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Subject: Re: Zero-width Window Null Move Search

Author: Stuart Cracraft

Date: 08:30:01 06/20/98

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>
>1.  This shoule *not* be a compute-intensive task.  If you aren't yet hashing
>your pawn structure scoring, you should, because whether a pawn is passed or not
>is a static piece of information that is independent of other pieces.  If you
>hash this, you will find that you find over 99% of your pawn structure
>evaluations are not needed because after you compute them one time, you will not
>re-compute this again, just use the hashed value.
>

While what you say is true about the 99%/1%, passed pawns that are
carefully calculated are not hashable. The reason is that a good calculation
of a hashed pawn includes who controls its queening square and
who controls the square directly ahead of it (facillitating its advance)
as well as other attacks along its path as well as blockading enemy pieces
on that path that would impede its progress. These extra items involve
calculations related to non-pawn pieces and therefore may be in
different positions where the passed pawn is on the same square. Hence,
unhashable. If you take the trivial case of just hashing the fact that it is
a passed pawn,  then yes, that is hashable. But not the full case which
is what you really need.

>2.  finding that a pawn is passed is trivial.  You could easily add a 64 bit
>word that contains a 1 for each white pawn, and then do the same for black.  If
>you update this after each move, asking "is a pawn on "SQ" passed" takes one
>64 bit operation, anding the opponent's pawns with a mask that shows where he
>must have pawns in order for your pawn on SQ to *not* be passed.

I used to carry a 64-bit quantity called "passed" in my hash struct. I recorded
all the passed pawns for both sides in it. However, as my passed pawn
handling got more advanced and involved more issues surrounding control
of squares on the passed pawns path, I discarded the passed in my
hash struct and have to calculate the passed pawns on every positional
evaluation. Since the other piece's locations and attacks can change,
I couldn't think of an alternative.

Anyway, I'm happy to say that my passed pawn handling is pretty good now.
I take the maximum case: a passed pawn that can definitely queen and
take fractions of that for all other cases based on how many detracting
factors are present (eg. enemy king not in square of pawn and enemy
has no pieces, control queening square, control squares on path,
control next square, blockading squares, etc.) All of these, if controlled
and blockaded by the enemy reduce my passed pawn's value substantially.
But if there are very few or none of these present, the bonus goes way up
and can amount to multiple pawns worth of positional bonus.

--Stuart



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