Author: Christophe Theron
Date: 06:52:37 07/31/04
Go up one level in this thread
On July 31, 2004 at 06:04:34, Ricardo Gibert wrote:
>On July 30, 2004 at 23:14:42, Christophe Theron wrote:
>
>>On July 30, 2004 at 23:05:05, Christophe Theron wrote:
>>
>>>On July 30, 2004 at 20:26:19, Uri Blass wrote:
>>>
>>>>On July 30, 2004 at 20:03:55, Christophe Theron wrote:
>>>>
>>>>>On July 30, 2004 at 06:47:39, Gerd Isenberg wrote:
>>>>>
>>>>>>On July 29, 2004 at 23:18:53, Walter Faxon wrote:
>>>>>>
>>>>>>>On July 29, 2004 at 17:34:11, Christophe Theron wrote:
>>>>>>>
>>>>>>>>On July 29, 2004 at 14:07:10, Robert Hyatt wrote:
>>>>>>>>
>>>>>>>>>On July 29, 2004 at 06:26:52, Gian-Carlo Pascutto wrote:
>>>>>>>>>
>>>>>>>>>>http://arxiv.org/ftp/cs/papers/0406/0406038.pdf
>>>>>>>>>>
>>>>>>>>>>I stumbled onto this when doing a search for Axon.
>>>>>>>>>>Not seen it mentioned here yet.
>>>>>>>>>>
>>>>>>>>>>They also have a paper about hashing out which I can't
>>>>>>>>>>download.
>>>>>>>>>>
>>>>>>>>>>--
>>>>>>>>>>GCP
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>Doesn't strike me as particularly interesting. IE it almost seems that they
>>>>>>>>>don't realize that most programs store positions in a repetition list as 64 bit
>>>>>>>>>Zobrist integers...
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>Actually I think it might be interesting.
>>>>>>>>
>>>>>>>>Recently, when I was rewriting the core of the Chess Tiger engine, I realized
>>>>>>>>that I could get even more speed by not computing the hash keys during the
>>>>>>>>quiescence search for example.
>>>>>>>>
>>>>>>>>In my case, it would have meant some more changes in the engine and the way I do
>>>>>>>>QSearch. But for some programs, it could be interesting.
>>>>>>>>
>>>>>>>>The problem then is how do you check for repetitions?
>>>>>>>>
>>>>>>>>If you allow checks and escape from checks in your QSearch, and if you actually
>>>>>>>>extend them in some way, you have to detect repetitions.
>>>>>>>>
>>>>>>>>So a lightweight, hash key free, repetitions detector is a must in this case.
>>>>>>>>
>>>>>>>>It could also be interesting for people who want to write a very small chess
>>>>>>>>program for portable units.
>>>>>>>>
>>>>>>>>But I think there is a better method than the one given in the paper. I would
>>>>>>>>use an array of integers, one per piece on the board. The array starts filled
>>>>>>>>with 0. Every time a piece is moved I would add the move vector to the integer
>>>>>>>>in the array.
>>>>>>>>
>>>>>>>>A repetition is detected when all the array is filled with 0 (nul vectors). It
>>>>>>>>is possible to use a "master vector" that receives all the individual vectors
>>>>>>>>after every move. One has to check the whole array only when the master vector
>>>>>>>>is nul, otherwise there cannot be a repetition.
>>>>>>>>
>>>>>>>>This method also works backwards (from the current move back to the last
>>>>>>>>irreversible move), but avoids any search in the concatenation list.
>>>>>>>>
>>>>>>>>It should be significantly faster than their method.
>>>>>>>>
>>>>>>>>Now I should write a paper. :)
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Christophe
>>>>>>>
>>>>>>>
>>>>>>>Will this detect when two like pieces have "traded places" in the repeated
>>>>>>>position?
>>>>>>
>>>>>>Good point.
>>>>>>
>>>>>>I don't see how the "New Approach" handles "traded places" as well, because the
>>>>>>list_of_moves doesn't contain piece information but only from/to squares.
>>>>>>
>>>>>>So occasionally the "New Appoach" may miss some repetitions, where rooks or
>>>>>>knights have traded places. Whether this is practically relevant is another
>>>>>>question.
>>>>>>
>>>>>>Gerd
>>>>>
>>>>>
>>>>>
>>>>>It will also catch the cases where pieces have just traded squares.
>>>>>
>>>>>Each piece is tracked individually by a vector summing up all of its moves. When
>>>>>all vectors are 0, all pieces have been moved back to their "original" square.
>>>>>
>>>>>The "master vector" is just a way to tell quickly if it is possible that there
>>>>>is a repetition, and in this case all the individual vectors must be checked.
>>>>>
>>>>>It is a "perfect" detector in the sense that it will not make any mistake.
>>>>>
>>>>>
>>>>>
>>>>> Christophe
>>>>If I understand correctly
>>>>it can miss some repetitions when 2 white rooks traded squares because in that
>>>>case not all vectors are 0 and vector of one rook is positive when vector of
>>>>second rook is negative.
>>>>
>>>>Uri
>>>
>>>
>>>
>>>Mmh... You are right.
>>>
>>>So it is not perfect in that sense.
>>>
>>>Someone has a solution for this?
>>>
>>>Actually I think the detector mentionned in the paper would have exactly the
>>>same problem.
>>>
>>>
>>>
>>> Christophe
>>
>>
>>
>>OK I have the solution.
>>
>>The master vector trick still works.
>>
>>When the master vector is 0 (nul vector), check the array.
>>
>>Instead of looking for 0 everywhere, the following conditions are accepted for
>>each individual piece vector:
>>* if it is 0: OK
>>* if it is not 0 and the content of (square_of_the_piece + vector) is the piece
>>itself on the current board (or a similar piece from the same side): OK
>>
>>(note that the first condition is just an optimization and can be removed)
>>
>>If all the vectors are "OK" then a repetition is detected.
>>
>>So the algorithm still works, but it is a little bit less elegant now. :)
>
>
>A lot less elegant when you incorporate ep status and castling rights.
You do not have to take these into account. A move that would change the
ep/castling status data is an irreversible move that stops the repetition
detection.
What you need is to have a flag in your move stack (or list) to mark
irreversible moves. You stop the draw detection algorithm (by 3-rep or
50-moves-rule) as soon as you find an irreversible move.
You also need to maintain this flag if you are using a draw detection method
based on Zobrist hash keys anyway.
Christophe
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