 0 0 0
S 1 1 0 0 0 0 0 0 0 0 0 0 0
+ A 2 3 4 5 6 7 8 9 T J Q K

Step 1: for each rank greater than or equal the highest rank used, set all suits in that rank to 0. you can get away with only checking higher cards because lower combinations will be checked by the lower starting points.

H 0 0 0 0 0 0 0 0 0 0 0 0 0
C 1 0 0 0 0 0 0 0 0 0 0 0 0
D 1 0 0 0 0 0 0 0 0 0 0 0 0
S 1 0 0 0 0 0 0 0 0 0 0 0 0
+ A 2 3 4 5 6 7 8 9 T J Q K

Step 2: Collapse to distinct rows

0 0 0 0 0 0 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0 0 0 0 0 0
A 2 3 4 5 6 7 8 9 T J Q K

Step 3: Climb back up determining first suit that match each distinct row, and choose the suits which match the distinct rows (identified by a *)

H 0 * 0 0 0 0 0 0 0 0 0 0 0
C 1 0 0 0 0 0 0 0 0 0 0 0 0
D 1 * 0 0 0 0 0 0 0 0 0 0 0
S 1 1 0 0 0 0 0 0 0 0 0 0 0
+ A 2 3 4 5 6 7 8 9 T J Q K

Now showing the repeat for rank 3

H 0 0 0 0 0 0 0 0 0 0 0 0 0
C 1 0 0 0 0 0 0 0 0 0 0 0 0
D 1 0 0 0 0 0 0 0 0 0 0 0 0
S 1 1 0 0 0 0 0 0 0 0 0 0 0
+ A 2 3 4 5 6 7 8 9 T J Q K

0 0 0 0 0 0 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0 0 0 0 0 0
A 2 3 4 5 6 7 8 9 T J Q K

H 0 0 * 0 0 0 0 0 0 0 0 0 0
C 1 0 0 0 0 0 0 0 0 0 0 0 0
D 1 0 * 0 0 0 0 0 0 0 0 0 0
S 1 1 * 0 0 0 0 0 0 0 0 0 0
+ A 2 3 4 5 6 7 8 9 T J Q K

Step 4: Once there are 5 cells set to 1, increment the total possible suit abstracted hands count by 1 and recurse up.

The total number of suit abstracted hands possible is 134,459. This is the code I wrote to test it out:

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication20
{
    struct Card
    {
        public int Suit { get; set; }
        public int Rank { get; set; }
    }
    
    class Program
    {
        static int ranks = 13;
        static int suits = 4;
        static int cardsInHand = 5;

        static void Main(string[] args)
        {
            List<Card> cards = new List<Card>();
            //cards.Add(new Card() { Rank = 0, Suit = 0 });
            int numHands = GenerateAllHands(cards);
    
            Console.WriteLine(numHands);
            Console.ReadLine();
        }
  
        static int GenerateAllHands(List<Card> cards)
        {
            if (cards.Count == cardsInHand) return 1;
    
            List<Card> possibleNextCards = GetPossibleNextCards(cards);
    
            int numSubHands = 0;
    
            foreach (Card card in possibleNextCards)
            {
                List<Card> possibleNextHand = cards.ToList(); // copy list
                possibleNextHand.Add(card);
                numSubHands += GenerateAllHands(possibleNextHand);
            }
    
            return numSubHands;
        }
    
        static List<Card> GetPossibleNextCards(List<Card> hand)
        {
            int maxRank = hand.Max(x => x.Rank);
            
            List<Card> result = new List<Card>();
    
            // only use ranks >= max
            for (int rank = maxRank; rank < ranks; rank++)
            {
                List<int> suits = GetPossibleSuitsForRank(hand, rank);
                var possibleNextCards = suits.Select(x => new Card { Rank = rank, Suit = x });
                result.AddRange(possibleNextCards);
            }
    
            return result;
        }
    
        static List<int> GetPossibleSuitsForRank(List<Card> hand, int rank)
        {
            int maxSuit = hand.Max(x => x.Suit);
    
            // select number of ranks of different suits
            int[][] card = GetArray(hand, rank);
    
            for (int i = 0; i < suits; i++)
            {
                card[i][rank] = 0;
            }
    
            int[][] handRep = GetArray(hand, rank);
    
            // get distinct rank sets, then find which ranks they correspond to
            IEnumerable<int[]> distincts = card.Distinct(new IntArrayComparer());
    
            List<int> possibleSuits = new List<int>();
    
            foreach (int[] row in distincts)
            {
                for (int i = 0; i < suits; i++)
                {
                    if (IntArrayComparer.Compare(row, handRep[i]))
                    {
                        possibleSuits.Add(i);
                        break;
                    }
                }
            }
    
            return possibleSuits;
        }
    
        class IntArrayComparer : IEqualityComparer<int[]>
        {
            #region IEqualityComparer<int[]> Members
    
            public static bool Compare(int[] x, int[] y)
            {
                for (int i = 0; i < x.Length; i++)
                {
                    if (x[i] != y[i]) return false;
                }
    
                return true;
            }
    
            public bool Equals(int[] x, int[] y)
            {
                return Compare(x, y);
            }
    
            public int GetHashCode(int[] obj)
            {
                return 0;
            }

            #endregion
        }

        static int[][] GetArray(List<Card> hand, int rank)
        {
            int[][] cards = new int[suits][];
            for (int i = 0; i < suits; i++)
            {
                cards[i] = new int[ranks];
            }

            foreach (Card card in hand)
            {
                cards[card.Suit][card.Rank] = 1;
            }
    
            return cards;
        }
    }
}

Hopefully it is broken up enough to be easily understandable.

Answer 4

I'm not a poker player, so the details of hand precedence are beyond me. But it seems like the problem is that you are traversing the space of "sets of 5 cards" by generating sets from the deck, when you should be traversing the space of "distinct poker hands".

The space of distinct hands will require a new grammar. The important thing is to capture exactly the information that is relevant to hand precedence. For example, there are only 4 hands that are royal flushes, so those hands can be described as the symbol "RF" plus a suit designator, like "RFC" for royal flush in clubs. A 10-high heart flush could be "FLH10" (not sure if there are other precedence characteristics of flushes, but I think that's all you need to know). A hand that is "2C 2S AH 10C 5D" would be a longer expression, something like "PR2 A 10 5" if I undestand your initial problem statement.

Once you have defined the grammar of distinct hands, you can express it as regular expressions and that will tell you how to generate the entire space of distinct hands. Sounds like fun!

Answer 5

Here is a simple and straightforward algorithm for reducing hands to a canonical one based on suit permutatoins.

1. convert hand to four bitsets, one per suit representing cards of the suit
2. sort the bitsets
3. convert bitsets back into hand

This is what the algorithm looks like in C++, with some implied Suit and CardSet classes. Note that the return statement converts the hand by concatenating the bitstrings.

CardSet CardSet::canonize () const
{
  int smasks[Suit::NUM_SUIT];
  int i=0;
  for (Suit s=Suit::begin(); s<Suit::end(); ++s)
    smasks[i++] = this->suitMask (s);

  sort (smasks, smasks+Suit::NUM_SUIT);

  return CardSet(
    static_cast<uint64_t>(smasks[3])                        |
    static_cast<uint64_t>(smasks[2]) << Rank::NUM_RANK      |
    static_cast<uint64_t>(smasks[1]) << Rank::NUM_RANK*2    |
    static_cast<uint64_t>(smasks[0]) << Rank::NUM_RANK*3);
}