[Anotheroneagain]

Even something very simple like listing piece positions in a given order should result in less. Let's say King, Queen, Rooks, Bishops, Knights, pawns. Give a position for each for both players in 6×32 (192) bits.

If a piece is taken, repeat the position of the king. If a pawn is promoted (and there are more than than the starting number of the piece it was promoted into) repeat the position of the piece it was promoted into.

1 bit for whose move it is. (193b)

3 bits for the en-passant column. (196b)

50 move rule (I suppose 7 bits, as you need it in ply?) 203b. Then the data for the extra pieces, in the order encountered if any. ?×6b (how many are possible? There are 16 pawns, but I suppose it isn't possible to promote all)

[thrw21]

My objective was to reduce average size, the example calculation I made is the max size

[Anotheroneagain]

My objective was to show that it's quite complicated for dubious gain. What do you think the average would be?

[thrw21]

  What do you think the average would be?

No idea, I don't know what kind data OP stores (mostly few pieces? or lots of pieces? etc). You can probably tailor your format if you have more insights. But no matter what, you can always have some gain compared to a naive approach. I would say storing a pos for each piece might be naive, since there are a lot of rules that allows you to exploit things

My 248 bit was the absolute worst, when you have 8 promotions on board and lost none of (non-pawn) pieces. For example the initial board would be about 190 bits as well and most would be lower as you capture pieces. I think that would be a worst case for you as well but I don't get this part

  If a pawn is promoted (and there are more than than the starting number of the piece it was promoted into) repeat the position of the piece it was promoted into

You mean you first need to tell the position of the piece it is promoted to and then another position for the position of the pawn (now promoted)? Then you have a pretty bad worst case as well

But you are right, I made an overcomplicated solution. Here is a simpler one that should perform better https://news.ycombinator.com/item?id=39066557

[Anotheroneagain]

>Then you have a pretty bad worst case as well

It would be really rare though, virtually all positions would be 192bits for the position as such.

I don't think you need en passant for each pawn, there can only be one, so you only need to mark the column, with 3 bits, and there always must be a column where it can't happen, that you can use where there isn't any. Or, maybe it's more, IDK.

[317070]

How would you enforce the three repetitions rule this way?

[Anotheroneagain]

I guess you can't do that without the full PGN. I guess the 50 move rule might be extraneous as well.

[Someone]

You don’t always need the entire move list.

Whenever any piece is taken (permanently decreasing the number of occupied squares), a pawn is moved, castling rights change, or an en passant opportunity isn’t taken, there won’t be any more repetitions of the board matching past board positions, so you can forget all earlier board positions.