Author: Dave Gomboc
Date: 10:34:57 06/14/99
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On June 14, 1999 at 12:34:48, William Bryant wrote: >On June 14, 1999 at 11:41:58, Dave Gomboc wrote: > >>On June 14, 1999 at 09:30:07, William Bryant wrote: >> >>>On June 13, 1999 at 21:10:57, Pat King wrote: >>> >>>>Before implementing null move, I had a simple aspirated AB search, and would >>>>research with a window of (Beta, Infinity) for fail highs and (-Inifinity Alpha) >>>>for fail lows. And life was good. With null move, however, it's not uncommon for >>>>my program to get stuck, alternately failing high and low, without ever properly >>>>resolving the score. What's up with that? Must the research be done >>>>(-Infinity Infinity)? Or is this a sign that I've a bug in the null move code? >>> >>>Pat, >>> >>> I just struggled with the same think, and I agree with Will that this should >>>be part of a FAQ. It was explained as follows, when switching from a alpha-beta >>>window to a beta-Infinity window, the search allows other possibilities that now >>>return a fail low score. It happens. I track it and I get a fail low following >>>a fail high about every 100 test positions or so. I can send you some positions >>>that seem to cause this. >>> >>> Now for practical advice gleaned from this group. Beleive the initial fail >>>high. If I fail low after a fail high, I ignore the fail low -- keep the fail >>>high. Therefore the move that failed high should become the new PV move >>>although you don't have a continuation and an exact score. >> >>I have seen the opposite advice as well (ignore the initial fail-high). I don't >>think a definitive solution has been recognized. >> >>Dave > >Dave, > What has you experience been. I ran a middle game mate position last night >that failed high - then low with a score of ~ 4.5. On the next iteration, with >a window of alpha = fail high score - window, beta = Infinity, it returned a >score of 7.something and a PV, so the fail high was clearly correct. > > I can send you the position and or the analysis is interested. I have not written a computer chess program, so you'd be better off asking someone who has for a specific experience. :-) If you do a search, it fails high, you do the re-search at the higher window, and it fails low, what does this say? Was the original fail-high incorrect? Was the fail-low incorrect? How can you tell? Dave
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