The hand is not the board

Hex Poker TD is a tower defense where poker hands build the towers. Tell an engineer that and watch them reach for the obvious conclusion: the cards must combine when they are next to each other on the board. Adjacent hexes, a flood fill, a hand detected in the grid. It is such a natural guess that I have had it explained back to me as though it were my own design.

It is wrong, and the wrongness is load-bearing. You select up to five cards from anywhere on the planet. There is no neighbour check. The scorer takes whatever you picked and runs plain, ordinary poker classification on it — ranks and suits, nothing spatial. Hex adjacency exists in the game, but it is used for enemy pathing and for asking what is near a wall. It never once decides whether a hand is a hand.

That separation is the most valuable line in the codebase, and I did not appreciate it until I tried to move the board. The rules layer owns one hundred percent of the poker logic and knows nothing about geometry. The board is a small interface — where are the cell centres, who are a cell's neighbours, is this one of the pentagons, find me a path. A sphere satisfies that interface. So does a flat hex grid. Both exist, both are real, and the poker mechanic runs unmodified on either, because it never asks what shape it is standing on.

Had I taken the obvious route and defined hands as adjacency patterns, the rules and the geometry would have fused. Every change to the board would have become a change to poker. The flat grid would have needed its own hand detector. Adding the twelve pentagons would have meant deciding what a straight means when a cell has five neighbours instead of six — a question with no good answer, invented entirely by a design mistake.

The scoring hides a small piece of arithmetic I am fond of. Straights are the annoying hand in every poker implementation, because the Ace is both high and low and the naive fix is a special case bolted on afterwards. Instead, map the thirteen ranks onto a cycle and ask a single question: of the four gaps between five sorted cards, is every one of them a step of one? On a cycle, the Ace-low wheel satisfies that with no special pleading — and so does every wrap-around run that a linear check would have missed. The Ace stops being an exception because the number line stopped having ends.

The lesson generalizes past card games. When you are choosing what a rule depends on, prefer the smallest thing that makes it true. Poker hands depend on ranks and suits. They do not depend on where the cards are sitting — and the moment I let them, I would have owned that dependency forever.