www.mikescher.com/www/statics/euler/Euler_Problem-054_explanation.md

Yep, I hate this problem.
Not only is there an enormous amount of input data that makes our program huge in size. But it contains a lot of different cases, rules and logic that needs be represented in our program

But none the less I tried to come up with an compact algorithm for scoring a set of cards

Get(@"https://projecteuler.net/project/resources/p054_poker.txt")
	.Where(p => GetScore(p.deck_1) > GetScore(p.deck_2))
	.Count()
	.Dump();

int GetScore(Card[] cards) {
	List<long> SUMS = new List<long>{ H*H*H*H*H, C*C*C*C*C, S*S*S*S*S, D*D*D*D*D };

	int[] array = new int[15];
	int score = 0;
	int flushSum = 1;
	int highCard = 0;
	int highGroup = 0;
	int straightIndex = 0;

	foreach(Card c in cards) {
		highCard = Math.Max(highCard, c.value);
		flushSum *= c.suit;
		if (array[c.value] > 0)
			highGroup = Math.Max(highGroup, c.value);
		array[c.value]++;
	}

	for(int i = 1; i < 15; i++)
	{
		score += (array[i]-1)*(array[i])*256;
		
		if (array[i] > 0 && array[i-1] > 0)
			straightIndex++;
	}
	score += highCard;
	score += highGroup * 15;

	if (straightIndex == 4)
		score += 2540;
	if (SUMS.Contains(flushSum))
		score += 2550;
		
	return score;
}

The different values are carefully crafted in such a way, that you can compare the score of two hands and get the winner

Card	Calculation	Score
High Card	{0-14} * [1]	= {0-14}
High Card (in Group)	{0-14} * [15]	= {0-210}
One Pair	2 * [256]	= 512 (+ HighCard) (+ HighGroup)
Two Pairs	4 * [256]	= 1024 (+ HighCard) (+ HighGroup)
Three of a Kind	6 * [256]	= 1536 (+ HighCard) (+ HighGroup)
Straight	[2540]	= 2540 (+ HighCard) (+ HighGroup)
Flush	[2550]	= 2550 (+ HighCard) (+ HighGroup)
Full House	10 * [256]	= 2560 (+ HighCard) (+ HighGroup)
Four of a Kind	12 * [256]	= 3072 (+ HighCard) (+ HighGroup)
Straight Flush	[2540] + [2550]	= 5090 (+ HighCard) (+ HighGroup)
Royal Flush	[2540] + [2550]	= 5090 (+ HighCard) (+ HighGroup)

One last side note: A royal flush is not really a independent rank. Because of the "highest card in the rank" rule a royal flush is always better than a straight flush (because the highest card is an ace)