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Subject: Detecting Draws using a Small Hash Table?

Author: Steve Maughan

Date: 05:25:54 02/20/02


The draw detection in Monarch uses a small hash table (65535 entries) to store
the path since a pawn move or capture.  When the search finds the current
position in the table it returns a draw score; if it's not found it store the
hash signiture carries on searching all the child positions and when complete
resets the hash entry to zero on exit.  This type of approach is outlined on
Bruce Moreland's excellent web page.  It seems to work well and detects draw
quite nicely.  However, I have been sent a game where Monarch allows a
repetition when it is clearly winning.  Having looked at the position, Monarch
sees the draw then 'loses' it.  Clearly this is unacceptable.  The problem crops
up when the draw is based upon positions already played in the game.  Monarch
usually sees the draw but since (many) other positions are also searched there
is a high likelihood that these historic board positions will be overwritten
with other search positions - hence Monarch loses the draw.

Has anyone else had the same problem?  Any obvious solutions?

My current thoughts are to only store the position in the table if the hash
entry is currently blank; and to only set it to blank if the stored hash is the
same as the current position.  This still allows for the possibility of not
detecting draws deeper in the tree but I guess the risk is greatly reduced.

Any thoughts appreciated!

Regards,

Steve Maughan

PS I used 0.5 Mb for this table any it doesn't seem to matter if I increase this
to 4 MB.  Is this sufficient?



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