c# check for a poker straight

Question

I am trying to write a poker hand evaluation method in c#. I have managed to do this for every poker hand using linq except a straight. For those that don't play a straight is made up of 5 cards with increments of 1 for each card. Ace can be high or low.

I have created an object called card which has a suit, rank and value (J = 11, Q =12 etc..). My method will be passed a list of this object containing 7 cards (hole cards and the board.)

Another thing to bear in mind is that a straight can only be made if the player has a 5 or 10.

See below my methods for other poker hands and please let me know if you have an idea for the straight method. Pseudo code would be fine also.

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public bool CheckPair(List<Card> cards)
{
    //see if exactly 2 cards card the same rank.
    return cards.GroupBy(card => card.Rank).Count(group => group.Count() == 2) == 1;
}

public bool CheckTwoPair(List<Card> cards)
{
    //see if there are 2 lots of exactly 2 cards card the same rank.
    return cards.GroupBy(card => card.Rank).Count(group => group.Count() >= 2) == 2;
}

public bool CheckTrips(List<Card> cards)
{
    //see if exactly 3 cards card the same rank.
    return cards.GroupBy(card => card.Rank).Any(group => group.Count() == 3);
}
public bool CheckStraight(List<Card> cards)
{
    // order by decending to see order
    var cardsInOrder = cards.OrderByDescending(a => a.Value).ToList();
    // check for ace as can be high and low
    if (cardsInOrder.First().Rank == "A")
    {
        // check if straight with ace has has 2 values
        bool highStraight = cards.Where(a => a.Rank == "K" || a.Rank == "Q" || a.Rank == "J" || a.Rank == "10").Count() == 4;
        bool lowStraight = cards.Where(a => a.Rank == "2" || a.Rank == "3" || a.Rank == "4" || a.Rank == "5").Count() == 4;
        // return true if straight with ace
        if (lowStraight == true || highStraight == true)
        {
            return true;
        }
    }
    else
    {
        // check for straight here
        return true;
    }
    // no straight if reached here.
    return false;

}

public bool CheckFlush(List<Card> cards)
{
    //see if 5 or more cards card the same rank.
    return cards.GroupBy(card => card.Suit).Count(group => group.Count() >= 5) == 1;
}

public bool CheckFullHouse(List<Card> cards)
{
    // check if trips and pair is true
    return CheckPair(cards) && CheckTrips(cards);
}
public bool CheckQuads(List<Card> cards)
{
    //see if exactly 4 cards card the same rank.
    return cards.GroupBy(card => card.Rank).Any(group => group.Count() == 4);
}

// need to check same 5 cards
public bool CheckStraightFlush(List<Card> cards)
{
    // check if flush and straight are true.
    return CheckFlush(cards) && CheckStraight(cards);
}

Answer 1

This might not be the best performing check, but I'd say it's very readable which is usually a good property.

Just grab 5 cards, skipping cards you've already seen every iteration and check for a straight for each sequence. An ordered sequence is a straight if it does not contain doubles and if the first and last cards difference is 5.

public bool CheckStraight(List<Card> cards)
{
     //maybe check 5 and 10 here first for performance

     var ordered = cards.OrderByDescending(a => a.Value).ToList();
     for(i = 0; i < ordered.Count - 5; i++) {
          var skipped = ordered.Skip(i);
          var possibleStraight = skipped.Take(5);
          if(IsStraight(possibleStraight)) {
               return true;
          }
     }
     return false;
}

public bool IsStraight(List<Card> fiveOrderedCards) {
     var doubles = cards.GroupBy(card => card.Rank).Count(group => group.Count() > 1);
     var inARow = cards[4] - cards[0] = 5; //Ace is 0

     return !doubles && inARow;
}