[thrw21]

After some more thinking and feedback:

  2 bits per square
  00 empty
  01 white pawn
  10 black pawn
  11 other piece
  =128 bits

4 bit index of white king on board (one of the others)

4 bit index of black king

for each "other piece" on board that is not a king: 1 bit color + 2 bit type

1 bit for current player

2 bits for each king if they moved (optional, only exists if king is at thair initial square. Otherwise it is moved)

1 bit for each pawn if they can be captured en passant (optional, these bits only exists for their pawns if there is a pawn at 4th row for white or 5th row for black)

(And i will ignore the draw rule)

[Anotheroneagain]

> =128 bits

Notice that this alone is more than a lot of positions with my "naive" implementation - 36 bits per side for the best case, with only the king, and at most one queen: 6b for the king, 6 bits for the queen, or its absence, 6 bits each for the absent rooks, bishops, knights and pawns.

[Anotheroneagain]

I thought of combining the two:

1 bit per square (empty/non empty) 64 bit.

Then we need at worst 5 bits (32 non empty squares or less) per piece to indicate the positions of the big pieces. (80 bits, typical worst case)

Then we only need 1 bit per pawn, to indicate color. (16 bit at most)

The typical worst case: 160 bits.