Author: Sune Fischer
Date: 08:43:21 01/19/04
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On January 18, 2004 at 22:39:23, Robert Hyatt wrote: > >I have heard that Shredder (and others) try to reconstruct the PV by probing the >hash table at the end of the search. This simply does not work with any degree >of accuracy. It works just fine. A while back I ran lots of test positions and compared the PVs between the two methods. What I generally saw was that they were identical, only the hash extracted PV usually were a few moves longer. I don't use the same replacement scheme as Crafty, so that may be part of the reason. The only thing I have noticed is that PV's tend to reach a maximum length, they rarely grow beyond 20 plies (with my replacement strategy anyway). My explanation is that once the PV has actually reached that kind of depth the search has been going on for so long that old PV entries have been replaced. >IE suppose you search and reach position A while searching the PV. > Later, at very shallow depths, you reach position A again and >overwrite it with different "best moves" depending on the depth remaining, >extensions triggered, etc. Now when you try to recover the PV from the hash >table, you get the right position A, but the wrong best move. And then the PV >looks funny. It doesn't happen every time, but if the PV is reconstructed >enough this way, it happens often enough. I tried this _years_ ago and ran into >the same problem. Never saw it in debugging. Saw it regularly when kibitzing >PVs on ICC. :) > >I now do it the correct way, backing the PV up along with the score... IMO the PV is a non-essential feature, having to do it "correctly" messes up the code and slows down the engine. That's just more than I want to spend on a fun-to-have feature. >Remember that this is speculation since I have never seen Shredder's source. But >recovering the PV in this way is simply going to produce errors, and there is >nothing that can be done about it. The first move and score will be correct, of >course. But beyond that, who knows, and the farther out, the greater the >probability of a bogus move. And? All engines produce not very bright move way out in the PV. You can't really rely on anything but the first few moves to be good, unless it ends in a mate or or tactical win. As long as you extract PV moves you won't see outright blunders, if you do you need to debug your search :) -S.
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