Programmer bug hunt challenge

[sje]

The simple workaround is to omit the en passant target square on the FEN unless a capture is possible.

[Dann Corbit]

I think that the standard should redo e.p. so that the square is not named unless capture is possible. For example, what is the practical difference between these two positions:
[D]r2qk2r/ppp2ppp/2n2n2/1B1pp1B1/1b1PP1b1/2N2N2/PPP2PPP/R2QK2R w KQkq e6 0 2

[D]r2qk2r/ppp2ppp/2n2n2/1B1pp1B1/1b1PP1b1/2N2N2/PPP2PPP/R2QK2R w KQkq - 0 2

The significance of the change is that a search for the either position above should also return the other, because they are the same position.

[mambofish]

Hmm... The position you give also indicates that both sides have castling rights. It does not state whether they can legally castle on their next move. In a FEN position the ep flag is viewed in the same way: as an indication that an ep capture on the named square might be available on the next move, not that an ep capture is necessarily legal.

If you change the rules for the ep field, you would presumably also have to change them for the castling field, to remain semantically consistent.

[sje]

If you change the rules for the ep field, you would presumably also have to change them for the castling field, to remain semantically consistent.

No, the castling availability potentially applies to all future positions, not just the next move.

[mambofish]

And what about the situation where the pawn move revealed a check on the enemy king? Even if an opposing pawn could theoretically capture e.p., this would also not be legal. Would you omit the e.p. square in this case as well from the FEN notation?

FEN says nothing about the legality of moves for the next player, and I personally think it should remain that way

[steffan]

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 :

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 :

if the corresponding first four fields of the FEN encoding for the positions are the same

[bob]

And what about the situation where the pawn move revealed a check on the enemy king? Even if an opposing pawn could theoretically capture e.p., this would also not be legal. Would you omit the e.p. square in this case as well from the FEN notation?

FEN says nothing about the legality of moves for the next player, and I personally think it should remain that way

I think the right answer is that an EP target is given only when an EP capture is possible. The pinned pawn issue doesn't matter to me as much as the original case. But I can handle it however you throw it at me so it really doesn't matter. Same problem with PGN games. Too many screw up castling and use zero-zero (among others). Some put a space after 21. Ne5, others just produce 21.Ne5. I've made my parsing handle these odd cases over the years to avoid confusion when it would fail on a PGN read...

But following the standard would be nice, IMHO...

[bob]

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 :

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 :

if the corresponding first four fields of the FEN encoding for the positions are the same

I don't see where repetition (or even the 50 move rule) would be a problem. For there to be a true EP target (according to my terminology) the position could _not_ have occurred previously in the game, because a pawn must be pushed the previous move to generate an EP target. And once the pawn is pushed, the 3-fold repetition counter starts over as does the 50 move rule. So how could there be an ambiguity? And it is certainly legal for two identical positions to be converted to FEN, where one has the EP target and one does not. Because the one that does not did not have a pawn push the previous move...

[hgm]

It seems to me that requiring the e.p. field in a FEN to be only present if there actually is a legal e.p. capture just puts a completely needless burden on the routine that converts board positions to FENs. No information is lost by putting a superfluous e.p. square. A possible reader for which the presence of an actual capture makes any difference almost certainly will have the intelligence and information to figure out for itself if the capture exists, if it wants to know that.

FENs are designed as a tool to make life easy. So I think the existing standard made the good choice.

[bob]

It seems to me that requiring the e.p. field in a FEN to be only present if there actually is a legal e.p. capture just puts a completely needless burden on the routine that converts board positions to FENs. No information is lost by putting a superfluous e.p. square. A possible reader for which the presence of an actual capture makes any difference almost certainly will have the intelligence and information to figure out for itself if the capture exists, if it wants to know that.

FENs are designed as a tool to make life easy. So I think the existing standard made the good choice.

Here's the reason it is bad: You have a position with a white pawn at e4 and no black pawns on the d or f file. If the last move was e4, you get an EP target of e3. If the last move was something else, the ep target would be blank. Yet the two positions are absolutely identical. Why should they have different FEN strings?

Whatever program writes the FEN string has some responsibilities anyway. The 50 move counter. Castling status. So why not simply get the EP target right so that there is no ambiguity as presently exists???