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Subject: Re: Search and eval in the endgame

Author: Tord Romstad

Date: 11:57:52 01/05/04

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On January 04, 2004 at 16:12:40, Russell Reagan wrote:

>On January 04, 2004 at 14:03:08, Tord Romstad wrote:
>
>Hi Tord,
>
>>Another
>>idea I have experimented with is to include passed pawn pushes in the qsearch in
>>pawn endgames.
>
>I think this is a good idea. I don't think of qsearch as "capture and check
>search." I think of it as "forcing move search." Pushing a passed pawn in the
>endgame is certainly a forcing move.

I agree that it looks like a good idea, but it didn't work very well when
I tried it.  However, I still think it should be possible to make the
idea more effective with some refinements.  Perhaps it is better to only
search the most dangerous-looking passed pawn pushes.  In most cases,
it is probably a waste of time to search the move e4-e5 if black's king
is on e7.

>The goal of qsearch is to hand off quiet positions to the evaluation function.
>If there is a passed pawn that can promote in a few plies, that isn't a quiet
>position at all! Your evaluation may be off by a whole queen or more in that
>situation.
>
>I think it is better here to take the time to resolve things correctly.
>Otherwise you can say, "my evaluation may be off by a queen here, but look at
>that full-width depth!" Search depth and nodes per second are only a means to an
>end.
>
>Maybe you will search one full-width ply less, but if you detect the passed pawn
>several plies earlier, is that not an improvement overall? Looking at it from
>the other side, is it possible that you will now miss something because of the
>slighly shallower full-width depth?
>
>
>>Endgame evaluation is also tricky, because the evaluation should be very
>>different
>>depending on the type of endgame.  I am tempted to write several different
>>evaluation
>>functions (one for pawn endgames, one for rook endgames, one for bishop vs
>>knight
>>endgames, one for endgames with unequal coloured bishops, and so on), but I am
>>afraid this would cause too big jumps when exchanges occur, make my static
>>exchange
>>evaluator too unreliable, and perhaps have other unfortunate side effects.  Is
>>the idea
>>still worth a try?
>
>I don't understand why this would make it unreliable. Wouldn't it make it more
>reliable? Your program might switch to another "plan" all of the sudden, but as
>long as you implemented your specific endgame evaluation correctly, it should be
>a better plan, right?

Hopefully.  I also don't understand this very well, but Bob and others have
warned against big evaluation discontinuities in the past.  I'll probably
give it a try and see if I notice any side effects.

Tord



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