static arithmetic encoding appears to store the initial board, "black to move" and 4 rook "can castle" flags and a 1-of-9 "can en passant" value in 22 bytes. "4

static arithmetic encoding appears to store the initial board, "black to move" and 4 rook "can castle" flags and a 1-of-9 "can en passant" value in 22 bytes. "4 pawns taken + 12 pawns promoted to queens", which I still think is the "complex-est" configuration that might actually be reachable in a valid game, fits in 25 bytes. A board with just 2 kings left on it encodes in 10 bytes (en passant and can castle flags are not needed).