Author: Uri Blass
Date: 04:35:38 01/15/03
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On January 15, 2003 at 07:24:44, David Rasmussen wrote: >On January 15, 2003 at 06:54:59, Richard Pijl wrote: > >>On January 15, 2003 at 06:12:23, David Rasmussen wrote: >> >>>Since null moves are illegal in the real game, it is not obvious how it affects >>>or should affect repetitions and the fifty move rule. >>>What I do now in Chezzz is just pretty much ignore the relation between these >>>concepts and formulate them as if they were formulated in isolation. >>>What is the correct way to deal with these things? >>>For instance: >>> >>>1. What to do with the fifty moves rule counter when doing a null move? >> >>I think in situations where you have not done any captures or pawn moves for >>30-40 moves nullmove pruning may yield little result as the programs often are >>'nullmoving' with real moves anyway. I haven't done experiments with it yet but >>I seriously consider turning nullmove off when the 50 moves counter reaches 35. >>But whatever you do, the overall effect in your engine is probably close to zero > >I've done exactly that, except it's 37.5 (75 ply) :) > >>... >> >>>2. When doing a null move, we obtain a new position, one that can never be >>> on the board immedeately after the currect position in a real game, but >>> it is treated as a "normal" position. What if a "repetition" is found of >>> two of these "imaginary" positions in the current line? We would return a >>> draw (that is what I do, since I treat these positions as all others, as do >>> most programs I've seen, Crafty for instance). >> >>I think that when you're getting a repetition in search (so it appears twice in >>search) you can safely return draw. If the first occurrence happens before a >>nullmove, there is no progress so it probably won't produce a cuttoff as it is >>likely the repetition (or better) can be reached by not null-moving. >> >>Richard. >> > >I'm not totally convinced by this. There are several cases, aren't there? Let's >just see it from white's perspectice, it will of course be symmetric for black: >doesn't all this depend on: >1. whether white is winning or losing at the root >2. whether the real score of the current position is below or above alpha or >beta >3. whether the last move was really good or really bad > >Also, what about stalemate detections? Null move pruning affects that too as far >as I can see. > >/David I do not see how null move pruning effect stalemate detections. Cases when my evaluation detects stalemate immediately after null move are very rare and I do not see how these cases can do demage to the program. Uri
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