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Check out Chess with Different Armies, our featured variant for July, 2024.
Check out Chess with Different Armies, our featured variant for July, 2024.
It seems to work now: I implemented a sort of killer heuristic in the 2-ply search needed to test for checkmate. This appears to speed up the search enough so that it can complete in this potentially unfavorable case.
What I did is this: when the reply to an investigated move captures the King (thus disqualifying the move and requiring the search for a legal one to go on), it remembers the square of the piece that made the capture. When it then investigates the next move, it first generates the moves of this piece alone, before generating all moves of the opponent. In positions with a pre-existing check most moves won't resolve that check, and are quickly refuted that way. (By generating moves of only a single piece, rather than of all pieces.)
In the rare case where there wasn't a pre-existing check, but it happened to first try a move that exposed the King, it could lead to extra work, as the moves of the King-capturing piece will be generated twice during investigation of the next move. But even there exposing the King could have been a result of moving away a pinned piece. And since moves are generated piece by piece the next move is then likely to have exposed the King in the same way, and will then be quickly refuted by the same King capture.