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H. G. Muller wrote on Wed, Jul 22, 2020 09:34 PM UTC in reply to Fergus Duniho from 01:41 PM:

OK, thanks. I never used PHP, so it takes a bit of getting used to. The 'let' must have been a sub-conscious confusion with BASIC. What also confuses me is that the # / var do not seem to be needed on origin, dest and such. Is that because these are really subroutines rather than variables?

The need to handle the implied side effects is a bit of a pain. XBetza could potentially define e.p. capture through Gryphon or Sissa moves. Also, to test the pseudo-legality of the latest move of the game, the preceding move would have to correctly set the e.p. rights. The best strategy seems to include a 'stripped' move generator in the GAME code, which generates only moves with implied side effects (i.e. e.p. capture, moves that create e.p. rights, and castlings). This would then be run for every simple move (the mentioned moves are never composit). This would be rather cheap, as most pieces don't have any such moves at all, so it would not generate anything when these move. In most CVs it would only generate something when a Pawn or a King moves, and it would then only generate the double-push if it was a 2nd-rank Pawn,  the e.p. capture if an e.p. square was created on the previous move, and in range, and sometimes one or two castlings. The few moves it generates could then be compared to origin and dest, and if one of them matches, all the side effects of it indicated by the move generator (i.e. e.p.-victim removal, Rook movement, or setting of the e.p. rights) would then be applied to the board.

For the latest move a complete move generator would generate all moves of the moved piece, and compare those to the input move. This is not optimally efficient, but it is easy and requires very little code that wasn't needed already for other purposes. And it is still very much cheaper than generating all legal moves in the final position for the purpose of setlegal: it has to be done only for a single piece, and the full legality has to be tested only for the move that matches the input. While setlegal requires the pseudo-legal moves of all pieces, and has to test each and everyone of those for legality. So there really wouldn't be any significant gain in optimizing it further, just a far larger code complexity.


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