Author: Roberto Waldteufel
Date: 10:44:11 04/08/99
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On April 08, 1999 at 12:11:09, Christophe Theron wrote: >On April 08, 1999 at 10:10:34, Roberto Waldteufel wrote: > >>In chess the side to move can stand pat even though there are captures >>available, since the ability to capture does not compel a capture to actually be >>made, but in checkers this is not allowed: if there is one (or more) capture >>available, then the side to move *must* make a capture, so standing pat is not >>an option. > >Hmhh... I had overlooked this. This is indeed not mainly a zugzwang problem. > > >>Originally I stopped the Qsearch as soon as the side to move had no >>captures, but I found that this led to errors if the other side was threatening >>a capture that could not be prevented, so now I continue the Qsearch in these >>cases as well, that is as long as either side has captures pending the search >>continues (full width). This does not add too much overhead, and the results are >>much more stable than before when I only extended if the side to move had >>captures pending without considering the other side's threats. > >An idea for a chess program? Never seen this before? ;) > > >>Usually in cases >>where the other side threatens a capture but the side to move has no captures, >>the side to move can meet the threat with at least one of its available >>(non-capture) moves, but in cases where nothing can escape from the threat, the >>Qsearch now scores the position accordingly. > >Why not do this for both sides? By detecting the opponent's threats your program >certainly plays safer (is able to see long range threats). By detecting also >your threats, it might be more agressive? > >Again, this is just a silly idea. I'm quite a stranger to checkers... > > > Christophe By a threat for the other side, I mean that, if it were in fact the other side's turn to play, he would have a capture. Therefore threats by side to move would mean that the side to move has one or more legal captures available, which is precisely the condition I started out with to trigger an extension. The enhancement was to extend if *either side* has pending captures. The whole qseearch is easier than in chess precisely because of the obligation to capture whenever possible - it is not really a qsearch in the sense we use that term for chess - rather it is an extension of the full width search. The only cases that escape this technique are so called "pitch" moves, where neither side has a capture, but the side to move deliberately offers a sacrifice of a piece (which must be accepted because of the obligation to capture whenever possible) in such a position that the material scrificed can either be regained with interest or an overwhelming positional superiority can be established by means of the sacrifice. If you ever turn your hand to another game, I can highly recommend checkers. It has far more depth and subtlety than I ever thought possible with such a limited set of moves compared to chess. The late Dr Marion Tinsley, thought by many to be the best (human) player of all time, was also quite a strong chess player in his youth. He compared the two games nicely thus: "Chess is like looking out over a limitless ocean, whereas checkers is like looking down a bottomless well". I understand from that quote that in checkers, because of the lower branching factor, you can look further ahead, but still sooner or later you must reach a horizon where a (possibly erroneous) positional evaluation must be made, just as happens in chess and other games. In my experience it is often more difficult to describe accurately what constitutes a positional advantage in checkers than it is in chess, although that might be because I am not very experienced at checkers, whereas I have played, studied and programmed competitive chess for many years. Best wishes, Roberto > > >>Another point is that when captures >>are available, the choice of moves is extremely limited (often only one legal >>capture), so such extensions are much less expensive than they would be in >>chess. >> >>Best wishes, >>Roberto
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