Author: Tord Romstad
Date: 04:04:16 01/20/04
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Hi Dennis, Thank you very much for your answer! I am very happy to recieve advice from one of the leading authorities on PN-like algorithms. :-) On January 20, 2004 at 05:54:11, Dennis Breuker wrote: > >I haven't worked on something similar, but have worked with PN search >and PN^2 search (see my thesis). I have thought about using PN search >for finding material wins. Yes, I've read and enjoyed your thesis, of course. Your idea of using PN search for finding material wins is already on the list of thing I would like to try out some time. One of my most recent changes in my engine is the following: When the side to move is ahead by a piece or more in material, and the opponent has no positional compensation, I use a null move search with my normal evaluation function replaced by a much simpler function. The idea is that the position is almost certainly won unless the opponent has a forced tactical win. Inside the null move search positional scores are not very important (I hope), and a primitive evaluation function should be enough. If the nullmove fails high by a big margin (which is usually the case), I return the value of the static eval (*not* the value of the null move search). If the null move fails high by only a small margin, it is too risky to trust the reduced-knowledge search, and I do another null move search with the full evaluation function. It is perhaps worth a try to replace the reduced-knowledge null move searches with a PN search for finding material wins. >In general, I think it is very difficult to use plain PN search >for finding an arbitrary goal. PN search is guided by mobility: >it prefers positions where the opponent has few moves, and you >have many moves. That is why it is good in finding forced mates. >If the opponent has few moves, there is a higher probability >that there is a mate somewhere. And that is why PN search is >not so good in finding mates where you have to make a >"silent move" (don't know the Engelish term for it) somewhere in >the sequence. >A forced mate like 1.Nf7+ Kg8 2.Nh6++ Kh8 3.Qg8+ Rxg8 4.Nf7 mate >is very easy to find for PN search, whereas a mate like >1.Bf6 any 2.Qh6 any 3.Qg7 mate is much more difficult to find >for PN search. > >This means that when you have other goals than mate, you must >guide the search for a most proving node by something else than >mobility... And that's not easy. It's not easy, but in certain cases I am not sure it is entirely impossible. Finding a clever domain-dependent way of assigning the initial proof and disproof numbers could help, don't you think so? Tord
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