Author: Marc Bourzutschky
Date: 03:01:16 02/05/03
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On February 05, 2003 at 04:25:46, GuyHaworth wrote: > >Maybe they differ because an a1-h8 rotation does not leave the position >unchanged when there are Pawns on the board. > >So ... they wouldn't be the same even if there was no e.p. rule. > >g The position does not have to be unchanged, only a 1-1 mapping needs to exist. The symmetry actually used is: 1. Flip the colors of all pieces 2. Flip the board (a1 <-> a8, h1 <-> h8, etc) including e.p. square 3. Change side to move For KPKP all positions in .nbb can be mapped to a position in .nbw this way, so .nbb and .nbw have the same information content. There must be another reason why Eugene keeps both files, probably more a convenience than anything else. This will get more interesting once he generates KRPKRP, etc. Note that probing code needs to be aware of the above transformation even for non-symmetrical endings because, e.g., endings such as KPKBP.NBW have to get mapped to KBPKP.NBB (including potential e.p. squares). -Marc
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