Author: Ricardo Gibert
Date: 10:10:43 05/02/02
Go up one level in this thread
On May 02, 2002 at 11:00:08, Will Singleton wrote: >On May 02, 2002 at 06:00:51, Ricardo Gibert wrote: > >>On May 02, 2002 at 01:54:29, Will Singleton wrote: >> >>>On May 02, 2002 at 00:02:53, Robert Hyatt wrote: >>> >>>>On May 01, 2002 at 21:53:01, Will Singleton wrote: >>>> >>>>>On May 01, 2002 at 11:41:54, Arwin Smit wrote: >>>>> >>>>>>Hi, >>>>>> >>>>>>I programmed a hash table for my chess engine. It seems to be faster >>>>>>and stronger now, but as far as I know the disadvantage of using a >>>>>>hashtable is that the engine will not always find the best solution >>>>>>anymore. And this is caused by not being able to see a 3-fold repetition >>>>>>draw anymore in some cases. A position may be in the hash table and >>>>>>returning a non-draw score, but it was reached in a different part of the >>>>>>search tree by repetition of positions. >>>>>> >>>>>>This worries me a bit since this is the first time I made a change to my >>>>>>engine causing it to be "non-perfect". >>>>>>Is the advantage bigger than the disadvantage? >>>>>>What is the best way to test which version is better anyway? Just let it >>>>>>play a lot of games against eachother? >>>>>> >>>>>>Arwin >>>>> >>>>>It's possible you can fix this by 1) not storing draw scores in the hash table, >>>>>and 2) testing for repetition prior to probing the hash (if you don't use the >>>>>main hash for your rep detection). >>>> >>>> >>>> >>>>There is really no way to close all the holes. It is possible that the >>>>path from the current position to the tip, which is found by a hash hit, >>>>repeats a position between the current position and the root. But it is >>>>impossible to know this without storing the complete path in a hash entry. >>>> >>>>Not storing draw scores is (IMHO) a bad idea. A draw score is as legitimate >>>>as any other score. Not storing them to avoid one problem simply creates >>>>another on the other side... And it still doesn't solve the first problem >>>>I gave, which means errors are going to occur, period... >>>> >>>>I simply ignore them... >>>> >>>> >>> >>>Hmmm... I don't know about that. You say that a position occurring between the >>>current position to the tip may contain a repetition of a position that occurred >>>between the current position and the root, and that the score of the current >>>position wouldn't reflect the draw without storing draws in the hash. My point >>>was that, if you don't hash draw scores, the current position would not appear >>>in the hash, and the draw would have to be found by the normal search. Which >>>seems to work fine in my prog. >> >>He not talking about false draw scores, he is talking about missing a draw by >>repetition. >> > >? So am I. > >>> >>>Let's take another example. Suppose you find a draw in a search, at ply 7 (draft >>>0), which gets stored as exact in the hash. Now you and the opponent move, and >>>in the next search, you find that draw score at ply 5 (draft 0). But it turns >>>out that the original draw score was from a two-fold repetition in the search, >>>and that two-fold rep doesn't exist in the current search. Hence, the draw >>>score in the hash is incorrect. >> >>The draw score is correct, because a forced repetition is anticipated. The draw >>score would not be there if it was avoidable. It does not matter that the >>current line does not contain a repetition...yet. >> > >This seems a bit "theoretical". If you want to give an example to explain your >point, I'd appreciate it. But it's clear that not storing draw scores has >little practical downside, if any. 1----x---r---y---R 2---/ / / / 3------/ / / 4---------/ / 5------------/ Line 1 evaluates as a draw due to position r being repeated at R. Now line 2 transposes at x. If you have a usable draw score stored at x, why on earth wouldn't you use it? The same goes for lines 3, 4 & 5. It does not matter whether r occurs in the respective line or not. With the tranpostion, given enough depth to analyze, you will only duplicate work already performed. If you don't have enough depth, you miss the draw as Hyatt points out. The case that *does* mess things up is the following: 1------x--r¹-- 2--r--/ Now when line 2 tranposes into line 1 at x and it uses the usable score found with the tranposition, it will fail to detect the repetition of r at r¹. Moreover, you should also note that line 1 has *also* failed to detect a repetition due to insufficient depth. If line 1 were extended, you should expect it to look like: 1------x--r¹--x¹--r² 2--r--/ With x repeating at x¹ and r¹ repeating at r². Naturally, this line would actually stop at x¹, the first repetition. If I've got this all wrong, please explain it to me, because I'm mistified by your idea of not storing such draw scores. > >>> >>>That was part of my thinking in not storing the draw scores. In any case, the >>>search will find any draws without the need for storing them. >>> >>>Will
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