Author: Patrick N
Date: 15:06:52 09/30/03
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On September 30, 2003 at 17:19:52, Russell Reagan wrote: >On September 30, 2003 at 17:03:46, Edward Seid wrote: > >>In the example where the terminal nodes are either -9999 or zero or +9999, it >>does make a difference. Setting alpha/beta to +/-10000 performs the same as >>+/-infinity. If using +/-9999, then it's possible for the score to equal either >>alpha or beta. Shouldn't that result in faster cutoffs or is my logic wrong? > >I think it is a good idea to make INFINITY equal to some value that is bigger >than your score fore MATE_SCORE. I know that Bruce Moreland does this in Gerbil >"in order to avoid unforeseen problems". He sets INFINITY to 40000, and mate to >32767. > >When you do an alpha-beta search, and the score that you get back is less than >or equal to alpha, or greater than or equal to beta, then you have to do a >re-search. Normally this is what you have to do during an aspiration search, and >during a "normal" search this isn't a problem because theoretically, your search >should not be capable of returning a value of +/- INFINITY. But if INFINITY >equals MATE_SCORE, then it is possible for a call to alpha-beta with alpha = >-INFINITY and beta = +INFINITY to return a score that is equal to the alpha or >beta value that you passed in to your alpha-beta function, and this will mean >you can't reliably detect mates, and who knows what other "unforseen problems" >might plague you. > I actually had a problem with this very recently. I am in the beginning stages of implementing a new engine and I originally had MATE_SCORE equal to -INFINITY. This was causing a problem when the engine detected that checkmate was imminent. The search was returning -INFINITY which was the same as a fail-low. This was causing the engine not to make any move at all. I fixed this by making INFINITY larger than -SCORE_MATE. Hopefully this information is useful to you. -Patrick
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