Author: Dennis Breuker
Date: 05:21:10 01/20/04
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On January 20, 2004 at 07:04:16, Tord Romstad wrote: >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 Still you have to be careful in positions with high potential (positional) value, like two connected pawns on the 6th/7th row, for instance. >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. Sounds like a nice idea. What are your results with this? >It is perhaps worth a try to replace the reduced-knowledge >null move searches with a PN search for finding material wins. If you can find a good way to initialize the proof and disproof numbers. >>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? Yes. I mean, I do think so. :) However... mobility still is an issue, since you do a summation over the (dis)proof numbers of all children. And less children usually means a lower value. Dennis
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