alt Hacker News26 bytes is 208 bits, about twice what you really need for a minimal encoding that has enough context (en passant, castling) to generate an accurate set of legal moves. I wrote a chess database tool back in the 90's (CDB) that used 96-bit encodings (if memory serves) to index all the positions reached in a collection of games so that one could see the moves made from any position, their frequencies, and their game outcomes. Good fun.
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eulgro • last Friday at 7:10 PM
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.
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96 bits is not nearly enough, as there are ~4.8 * 10^44 > 2^148 legal chess positions (with side to move/castling/ep info) [1].
Chess Position Ranking provides a 153 bit encoding but it's very slow to decode.
If the encoding only needs to work for the set of positions occurring in some database, then there's almost no limit to the number of coding optimizations one can make (until the encoding just becomes an index in the set of all unique db positions).
[1] https://github.com/tromp/ChessPositionRanking