Author: Robert Hyatt
Date: 04:43:59 08/02/98
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On August 02, 1998 at 03:35:04, Ed Schröder wrote: >>Posted by Howard Exner on August 01, 1998 at 18:48:04: > >>When analysing positions with Rebel 8 it often happens that the >>correct move is locked onto in conjunction with a plus sign appearing >>beside the move. eg. Qh6 (+). > >Chess programmers call this a "fail-high". It means for the move in question >that the score is out of the "alpha-beta-window". Therefore the move must >be re-searched with a new "alpha-beta-window" to get the correct score >for the move in question. > >>The actual move however can take a long >>time before it becomes the first choice among the other move choices. > >It usually takes a bit longer because of the wider "alpha-beta-window". > >>Why is there such a long delay in deciding to make this the first choice >>when almost always the move turns out to be correct? Did this occur in >>Rebel 9 also? Will the new Rebel have that long delay between marking >>the move with the plus sign and actually making the move the top choice? > >In Rebel10 when a "fail-high" or (+) appears Rebel will immediately mark >the move as "best-move" and THEN start the re-search to figure out the >correct score. This because of the growing popularity of finding key-moves >in the so called computer test sets. As far as I know all (most?) chess >programs act like this. > >This change will result in finding key-moves average 20% faster and >therefore Rebel10 will perform a little bit better in computer test sets. > >Still you will notice that sometimes (say in 5% of the cases) the "fail-high" >or (+) is NOT rewarded by the "re-search". In this cases Rebel10 will >stick to the previous "best-move". > >I have noticed that other chess programs (all??) do not show this >behavior. When they "fail-high" the move always becomes the >"best-move". > >Is the latter true or are there programs around who behave like Rebel >in this respect? > >- Ed - I've always done this. The problem is that on some occasions, the null- move search can cause a "false-fail-high". The re-search produces a score that was worse than the old search, or, more commonly, actually fails low. I reject such fail highs if the re-search is worse than the old best (I catch this by searching with (old-best, +infinity), so that if I fail low, I know it is worse than the old best move and it gets rejected outright. I've had versions where this was "broken" for various reasons, and they always produced an occasional bizarre-looking move that could be an outright blunder, or just a slightly weaker move...
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