Author: Sune Fischer
Date: 12:07:11 01/21/04
Go up one level in this thread
On January 21, 2004 at 14:28:14, Robert Hyatt wrote: >>But actually, your example is interesting as well. >>Say we concatenate two PVs, one very much shorter than the other (from a much >>less deep search), why should that give us problems? > >Simple. One could search below position H and produce a best score and best >move. The second could produce a fail-low. No best move. Incorrect. That would mean it wasn't a PV node, which I've assumed it is. I can't speak for you, but tend to update the search flag also when I record a new entry. ;) So you do have an exact score and a move from both, my question, which I'm about to repeat for the 100th time, is "why would that be nonsense"? >Or a different >best move. What's the harm in that? >You have the score from the first search below H, and either the >best move from a different search below H with a different depth, Sure, but we are talking about the PV's here, not the score. >or else >no move there at all... *PV* nodes! >>They are both PVs so seperately they are ok, why shouldn't they be ok together? >> > >PV A goes with score A. > >PV B goes with score B. > >but in the case above, PV B goes with score A. The score and pv does not necessarily go together, that's true. But the PV shouldn't be nonsense in itself. >I can't debug that. Well we all have our limitations ;) >>I suppose it is possible the window is way off in one case (e.g. a mate window) >>and somehow getting mated turns into a PV which we then glue up with a PV from a >>different window showing a draw score.. :) > >That is one of many odd cases that happen, yes. But if you look back over >the CC literature, you will find lots of questions about PVs when they are >constructed in this fashion. Deep Blue was but one case where oddball moves >showed up because they had no choice in how to get their PV since the chess >processors did not supply one. Hand waving Bob, please come up with a tangible example! Don't talk me wrong, show me wrong. :) >>But I suspect this would be unusual, basicly I don't see how the same position >>can produce such different yet _exact_ scores?? >> > > >DIfferent "depth remaining". Caused by two different orders of moves leading >up to the position, each having a different number of checks. Sit at a board >and set up the pieces so that you look at a root position where you want to play >two moves by either side to get to my position "H" as described previously. IE >perhaps Bc3+ Kf5 Kh2 Kf4. One check, one extension. What happens when you >search Kh2 Kf5 Bc3 Kf4? One less check, one less extensions, a completely >different subtree below that position since it will be one ply shallower the >second time around. If your point is that you can search the same position with different remaining depths due to transpositions then...: I know that! It doesn't answer my question which very eloquently frased is: "so what?". It leads to a different position than the one related to the score, but that doesn't explain "buggy PV's". >>>The hash is path-independent. The tree below a node is _not_ path >>>independent, from repetition to 50-move-rule to search extensions that >>>may or may not be done depending on the order of moves in the path. >> >>Yes that's true, that is always an issue with the hash. >>I think it is a small issue in this context though, "grasping for straws" come >>to mind :) > >Nope. I sure hope you _are_ grasping for straws, because your program as well is mine is based on path independent scores from the hash. If this is enough to cause serious problems for the PV then imagine what it must be doing to our search! I think we can only assume (and hope) that it can be ignored. > Just explaining a problem that has been well-known for a _long_ time. >Surely you don't think that this idea came up recently? People have already >criticized DB's PVs because they had strange moves in them for this very >reason. At one point HiTech did this I believe. I can recall others talking >about the same odd behavior back in the late 70's/early 80's as memory became >big enough to make hashing more viable... I think you are comparing apples and oranges now. I'm not trying to construct a PV from a hardware produced search. -S.
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