Chess - Programming - Polyglot Book Format - Enpassant

Enpassant is part of the Key

It is the exclusive or of entries from the RANDOM_ENPASSANT sub-array.

The offset is the file of the double-pushed pawn resulting in enpassant (counted from 0(=a) to 7(=h)).

NOTE: If this does not apply then enpassant=0.

This is different from the FEN standard.

In the FEN standard the presence of an “enpassant target square” after a double pawn push is unconditional.
It is irrelevant if the potential enpassant capturing move is legal or not.
- Examples where it would not be legal are when the capturing pawn is pinned or when the double pawn push was a discovered check.