Position Causes Stockfish and Komodo To Crash

[Sesse]

In my browser the diagram in your post has extra squares i1, j1, k1, l1 with a black bishop on j1 and a black rook on k1. But in this reply those extra squares seem to disappear...

Yeah, I noticed so myself. They didn't appear in preview when I wrote the post, however…

[Michel]

An interesting issue here https://github.com/official-stockfish/S ... ssues/3270

SF move generation assumes (correctly it seems) that in a game an escape move can never be an ep move. But of course one can create positions (without parent) in which there is an escape move which is indeed an ep move.

[d]8/8/8/1k6/3Pp3/8/8/4KQ2 b - d3 0 1

[Sesse]

This just makes me even more convinced you'll need to just brute-force the possible unmoves to see if there are any.

[Ajedrecista]

Hello Michel:

An interesting issue here https://github.com/official-stockfish/S ... ssues/3270

SF move generation assumes (correctly it seems) that in a game an escape move can never be an ep move. But of course one can create positions (without parent) in which there is an escape move which is indeed an ep move.

[d]8/8/8/1k6/3Pp3/8/8/4KQ2 b - d3 0 1

What about?

[d]8/8/8/4k3/2p5/8/3P4/4K3 w - - 0 1

Then 1.- d4+, cxd3 e.p. is possible and perfectly legal. It is true that the own moving pawn gives check, which might not fall in the same category as the example of Ke1, Qf1, Pd4; kb5, pe4 without legal previous move of d2-d4 because the black king was already in check by the white queen and the white pawn itself does not give check. A definition of 'escape move' would be useful. Avoiding check given by any opposite piece regardless if it has just moved or not?

Regards from Spain.

Ajedrecista.

[Michel]

Hello Michel:

An interesting issue here https://github.com/official-stockfish/S ... ssues/3270

SF move generation assumes (correctly it seems) that in a game an escape move can never be an ep move. But of course one can create positions (without parent) in which there is an escape move which is indeed an ep move.

[d]8/8/8/1k6/3Pp3/8/8/4KQ2 b - d3 0 1

What about?

[d]8/8/8/4k3/2p5/8/3P4/4K3 w - - 0 1

Then 1.- d4+, cxd3 e.p. is possible and perfectly legal. It is true that the own moving pawn gives check, which might not fall in the same category as the example of Ke1, Qf1, Pd4; kb5, pe4 without legal previous move of d2-d4 because the black king was already in check by the white queen and the white pawn itself does not give check. A definition of 'escape move' would be useful. Avoiding check given by any opposite piece regardless if it has just moved or not?

Regards from Spain.

Ajedrecista.

Thanks. It seems I was wrong. In your position SF does find the ep move. Then I wonder why it does not consider the ep move in the parentless position...

[Ras]

SF move generation assumes (correctly it seems) that in a game an escape move can never be an ep move.

Consider the following position:

[d]8/6Q1/8/7k/7p/8/6P1/7K w - - 0 1

If White now plays g2-g4+, then the only legal move for Black will be hxg3 e.p., and Stockfish 11 and 12 find that move if given as:

Code: Select all

position fen 8/6Q1/8/7k/7p/8/6P1/7K w - - 0 1 moves g2g4

Both Stockfish versions also find that if the subsequent position with Black to move is given directly as fen with e.p. square, i.e. like:

Code: Select all

position fen 8/6Q1/8/7k/6Pp/8/8/7K b - g3 1 1

[Joerg Oster]

Hello Michel:

An interesting issue here https://github.com/official-stockfish/S ... ssues/3270

SF move generation assumes (correctly it seems) that in a game an escape move can never be an ep move. But of course one can create positions (without parent) in which there is an escape move which is indeed an ep move.

[d]8/8/8/1k6/3Pp3/8/8/4KQ2 b - d3 0 1

What about?

[d]8/8/8/4k3/2p5/8/3P4/4K3 w - - 0 1

Then 1.- d4+, cxd3 e.p. is possible and perfectly legal. It is true that the own moving pawn gives check, which might not fall in the same category as the example of Ke1, Qf1, Pd4; kb5, pe4 without legal previous move of d2-d4 because the black king was already in check by the white queen and the white pawn itself does not give check. A definition of 'escape move' would be useful. Avoiding check given by any opposite piece regardless if it has just moved or not?

Regards from Spain.

Ajedrecista.

Thanks. It seems I was wrong. In your position SF does find the ep move. Then I wonder why it does not consider the ep move in the parentless position...

Why generating an illegal move?

[Ajedrecista]

Hello again:

Thanks. It seems I was wrong. In your position SF does find the ep move. Then I wonder why it does not consider the ep move in the parentless position...

The key could be in movegen.cpp. There is a three-line comment at current lines 159-161:

Code: Select all

// An en passant capture can be an evasion only if the checking piece
// is the double pushed pawn and so is in the target. Otherwise this
// is a discovery check and we are forced to do otherwise.

Which might explain the feature. A programmer opinion is valuable since I am not a programmer.

Regards from Spain.

Ajedrecista.

[Michel]

Hello Michel:

An interesting issue here https://github.com/official-stockfish/S ... ssues/3270

SF move generation assumes (correctly it seems) that in a game an escape move can never be an ep move. But of course one can create positions (without parent) in which there is an escape move which is indeed an ep move.

[d]8/8/8/1k6/3Pp3/8/8/4KQ2 b - d3 0 1

What about?

[d]8/8/8/4k3/2p5/8/3P4/4K3 w - - 0 1

Then 1.- d4+, cxd3 e.p. is possible and perfectly legal. It is true that the own moving pawn gives check, which might not fall in the same category as the example of Ke1, Qf1, Pd4; kb5, pe4 without legal previous move of d2-d4 because the black king was already in check by the white queen and the white pawn itself does not give check. A definition of 'escape move' would be useful. Avoiding check given by any opposite piece regardless if it has just moved or not?

Regards from Spain.

Ajedrecista.

Thanks. It seems I was wrong. In your position SF does find the ep move. Then I wonder why it does not consider the ep move in the parentless position...

Why generating an illegal move?

Why do you consider the ep move illegal?

[Joerg Oster]

Thanks. It seems I was wrong. In your position SF does find the ep move. Then I wonder why it does not consider the ep move in the parentless position...

Why generating an illegal move?

Why do you consider the ep move illegal?

Because this kind of move can NEVER occur in a legal game of chess.
So I can only conclude it must be an illegal move.

The illegal position makes it look like a valid move, doesn't it?