Author: Andrew Dados
Date: 19:03:27 08/10/99
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On August 10, 1999 at 21:35:12, Robert Hyatt wrote: >On August 10, 1999 at 19:56:10, Marc Plum wrote: > >>A while back I ran some multiple engine tournaments within the Nimzo99 >>interface. One thing that I noticed was that some programs would make >>meaningless underpromotions. That is, in a position where a promoted pawn would >>be immediately exchanged anyway, the computer might promote to a bishop or rook >>rather than a queen. I had occasionally encountered the same thing in my own >>games with computers; I also found a small number of computer games like this >>when doing a database search for underpromotions. I don't have any statistics >>to present; I'm just noting that this happens not infrequently. >> >>When a human player does this, he is probably just being whimsical, or it could >>be a psychological ploy. I wonder, though, why a computer would do it. Is it >>just a random thing? Does the computer reason that losing a bishop is less bad >>than losing a queen, even though the resulting position is the same? Or do >>computers like messing with people's minds too? >> >>Marc Plum > >Actually at times there is a valid reason. If (say) d8=Q is a check, and d8=R >is not, then the program can choose whichever one maximizes the evaluation. How >could they be different? Remember that one is a check and will extend the >search while the other is not. So if searching one extra ply discovers >something interesting, then =Q will get played. If searching one extra ply >discovers something bad, then we avoid seeing the 'bad' by playing =R. > >Cute, eh? :) However there are cases when underpromotions are *totally* meaningless and still made by some programs. While hashtable should prevent re-searching a8=N Bxa8 because there should be score for a8=Q Bxa8, it looks like either hash entry can get overwritten or some programs don't calculate exact same signatures in those 2 lines. Or else? (Note that hashtable hit returns 'true' score, because a8=Q was searched full-width to become PV. It can be that some programs don't store 'true score' in hash so researching with 'new' bounds may produce new score). -Andrew-
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