I've found that the rules can be expressed quite elegantly and compactly in a language that supports pattern matching. I have a Haskell implementation [1] which

I've found that the rules can be expressed quite elegantly and compactly in a language that supports pattern matching. I have a Haskell implementation [1] which is just under 300 lines of code including the parsing of SAN moves (standard algebraic notation) which is used in PGN files. [1] https://github.com/ArnoVanLumig/chesshs https://github.com/ArnoVanLumig/chesshs