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Subject: Re: how large should a pawn hashtable be?

Author: Tord Romstad

Date: 02:32:02 02/12/04

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On February 12, 2004 at 04:40:34, martin fierz wrote:

>On February 12, 2004 at 04:23:54, Tord Romstad wrote:
>
>>My entries are much bigger; currently 128 bytes.  I store all of the above,
>>as well as information like total number of pawns on black/white
>>squares for each side, total number of blocked pawns on black/white squares,
>>weak squares, open files, pawn storm stuff for use in positions with
>>castling to opposite sides, and a classification of the central pawn
>>structure.
>
>yes, obviously one can store lots of stuff. i also have some code on most of
>your topics above in my engine, but i forgot when i wrote my post :-)
>i can think of many other things too.

The problem, at least for a weak chess player like me, is to find good
ways to use all the information.  When I implemented the pawn hash table,
I had ambitious plans of writing special search and eval code for each
of the most common central pawn structures.  For instance, when the
pawn structure is classified as FRENCH, the engine would know something
about the most important thematic moves in the French defence, and
extend (or perhaps just avoid to prune or reduce) these moves.  Evaluation
terms specific to one particular central pawn structure could also be
added (rules like "in the SICILIAN_PELIKAN pawn structure, white wants
to exchange the minor pieces, especially black's light-squared bishop").

I still haven't written any such code, however.  The classification of
the central pawn structure is currently not used at all, except that
I evaluate differently depending on whether the centre is open or closed.
Before I can add something more sophisticated I need to improve my own
chess knowledge.

> and thanks to your information...
>
>>In my opinion, there is no reason to be afraid of having big pawn hash
>>table entries, because you need very few of them.  The pawn structure
>>doesn't change that often.  I've found that 256 entries is enough in
>>my engine.  Making the table bigger gives only a tiny speedup.
>
>... that even 256 entries are enough, i won't mind putting all that stuff in!
>thanks for sharing.

You should note that I don't hash the kings, though.  If you do, you will
certainly need a much bigger table.

Tord



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