The Opening Book book.txt ignores possible en passant within its Fen-Strings.
e.g. the second Fen String within the txt file is
rnbqkbnr/pppppppp/8/8/3P4/8/PPP1PPPP/RNBQKBNR b KQkq -
which is the position after white plays d2d4 - but d2d4 means there is an en passant possible for the white pawn and thus the Fen String should be
rnbqkbnr/pppppppp/8/8/3P4/8/PPP1PPPP/RNBQKBNR b KQkq d3 0 1
according to FEN Notation
Given this converting the book.txt to zobrist hashes for a faster lookup failes for all position where en passant is possible.
The Opening Book book.txt ignores possible en passant within its Fen-Strings.
e.g. the second Fen String within the txt file is
rnbqkbnr/pppppppp/8/8/3P4/8/PPP1PPPP/RNBQKBNR b KQkq -
which is the position after white plays d2d4 - but d2d4 means there is an en passant possible for the white pawn and thus the Fen String should be
rnbqkbnr/pppppppp/8/8/3P4/8/PPP1PPPP/RNBQKBNR b KQkq d3 0 1
according to FEN Notation
Given this converting the book.txt to zobrist hashes for a faster lookup failes for all position where en passant is possible.