The problem is that the board state requirement makes searching for specific moves hard. But if you allow board state, you can do less than 14. 3 bits for the

The problem is that the board state requirement makes searching for specific moves hard. But if you allow board state, you can do less than 14. 3 bits for the type of piece, 6 for the destination, 2 extra bits. (Promotion if destination is last/first line, disambiguation for pawns in en passant) You're still at about twice the theoretical minimum, I think. I vaguely recall an upper bound of 6 bits?