[rxnwnedz] Shredder-FEN castling

Shredder-FEN permits expressing bizarre castling not permitted in Chess960.

let white's first rank be

R...K..R

and let the Shredder-FEN castling availability string be HA.  nothing unusual here: after e1h1 (castling in Chess960 is expressed as king takes rook), the position is

R....RK.

next, consider the same start position, but let the Shredder-FEN castling availability string be AH.  after e1h1, the position is

R.KR....

AH means the H rook is the queenside (A-side) castling rook, because it was given second in the castling availability string.  similarly, after e1a1, the position is

.....RKR

unfortunately, this "inverted" castling is only possible if neither rook has moved.  the castling side of an inverted rook reverts to normal if the other rook moves. for example, let the position again be

R...K..R

and the castling availabilty string again be AH, meaning the A-rook is the kingside castling rook, and the H-rook is the queenside castling rook.  if the A rook moves or is captured, the castling availability string becomes H, and the only way to tell which side castling the H rook is is to look at the relative position of the king.  the H-rook therefore tranforms into a normal kingside castling rook because it is to the right of the king.  similarly, if the H-rook moves, the A-rook transforms into a normal queenside castling rook.

given that we are inventing fairy chess variants, we can easily imagine a variant in which this transformation does not occur.  we would need to extend Shredder-FEN, perhaps with a non-letter character in the castling availability string is a placeholder indicating castling is not available in that direction.

more bizareness: the king does not need start between the rooks.  for example, let the position be

.....KRR

and let the castling availability string be HG.  then, after f1g1, the position is

..KR...R

we have executed queenside castling with the G rook, listed second.

another example: let the position be

..KR...R

and the castling availability string be HD.  then, after c1d1, the position remains completely unchanged, other than castling availability has been lost.

previously more abuse of Shredder-FEN, permitting white and black to have different rook starting locations.