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Steffan Westcott

Joined: 11 Mar 2006
Posts: 28
Location: Midlands, England

Post subject: Re: Programmer bug hunt challenge    Posted: Sat May 05, 2007 11:17 am

 Dann Corbit wrote: I think that the standard should redo e.p. so that the square is not named unless capture is possible.

There is a potential difficulty with that, the clause "unless capture is possible" needs stricter definition. What if the en passant capture is a pseudo-legal move but not a genuinely legal move i.e. it leaves the moving side in check?
[D]1k6/3r4/8/3Pp3/8/3K4/8/6R1 w - e6 0 2
At present, it is not necessary to check for legal moves (psuedo-legal or otherwise) to write a FEN string. If the proposed change is to only name the en passant square when genuinely legal en passant capture(s) are possible, then programs writing FEN strings must check for the legality of moves i.e. become more complex.

On a related issue, for the purposes of 3-time repetition, section 9.2 of the FIDE Laws of Chess deem that a position is repeated only if :
 Quote: if the same player has the move, pieces of the same kind and colour occupy the same squares, and the possible moves of all the pieces of both players are the same. Positions are not the same if a pawn that could have been captured en passant can no longer in this manner be captured or if the right to castle has been changed temporarily or permanently.

The part I want to draw attention to is the clause "the possible moves of all the pieces of both players are the same".

With the current FEN standard, it is possible to have 2 chess positions which are considered a repetition by the FIDE laws, but have different en passant square encoding in FEN. For example:
[D]r3k3/8/8/4p3/4P3/8/8/1R2K3 w - - 0 2
[D]r3k3/8/8/4p3/4P3/8/8/1R2K3 w - e6 0 2

If we imagine the FEN standard were changed to only name the en passant square when genuinely legal en passant capture(s) are available, the last example shown is not possible. The conditions for position repetition then become exactly equivalent to :
 Quote: if the corresponding first four fields of the FEN encoding for the positions are the same
 Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First
Subject Author Date/Time
ed Fri May 04, 2007 9:26 am
Reinhard Scharnagl Fri May 04, 2007 10:09 am
Rob Fri May 04, 2007 10:31 am
Tony Fri May 04, 2007 10:49 am
Tony Fri May 04, 2007 10:55 am
Rob Fri May 04, 2007 11:25 am
Tony Fri May 04, 2007 12:02 pm
Reinhard Scharnagl Fri May 04, 2007 12:01 pm
ed Fri May 04, 2007 1:51 pm
Uri Blass Fri May 04, 2007 4:46 pm
H.G.Muller Fri May 04, 2007 4:54 pm
Stan Arts Fri May 04, 2007 5:27 pm
Steffan Westcott Fri May 04, 2007 7:24 pm
Robert Hyatt Fri May 04, 2007 8:22 pm
Steven Edwards Sat May 05, 2007 12:28 am
Dann Corbit Sat May 05, 2007 1:36 am
mambofish Sat May 05, 2007 11:11 am
Steven Edwards Sat May 05, 2007 11:13 am
mambofish Sat May 05, 2007 11:15 am
Robert Hyatt Mon May 07, 2007 10:58 pm
Re: Programmer bug hunt challenge Steffan Westcott Sat May 05, 2007 11:17 am
Robert Hyatt Tue May 08, 2007 12:41 am
H.G.Muller Tue May 08, 2007 12:11 pm
Robert Hyatt Tue May 08, 2007 4:35 pm
H.G.Muller Tue May 08, 2007 4:50 pm
Robert Hyatt Tue May 08, 2007 5:42 pm
H.G.Muller Tue May 08, 2007 7:26 pm
H.G.Muller Tue May 08, 2007 7:26 pm
Robert Hyatt Fri May 04, 2007 8:20 pm
Robert Hyatt Fri May 04, 2007 8:19 pm
Peter Fendrich Fri May 04, 2007 5:34 pm
Steven Edwards Wed May 09, 2007 11:14 am
NKOTB Wed May 09, 2007 11:53 am

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