Author: Ricardo Gibert
Date: 11:58:28 11/17/00
Go up one level in this thread
On November 17, 2000 at 13:33:50, Heiner Marxen wrote: >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. It is the horizontal pin that I had in mind. A naive (mistaken) algorithm would fail to detect the existence of a pin. > >E.p. captures are somewhat tricky, yes. > >Heiner
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