🕸📝Fergus Duniho wrote on Sun, Jan 4, 2009 11:41 AM EST:
Thank you for the fixes and explanations. But you have overlooked that in
sub 'enpassant' there is twice 'filename' used.
It should be 'filename #to rankname #from'.
Thanks for mentioning that. It's now fixed.
To check if any piece or any empty square is attacked, one could use the
following subroutine:
This subroutine may work for Chess, but it would not work for games with other divergent pieces, such as Cannons. The best way to handle attacks on empty spaces that a King must pass through to castle is to move the King to each square before checking whether it is attacked, which I have now done. This works equally well for any divergent piece.
A better method than your subroutine to check whether an empty space is attacked is to temporarily fill an empty space with a piece then check for attacks on it. With an enemy Cannon on a player's first rank, this would not work accurately for castling, though it would give the right result for the wrong reason. Let's say that the first rank just has an unmoved King and Rook with an enemy Cannon on the other side of the King. Although the Cannon attacks the spaces between the King and Rook, it is not accurate to say that the King would be passing through check from the Cannon if it castled. After all, the Cannon attacks the space just to the other side of the King, using the King as the screen, but the King may legally move there, because there is no screen between the King and the Cannon. Still, castling would put the King in check, because the Rook would become a screen. So, castling would still be illegal. In most instances, using an attacked subroutine that temporarily places a piece on an empty space would work for castling. But if you had a game in which Rooks can block Cannon attacks, then castling would be legal, and this method of checking for attacks would not work for castling. Alternately, if you had a piece that can check the King but not any other piece, or a piece with an attack that only affects the King, such as the Kings' long range attacks on each other in Eurasian Chess, then whether or not this subroutine worked well with castling would depend in part on which piece is temporarily placed on the empty space.
Note that the changes I made to the castle subroutine in the chess2 include file are not needed in the chess include file, because that include file uses separate functions to determine whether the King is attacked.
Thanks for mentioning that. It's now fixed.
This subroutine may work for Chess, but it would not work for games with other divergent pieces, such as Cannons. The best way to handle attacks on empty spaces that a King must pass through to castle is to move the King to each square before checking whether it is attacked, which I have now done. This works equally well for any divergent piece.
A better method than your subroutine to check whether an empty space is attacked is to temporarily fill an empty space with a piece then check for attacks on it. With an enemy Cannon on a player's first rank, this would not work accurately for castling, though it would give the right result for the wrong reason. Let's say that the first rank just has an unmoved King and Rook with an enemy Cannon on the other side of the King. Although the Cannon attacks the spaces between the King and Rook, it is not accurate to say that the King would be passing through check from the Cannon if it castled. After all, the Cannon attacks the space just to the other side of the King, using the King as the screen, but the King may legally move there, because there is no screen between the King and the Cannon. Still, castling would put the King in check, because the Rook would become a screen. So, castling would still be illegal. In most instances, using an attacked subroutine that temporarily places a piece on an empty space would work for castling. But if you had a game in which Rooks can block Cannon attacks, then castling would be legal, and this method of checking for attacks would not work for castling. Alternately, if you had a piece that can check the King but not any other piece, or a piece with an attack that only affects the King, such as the Kings' long range attacks on each other in Eurasian Chess, then whether or not this subroutine worked well with castling would depend in part on which piece is temporarily placed on the empty space.
Note that the changes I made to the castle subroutine in the chess2 include file are not needed in the chess include file, because that include file uses separate functions to determine whether the King is attacked.