Halfmove count wraps at 63

[mbabigian]

To reproduce import fens using "learn convert_bin" then rename the output file and reverse with "learn convert_plain" Check the output file. Look at fens that had a halfmove count greater than 63. 64 becomes 0, 65 becomes 1, etc.

Looks like an overflow.

Adding this issue to document the behavior.

[nodchip]

Thank you for reporting. I will take a look it after NNUE is integrated into the official Stockfish branch.

[gcp]

The bug is here: https://github.com/nodchip/Stockfish/blob/master/src/extra/sfen_packer.cpp#L198

It's written as 6 bits, but the count is in halfmoves so it can go up to 100.

To keep data files compatible it will probably have to be clamped.

[Sopel97]

Okay, I'm looking at this now and trying to find a way to fix this without breaking backwards compatibility. It looks like it can be fixed by adding one more bit to the bit stream at the end. This would be completely backwards compatibile.