Author: Heiner Marxen
Date: 10:33:50 11/17/00
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On November 16, 2000 at 19:18:10, Ricardo Gibert wrote: >On November 16, 2000 at 17:24:37, Lenard Spencer wrote: > >>On November 16, 2000 at 04:25:53, Ricardo Gibert wrote: >> >>>On November 16, 2000 at 02:51:47, Tony Werten wrote: >>> >>>>On November 15, 2000 at 20:40:16, Lenard Spencer wrote: >>>> >>>>>This question may probably be best answered by the problemists, but if what I'm >>>>>thinking is correct, it may be possible to make looking for double checks go a >>>>>lot faster than the brute force approach of looking all over the board for more >>>>>than one checker. >>>> >>>>The way I use it: >>>>first, can the piece just moved attack the king (lookup table)? If so get the >>>>direction in which it needs to travel (same lookup table) and check if there are >>>>any other pieces blocking. >>>> >>>>second, can a rook or bishop attack the king from the fromsquare of the moved >>>>piece. If so get the direction, then travel from the king in the direction of >>>>the fromsquare until you go off the board (no discoverd check) or bump into a >>>>piece (if piece=rook,bishop,queen then it's a discovered check) >>>> >>>>if ( first and second) then doublecheck:=true; >>>> >>>>Tony >>> >>>How about this position: >>> >>>[D]8/8/7k/6pP/8/4B3/7R/7K w - g6 >>> >>>The move 1.hxg6 is double check, but it is not clear to me how your algorithm >>>catches this. >>> >> >>In this example, the pawn move delivers a double check, but the pawn itself is >>not a checking piece. But it does serve to illustrate just how tricky it can >>be. > >Yes, that was the point of my post. It is unusual, because it is a "double >discovered check" to put it more precisely. The loss of the Black pawn discovers >a check from the Bishop and the capture by the White pawn discovers a check from >the Rook. > >There is a similar "joker" concerning pins too, which a naive algorithm may >miss. Yes and no: an intended ep capture cannot be illegal by simply uncovering a check through the captured pawn: vertically the line (column) is not opened (the moving pawn closes it again), and diagonally undiscovering a check would imply that in the move before the other side could have captured the king instead of moving that pawn. Horizontally we can have a pin through *both* pawns (as well as an undiscovered check through both of them), but I cannot see how we could have two simultaneous pins for an ep capture. E.p. captures are somewhat tricky, yes. Heiner
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