Author: Robert Hyatt
Date: 09:54:49 08/02/98
Go up one level in this thread
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. one other note. If you are going to behave like this, you might consider the "full" PVS implementation that at least two programs used in the late 70's/early 80's.. belle and cray blitz. when you fail high, mark that move as best but *don't* research. If a second move fails high, you first research the first move with a relaxed beta value, then you search the second to see if it fails high. If not, keep going and stick with the first fail-hi move. If the second fails high, don't research, just mark as best and see if another will fail high. What this really does is help those positions where A is best at odd plies and B is best at even plies, because you never have to do a research. I didn't like it due to lack of something to ponder, and you won't get a best ply=2 move from the hash table either. But I have alternative ways to find something to ponder, and with "internal iterative deepening" not having a PV at the start of an iteration is not too harmful either... > >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 -
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