Author: Ulrich Tuerke
Date: 10:16:38 06/23/98
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On June 23, 1998 at 12:39:03, Bruce Moreland wrote: > >On June 23, 1998 at 03:58:26, Ulrich Tuerke wrote: > >>I'm afraid this can imply some problems for the search. Assume >>that you have flagged a position as a "draw" in your transposition >>table because the 50 move rule had applied. If this position is later >>reached at a lower depth via a transposition, then the hash table >>would return a "draw". But may be, it really isn't because at lower >>depth the search might be still below the 50 move threshold ? >>This could result in real foolish moves returned from the t-table. >>IMHO, similar remarks hold for the draw by repetition case. >> >>I think that I had once detected problems through storing these >>kinds of positions in the hash table. Now, I don't store them. >> >>Did I get somehing wrong ? > >No, but you didn't get everything right, either. > >Draw scores don't just affect one node, they can propagate up the tree, and they >can cause other nodes to have a different value, without that value necessarily >being the draw score. > >So as long as you have a transposition table that doesn't take into account the >path used to get to a node, there will always be problems. > >And taking into account the path used to get to a node defeats the purpose of a >transposition table. > >So if you don't store 50-move and rep draws at the point you encounter them, >maybe this will make things work better, but you can't say, "whew, it is perfect >now", either. I know that it's not perfect. I have also taken care that a parent node of such special draw node which became the current best cause of the draw evaluation will not be stored either in the hash table. But of course, this is still not perfect. But according to my experience it's better than just to ignore the problem. I am pretty sure that in past I occasionally have had serious trouble in my program resulting from 50 move rule draw scores in the hash table. I am sure that the risk to obtain some catastrophic move is in case of the 50 move rule is really there and may be far larger than in the repetition case. So I changed it and it seems to work better; just a lot of pragmatism required to maintain a chess program: try and see if it's useful, nothing for theorists or perfectionists. Anyway, thanks a lot for the feedback here. > >bruce
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