board representation should include which player's turn it is

[nellore]

Another consideration: a board does not in general determine which player's turn it is, right? And a board B where it's black's turn is strategically inequivalent to B where it's white's turn, so we should include which player's turn it is in the board's representation (perhaps as an extra component).

Incidentally, we are currently using the "bitboard" representation from https://en.wikipedia.org/wiki/Board_representation_(chess).

[nellore]

Same probably goes for castling availability and en passant. Maybe halfmove clock and fullmove number from the FEN can be ignored though -- maybe all from this comment can be ignored for now, but whose turn it is seems important.