Author: Robert Hyatt
Date: 20:08:59 07/29/02
Go up one level in this thread
On July 29, 2002 at 12:56:33, J. Wesley Cleveland wrote: >On July 29, 2002 at 11:00:30, Robert Hyatt wrote: > >>On July 29, 2002 at 00:28:05, J. Wesley Cleveland wrote: >> >>>On July 28, 2002 at 13:02:01, Robert Hyatt wrote: >>> >>>>On July 27, 2002 at 15:06:23, J. Wesley Cleveland wrote: >>>> >>>>>On July 25, 2002 at 20:13:45, Robert Hyatt wrote: >>>>> >>>>>>On July 25, 2002 at 19:24:06, J. Wesley Cleveland wrote: >>>>>> >>>>>>>I see that crafty does not store lower bounds of MATE-n in the hash table, >>>>>>>rather changes them to MATE-300. Bob wrote that he had search instabilities >>>>>>>before he did this. Normally, this does not matter, but I think it makes crafty >>>>>>>considerably slower in finding mates, as it only gets cutoffs on exact scores. >>>>>>>Do other people have experience in this ? >>>>>> >>>>>> >>>>>>Note that all this does is slightly decrease search efficiency. I do store >>>>>>_exact_ mate scores as they should be stored. I store "bounds" that are based >>>>>>on MATE as MATE-300. The penalty is _very_ small unless you have a position >>>>>>where almost everything leads to a forced mate of some sort... >>>>> >>>>>The place where I notice it is in engame analysis with EGTBs, where after a long >>>>>time the PV is scored as Mate in 38 or so, and then it takes a *very* long time >>>>>to prove the other root moves are worse. >>>>> >>>>>A related question: >>>>>If the score in the hash table is MATE-300 and this would cause a cutoff, >>>>>shouldn't you cut off even if the draft is not deep enough ? >>>> >>>> >>>>I could but I don't. That would prevent finding a _shorter_ mate the next >>>>iteration. >>> >>>But wouldn't you only care about a shorter mate if the _value_ would not cause a >>>cutoff ? >> >> >>There are two issues here: >> >>1. absolute mate scores. I store those correctly, as is, corrected for the >>distance from the current position to the actual mate. >> >>2. mate bounds. I found problems with those, and simply changed any mate >>bound to mate-300. > >>It is still large enough to cause cutoffs against any >>possible material gain or loss. But not large enough to confuse a real mate >>search where the scores are absolute but the bounds are not... > >Let me give an example. Assume that while searching at a given ply, alpha is >1805 centipawns. When searching after a move for black, the hash table has a >lower bound of MATE-300 but the draft is less than the depth. Why would you not >want to cut off without searching here ? Wouldn't you search *exactly* the same >moves (assuming no hash table overwrites), and return the same MATE-300 value ? > You can do this. I used to do this in an even stronger form in Cray Blitz, but I dumped it later. All you need to do is set the "draft" to infinity for any mate score or bound... The down-side is that you will _never_ find the shortest mate if you find another mate first. >On the next ply alpha is MATE-21. Now the score will not cause a cutoff and the >position is re-searched normally.
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