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H. G. Muller wrote on Thu, Nov 14, 2019 10:32 PM UTC:

We might miscommunicate, because I am thinking in terms of a pseudo-legal-move search, while you might want to determine move legality before searching them. For me an illegal move is simply a move that receives score -INFINITY when I search it. In an engine for normal chess this happens when the next ply captures your King, which gets awarded +INFINITY, and is then negated. Incidentally -INFINITY is also the start value of bestScore, and if it is left at -INFINITY all of the moves must have been illegal. That means mate or stalemate, and if stalemate is a loss both can get the same score, and -INFINITY would be a very suitable value, and you don't really have to do anything other than just returning the maximum move score.

IMO independently trying to establish legality of moves in a search is just a waste of time, as moves are legal more often than not. Just do the search, and when the move is illegal this will discover it soon enough, and will return -INFINITY. But most of the time there is nothing to discover. As long as an illegal move will get a score lower or equal than anything else, the search will work fine.

If you want to test legality of an input move, just do a sufficiently deep search on it (1 ply + QS in normal chess, 2 ply + QS in Marseillais), and reject it if the score is -INFINITY. You don't need any special code for that. I you want to make a list of legal moves (e.g. for the purpose of highlighting taret squares), just write a small loop to subject every prseuo-legal move of interest to such a search.


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