Computer Chess Club Archives




Subject: Why to use compatible X-FEN (in Chess960)

Author: Reinhard Scharnagl

Date: 23:15:26 07/17/05

There seems to be a need to replace some unqualified statements arising in some
fora to FEN compatibility according Chess960 by something more objective.

There are people requesting a compatibility of Chess960 and traditional chess in
both directions. But that is obviously nonsense, because in Chess960 positions
exist never occuring in common games. And there are people thinking this
discussion would be a question of winning a battle, whereas in fact this is a
big endeavor to preserve compatibility, keeping Chess960 what it is: a
compatible superset to traditional chess.

Thus, because Chess960 is bigger, full compatibility means:

A) Any posible situation has to be uniquely encoded (that is: any position,
which also exists in traditional chess, has to be 100% identically encoded like
done in already used FEN). (P.S.: This also is necessary in respect to the PGN
specification requiring a uniquely encoding of games and thus of FEN strings.)

This is done by X-FEN but not by Shredder-FEN, concerning positions with
castling rights only related to traditional K-R placements, e.g. SP 518, which
is the usual starting array of traditional chess. Also 18 pseudo-FRC starting
positions are affected and a numberless set of additional thinkable positions.

B) The outer form of created FEN strings should look like traditional FEN.

Taking all 960 starting arrays of Chess960 as an example X-FEN uses 'KQkq' in
every case to encode existing castling rights, whereas Shredder-FEN produces a
set of very different and strange looking castling tags. This is related to not
using X-FEN's well established assumption that castling rights by default are
related to the outermost rook of an affected side.

C) Chess960 games already stored in databases, used Chess960 representations in
online chess servers, the ability of PGN viewers already enhanced to also
display Chess960 PGNs, and a lot of existing GUIs and applications should kept
working instead of being devaluated.

Because X-FEN is the base of all those approaches, Shredder FEN is disturbing
those pioneer activities without any need. To preserve compatibility instead
should have priority number one.

I warmly recommend Shredder to also support X-FEN instead of its incompatible
approach blindly ignoring all current activities to support Chess960. (P.S.:
X-FEN formerly has been known also as FRC-FEN.)

Regards, Reinhard.

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