Author: Bruce Moreland
Date: 09:59:20 10/01/98
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On October 01, 1998 at 07:58:57, Roberto Waldteufel wrote: >The only problem I have encountered with method 2 is that draws correctly >identified by the draw hash table will have an adverse affect on the scores of >ancestor nodes in the tree when they are entered into the transposition table >later on, and so a later hit on one of these ancestor nodes may give misleading >information. I don't really see a way to avoid this - it seems at least to be a >rare event. When you detect a repetition, just return instantly. I don't know how your program is structured, but in mine, I can just return out of the search function at that point, and conveniently forget to store the result. Of course, if the draw score is a fail low, you'll get a fail high when you return, and you'll store that score. Which brings up a point I don't fully understand. People are concerned about hashing draw scores because they don't want incorrect information in their hash table. But you will always get incorrect information in your table, even if you have a constant value for a draw, and your hash store function has a hook in it that says never hash this score. When you detect a repetition, sometimes you'll have to select a different move in order to avoid it, and the score of this gets hashed. Other times, when you find this position, and you have a choice between the move that was a repetition in the other position, and the move you chose instead, now you'll want to choose the repetition move, because it's no longer a repetition. So what you get is an erroneous score in the hash table, but it's not the draw score in the table, it's the score of that second move. You get similar problems when you start doing extensions and pruning that are based upon alpha. In one place you'll cut off and in another place you won't. So you're always going to have the general problem, I think, of positions that fail high when you search them, then fail low when you re-search them. I just ignore this problem and cross my fingers, and this seems to work fine. bruce
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