  We present an algorithm that correctly updates the Forsyth-Edwards Notation
(FEN) chessboard character string after any move is made without the need for
an intermediary array representation of the board. In particular, this relates
to software that have to do with chess, certain chess variants and possibly
even similar board games with comparable position representation. Even when
performance may be equal or inferior to using arrays, the algorithm still
provides an accurate and viable alternative to accomplishing the same thing, or
when there may be a need for additional or side processing in conjunction with
arrays. Furthermore, the end result (i.e. an updated FEN string) is immediately
ready for export to any other internal module or external program, unlike with
an intermediary array which needs to be first converted into a FEN string for
export purposes. The algorithm is especially useful when there are no existing
array-based modules to represent a visual board as it can do without them
entirely. We provide examples that demonstrate the correctness of the algorithm
given a variety of positions involving castling, en passant and pawn promotion.
