gambit

A chess engine only looks like one project. It is three subsystems, each able to ruin the whole thing in its own way, and only together do they produce something that plays sensibly. gambit is about making those three parts agree with each other well enough that the result starts to resemble a real opponent, not a random move generator.

Scala 3 gives an expressive language here for modeling the board, moves and the tree of variations. But the real challenge is not syntactic, it is algorithmic: a flawless generator, a fair evaluation and a search that does not compute forever.

A move generator that lets one illegal move slip through per million positions will make the engine play something impossible. An evaluation that weighs pieces wrong will steer it confidently into a loss. A search without pruning will think for hours about a move that should take a second.

None of these three can be dismissed. An engine weak in any one of them will lose, and for a specific, pinpointable reason, not a general weakness.

Naive search examines every possible continuation, and there are billions of them just a few moves deep. Alpha-beta pruning notices that if one of the opponent's replies already refutes a line, the rest of their replies need not be computed. With good move ordering it cuts the space so hard that the engine reaches deeper in the same time.

This is not a minor optimization but the difference between an engine that sees three moves ahead and one that sees six. In chess that depth translates directly into playing strength.

Move generator correctness is verified with so-called perft: you count all positions reachable at a given depth and compare against numbers the whole chess community knows by heart. If your number is off, you have a bug somewhere, and it shows you exactly at which depth the divergence begins.

The result is a complete engine that takes a position and returns a move: the generator lays out every legal continuation, the evaluation says which leads somewhere better, and alpha-beta searches as deep as time allows. Three subsystems that are only parts on their own together make an opponent that can play.