[eulgro]

Given the current upper bound on legal chess positions is 7.7e45 ≈ 152.4 bits, you either have found a better upper bound or your memory doesn't serve.

[wongarsu]

They didn't try to encode all legal positions though, only ones that were actually reached in their database of games. It sounds very plausible to me that this allows a lot of simplifying assumptions that cut the state space by about 60 bits

[mlyle]

I can put it all in, say, 24 bits, if my database is small. 140k games, 120 positions each. log(140000*120)/log(2) ~~ 24.001, and surely there will be some duplication.

The encoding is just the index number of the game + move that resulted in that position.

[pklausler]

The duplication is the problem if you want to use positions as DB keys.

[bombcar]

Now I'm wondering if it is a "legal" chess position to get the pieces to swap sides ... a solver to find how to do it would be amusing.

[mlyle]

Obviously not possible -- how would pawns pass each other without captures?

[bombcar]

Ah! True, I worked out how you could have OTHER pieces get around a pawn, but pawns themselves can't get past each other.

[NitpickLawyer]

Not what you asked for, sorry, but it reminded me of this gem - https://www.youtube.com/watch?v=C5JVFCouXIU