Author: Dave Gomboc
Date: 18:23:39 02/01/03
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On January 31, 2003 at 18:27:10, David Rasmussen wrote: >It doesn't really make sense in FEN to be able to define the halfmove clock (or >whatever they call it), without being able to define the positions that went >before. For example, in this position: > >[D]4q1kb/1pr2p1p/p3p1pP/P2pP1N1/3P1QP1/1P3R2/5P1K/8 w - - 27 100 > >The halfmove clock is 27. I understand that it is important for knowing when the >50 moves limit has been reached, and that's why it's there. But to specify the >position, or the "environment" of the game totally, you will have to say what >the last 27 positions were too, so you can also detect repetitions. If you just >give this FEN to an engine, it might suggest a move that leads to a repetition, >without realizing it. >Practically, this undefinedness can be a source of bugs in an engine. I know, >because I had one :) >My program checks for repetition at the root and in search (although >differently) by checking at most the last 27 positions in this case (every >other, to only check ones with the same player to move etc.). In a normal game >started from the initial position, the invariant always hold that the number of >halfmoves played over the board, is at least as great as the 50 moves counter. >That is, if the halfmove clock is 27, we have at least played 27 moves over the >board, of course. When a position like the one above gets loaded, the number of >moves over the board is 0. So we check the position with index -2, -4, ... -26, >in the array of hashkeys of positions that have already been played. I know that >there are ways around this, but only rather meaningless ones, like saying that >27 positions have been encountered in this case, and the hash signature of them >was 0 etc. Because we don't know what the positions were. > >/David I don't think it's an oversight -- I think this was recognized during its development. You have to draw the line somewhere, and SJE drew it where he thought the best compromise was. Dave
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