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Subject: Re: A New Approach to Draw Detection by Move Repetition in Computer Ches

Author: Christophe Theron

Date: 20:14:42 07/30/04

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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. :)



    Christophe



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