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Greg Strong wrote on 2022-12-21 UTC

FEN Notation

The Bishop Conversion Rule requires additional state information to be preserved in the FEN for a position.  The purpose of this post is to document how I have encoded this information in ChessV.

The extended FEN format is:
{array} {current player} {castling} {en-passant} {bishop-conversion} {half-move clock} {turn number}

A new component has been inserted between the notation of the en passant square and the half-move ("50 move") counter.  The value for this field at the start of a game is "CGcg".  Like with castling privileges, the upper-case letters are for white and the lower-case letters are for black.  The letter is the file of the bishop, and its presence indicates that the bishop has the ability to convert.  Unlike castling privilege, however, bishop conversion can be mandatory. This is indicated by a "+" after the letter.  For example, if white moves the c-file bishop without converting, it become mandatory to convert the g-file bishop.  Thus, the notation becomes "G+cg".

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