OK I see what you mean, I admit this wasn't made completely clear in the post. For en passant using the own king's position, the file where the pawn can be captured en passant is NOT reordered whereas other pawns, captures and promotions are reordered.
Yes using the back row works too! I was trying to see if we can get an improvement on the 18 additional bits needed from the post (or 14 additional bits by taking advantage of knight and bishop ordering).
These discussions have been great, very much enjoying seeing the incremental improvements!
Edit: I did another pass, item 4 in the notes did mention this.
> [4] For en passant we need the pawn to remain on its home file. Hence we exclude the pawn from this step if it can be captured en passant. Captures can appear on any file.
This is detailed in the Python code. https://github.com/savarin/bitpacker/blob/239d68dcd3ec5db67e...
Yes using the back row works too! I was trying to see if we can get an improvement on the 18 additional bits needed from the post (or 14 additional bits by taking advantage of knight and bishop ordering).
These discussions have been great, very much enjoying seeing the incremental improvements!
Edit: I did another pass, item 4 in the notes did mention this.
> [4] For en passant we need the pawn to remain on its home file. Hence we exclude the pawn from this step if it can be captured en passant. Captures can appear on any file.