Author: Dann Corbit
Date: 16:17:36 12/20/05
Go up one level in this thread
On December 20, 2005 at 18:51:18, Frederik Tack wrote: >On December 20, 2005 at 17:59:20, Dann Corbit wrote: > >>On December 20, 2005 at 17:42:40, Dann Corbit wrote: >> >>>On December 20, 2005 at 17:11:41, Frederik Tack wrote: >>> >>>>I'm looking for the best way to implement a repetition detection in the search >>>>tree for my engine without loosing performance. >>>> >>>>I've found some articles on the internet suggesting to create a repetition table >>>>using zobristkeys. I have now implemented this and it works fine, but i've lost >>>>about 10% performance. >>>> >>>>I have now come up with a new idea that should have no impact on performance. >>>>However, i'm not sure if what i'm about to do is completely correct. Here's the >>>>idea. Before starting the analysis, i store every position i've had on the board >>>>from the move history in the transposition table with an evaluation of 0 and a >>>>depth of maxdepth + 1. During the analysis, i use a replacement scheme in the >>>>transposition table that will not replace the position if the depth in the tree >>>>is smaller than depth in the table. Since i've stored the historic positions >>>>with maxdepth + 1, they will never get replaced. This should score every >>>>position that has been on the board before with a draw score. >>>> >>>>The fact is i haven't found any article on the internet discussing this way of >>>>working, so i fear it may have some drawbacks i'm not aware of. Has any1 tried >>>>implementing repetition detection this way or has any1 an idea why this wouldn't >>>>work? >>> >>>That sounds similar to what most everyone does. Usually, you keep a list of all >>>the positions you have played [or are planning to play enough to shove it in the >>>pv] OTB up to now (usually called the move_stack or thereabouts). Then, you >>>just scan the move stack to see if there is any strange feelings of Deja Vu. >>> >>>That way, you can clear your hash table if you want to. >> >>If you are scanning your whole move stack, you can shorten it up a lot by only >>going back to the last non-reversible move. If (for instance) there is a pawn >>push or a capture or a promotion, etc. then we know for sure that the positions >>from there backwards are no longer in the realm of repeat candidates. >> >>A ten percent performance hit seems REALLY high. How are you checking for >>repeats? I would expect it to be hard even to measure it. > >Tx for the reply. > >The way i've currently implemented it is by using a repetition table (similar to >a transposition table. It's a table with a 16-bit index, a 64-bit zobristkey and >a counter for the number of times the position occurred. The problem with this >approach is i loose performance because i have to access this table in every >node in the tree. The first lines in my negamax function look like this : > >function Negamax(const PositionPtr: PPosition; Depth: TSearchDepth; > Alpha, Beta: TEvaluation): TEvaluation; >begin > inc(Nodes); > if RepTable[PositionPtr^.ZobristKey and RepetitionTableSize].Counter > 1 > then Result := evDraw // 3-fold repetition > else ... > >Concerning the new implementation i suggested : >If i understand correctly, you suggest scanning the move stack during the >analysis. However, my suggestion does not involve any scanning during analysis. >Before starting the analysis, i would just store all the moves from the move >stack in the transposition table with an evaluation of 0 (indeed, i could >optimize this so i store only from the last non-reversible move on). Then i just >start the analysis and don't do any scan or anything special. Positions that are played are either history positions or positions in your pv. If you store both the history positions and your pv in a move stack, then when you add something to the move stack, you can scan backwards at most 100 positions to count up all the times that it has occurred. You can obviously make a little hash table for the move stack also, so that access is approximately O(1) to find something. A 12000 entry table should have very little chaining, even in the theoretical maximum game. A skiplist could also be a nice way to store them. You could create a skiplist with unique nodes but that also has a counter in it for repeats. Even the linear search of up to 100 nodes only happens when you put something into the move stack. So I guess that I am suggesting: Keep a list of things that you have played or think you might play. When you put something into the list, at the same time update the counter for frequency. The list should have stack semantics for pushing and popping moves and also random access of at worst log(stacksize) If you want to be fussy about it, your repetition stack never needs more than the last 100 movestack items in it. >I also don't quite understand what you mean with 'That way, you can clear your >hash table if you want to' I thought you were talking about storing repetitions in the regular hash table.
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