Chess, from scratch

Board model, per-piece move generation, check and mate detection, pawn promotion. No chess library; every rule is written by hand.

The part that fought back the most was the "you can't leave your king in check" filter: for every candidate move I copy the board, play the move, and look at whether the king is safe.

The Java core lives on desktop; the JS build runs on this site with no backend at all. I still poke at it. Castling and en passant are next on the list.