Author: J. Wesley Cleveland
Date: 09:56:33 07/29/02
Go up one level in this thread
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 ? 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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