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Subject: Re: Incomplete egtbs can be harmful

Author: Uri Blass

Date: 13:21:13 11/06/01

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On November 06, 2001 at 10:31:05, José Carlos wrote:

>On November 06, 2001 at 09:45:35, Uri Blass wrote:
>
>>On November 06, 2001 at 09:35:13, José Carlos wrote:
>>
>>>On November 06, 2001 at 07:23:07, Gian-Carlo Pascutto wrote:
>>>
>>>>On November 06, 2001 at 07:18:29, Leen Ammeraal wrote:
>>>>
>>>>>My program sees that black deserves a very high score,
>>>>>derived from the egtb, but fails to make the trivial
>>>>>move 1. ... g1Q because it does not find an entry
>>>>>for the resulting position in the egtb, and because
>>>>>the computed score after this promotion move
>>>>>is lower than the egtb score retrieved after the
>>>>>move Rg3, so the latter move is made and the
>>>>>game results in a draw instead of in a win for
>>>>>black. Has anyone encountered similar problems?
>>>>>It seems to me that for any egtb file with
>>>>>pawns, there should be a corresponding file
>>>>>for the case that one of the pawns has turned
>>>>>into a queen. If not, the result may be
>>>>>completely wrong, as in the above example.
>>>>>Leen
>>>>
>>>>This is wellknown.
>>>>
>>>>A possible solution is to turn off egtb's if you are in an egtb
>>>>position and you note that the distance to mate does not shorten
>>>>anymore.
>>>>
>>>>--
>>>>GCP
>>>
>>>  I don't understand. If the draw-by-repetition and draw-by-50-moves-rule tests
>>>are done correctly, this is, _before_ probing, the program will surely move the
>>>rook stupidly during 50 moves. Then it will see the draw, and chose another
>>>move. I don't see how the program can accept the draw here.
>>>
>>>  José C.
>>
>>It is possible that the program's move is only the 99 ply with no conversion so
>>the program looks at the tablebases and does not consider it as a draw and the
>>draw is in the 100th ply that the program does not consider because it looks on
>>the tablebases.
>>
>>It is also possible that in the 100th ply the program finds that conversion does
>>not win(there is another win by tablebases but it is a draw by the 50 move
>>rule).
>>
>>Uri
>
>  Ok, I get it. After _my_ move I get a tb hit when it's ply 99. Then my
>opponent moves and it's ply 100, so it's a draw after my opponent's move.
>Correct.
>  Bob gives a solution in other post, but there's another possibility. When you
>get a tb hit, check if it is ply 99. If it is, anything exept checkmate is a
>draw. Well, you lose some time checking the 100 ply counter, but it's a tiny
>time compared to the tb probe.
>
>  José C.

It is not so simple

The next possible situation can also happen

It is ply 97 you play a move that is a win by tablebases.
The opponent reply with check so you cannot play a conversion in the next move
and inspite of the fact that the position is a tablebase win you get a draw by
the 50 move rule.

Uri



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