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Marseillais Chess. Move twice per turn. (8x8, Cells: 64) (Recognized!)[All Comments] [Add Comment or Rating]
H. G. Muller wrote on Fri, Nov 15, 2019 07:08 PM UTC:

About the FEN: is this really needed? Why would one ever want to have a FEN for a position after the first move? In orthodox Chess we also do not have a special form of the turn indicator for positions half-way a castling, where the King has already moved and the Rook is yet to move. Or in Chu Shogi, for after the Lion made the first leg of a locust capture. The exceptional case for the initial position can already be recognized from the full-move counter in the FEN.

[idea!] Why not use a fraction on the full-move counter if you want to do this?  E.g. move 10.5 would mean after the first half of the 10th turn of the player on move.

As to legal move generation:

It seems to me that a Marseillais Chess cannot be implemented without some dedicated code that would be useless in single-move variants. One task for this code would be to score a stalemate after the first move different from one before it, namely as an illegal position (whatever score you want to use for that) rather than a draw. A second task should be to treat first-moves  that get such a score as illegal moves, i.e. ignore them, and correct the legal-move count that might have been made during move generation for it. This would need a 'delayed' mate detection to handle the case where this count hits zero because all 1-move-legal moves turn out to cause self-stalemate the player, aborting the node with a checkmated or a draw score depending on the in-check status.

It should be clear that you cannot judge the legality of a turn before you have seen the full turn. So that would be 2 ply in Marseillais, and a progressive number of ply in Progressive. If you wouldn't try the last ply of the turn, you could never know if the turn can be legally completed. Unless of course the rules guarantee there always is a legal move irrespective of the position, e.g. because passing on the second turn is always allowed. If you do a full judgement on the legality of the position after the turn, one turn should be enough.

The reason I mentioned QS was because I assumed this would be required to detect the fact that the previous ply exposed the King. The assumption was that the root node of QS would start generating moves, and detect that one of the moves captured the King before really searching any. And then abort with a very happy score. So it would always be a 1-node QS, or a dedicated King-capture test, if you want. Depending on the variant there could be simpler ways to detect King capture than generating all moves and see what they hit. E.g. making all possible retrograde captures from the King, and see if they hit an enemy that reciprocates one (sometimes referred to as the 'super-piece method').  Depending on the complexity of the moves in the variant the super-piece method might be cheap or very expensive. (E.g. bent sliders, Cannons or Fire Demons are a pain.) Just generating all prograde capture moves, which you must be able to do anyway, is always a sure method, though.

How much extra you need compared to the turn you want to judge the legality of unavoidably depends on how complex the conditions for legality can be in the variant at hand. Take the Shogi Pawn-drop-mate ban. To detect it you must be able to see that that the Pawn drop checks (which is of course trivial), but that every conceivable reply move is illegal (exposes your King, or perhaps (worse) completes a perpetual checking cycle). So that is two turns plus a King capture test.

But it can be worse. Imagine a variant where it is illegal (rather than just losing) to expose yourself to mate-in-1 (rather than just to King capture). To judge a move you then have to take into account all replies that check, and then all evasions to any of those, to see if these expose the King. That is 3 ply + 'QS' (or 2-ply + mate-test). Imagine a variant where it is illegal to check when you cannot continue checking until you checkmate. (Tsume Chess?) Far fetched? Take the chained release moves of Paco Shako, implemented as a multi-move turn. It is illegal to release a piece from an embrace (by jumping into it) if the released piece cannot find an empty square to go to, an enemy to enbrace, or trigger another legal release. You will have to verify that at least one chain of releases (no matter how long) exists that terminates. There is just no predicting how many ply it can take.