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 ways to choose 5 cards from a 52 card deck. However, since suits are interchangeable in poker, many of these are equivalent - the hand 2H 2C 3H 3S 4D is equivalent to 2D 2S 3D 3C 4H - simply swap the suits around. According to wikipedia, there are 134,459 distinct 5 card hands once you account for possible suit recolorings.

The question is, how do we efficiently generate all these possible hands? I don't want to generate all hands, then eliminate duplicates, as I want to apply the problem to larger numbers of cards, and the number of hands to evaluate fast spirals out of control. My current attempts have centered around either generating depth-first, and keeping track of the currently generated cards to determine what suits and ranks are valid for the next card, or breadth-first, generating all possible next cards, then removing duplicates by converting each hand to a 'canonical' version by recoloring. Here's my attempt at a breadth-first solution, in Python:

# A card is represented by an integer. The low 2 bits represent the suit, while
# the remainder represent the rank.
suits = 'CDHS'
ranks = '23456789TJQKA'

def make_canonical(hand):
  suit_map = [None] * 4
  next_suit = 0
  for i in range(len(hand)):
    suit = hand[i] & 3
    if suit_map[suit] is None:
      suit_map[suit] = next_suit
      next_suit += 1
    hand[i] = hand[i] & ~3 | suit_map[suit]
  return hand

def expand_hand(hand, min_card):
  used_map = 0
  for card in hand:
    used_map |= 1 << card

  hands = set()
  for card in range(min_card, 52):
    if (1 << card) & used_map:
      continue
    new_hand = list(hand)
    new_hand.append(card)
    make_canonical(new_hand)
    hands.add(tuple(new_hand))
  return hands

def expand_hands(hands, num_cards):
  for i in range(num_cards):
    new_hands = set()
    for j, hand in enumerate(hands):
      min_card = hand[-1] + 1 if i > 0 else 0
      new_hands.update(expand_hand(hand, min_card))
    hands = new_hands
  return hands

Unfortunately, this generates too many hands:

>>> len(expand_hands(set([()]), 5))
160537

Can anyone suggest a better way to generate just the distinct hands, or point out where I've gone wrong in my attempt?

Answer 1

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 2

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 3

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 4

Your problem sounded interesting, so i simple tried to implements it by just looping over all possible hands in a sorted way. I've not looked at your code in details, but it seems my implementation is quite different from yours. Guess what count of hands my script found: 160537

• My hands are always sorted, e.g. 2 3 4 4 D
• If there are 2 equal cards, the color is also sorted (colors are just called 0,1,2,3)
• the first card has always color 0, the second color 0 or 1
• A card can only have the color of an previous card or the next bigger number, e.g. if card 1+2 have color 0, card three can only have the colors 0 or 1

Are you sure, the number on wikipedia is correct?

count = 0
for a1 in range(13):
    c1 = 0
    for a2 in range(a1, 13):
        for c2 in range(2):
            if a1==a2 and c1==c2:
                continue
            nc3 = 2 if c1==c2 else 3
            for a3 in range(a2, 13):
                for c3 in range(nc3):
                    if (a1==a3 and c1>=c3) or (a2==a3 and c2>=c3):
                        continue
                    nc4 = nc3+1 if c3==nc3-1 else nc3
                    for a4 in range(a3, 13):
                        for c4 in range(nc4):
                            if (a1==a4 and c1>=c4) or (a2==a4 and c2>=c4) or (a3==a4 and c3>=c4):
                                continue
                            nc5 = nc4+1 if (c4==nc4-1 and nc4!=4) else nc4
                            for a5 in range(a4, 13):
                                for c5 in range(nc5):
                                    if (a1==a5 and c1>=c5) or (a2>=a5 and c2>=c5) or (a3==a5 and c3>=c5) or (a4==a5 and c4>=c5):
                                        continue
                                    #print([(a1,c1),(a2,c2),(a3,c3),(a4,c4),(a5,c5)])
                                    count += 1
print("result: ",count)

Answer 5

Look at Pokersource. The problem gets even worse when you're considering completing hands given some cards already drawn.

The guy behind PokerStove did a great job in this direction, but the source is disclosed.

Answer 6

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);
}