Computer Chess Club Archives




Subject: triangular pv vs. hash move pv

Author: Stuart Cracraft

Date: 21:08:20 09/10/04


I added keeping a triangular pv in main search and quiescence
to compare it with the output of my walk-the-hashtable-pv.

The two differ frequently but quite often are also mostly
identical all the way through.

Which should I trust? Seems like the hash table is getting
overwritten with other variations (not sure why). What
kind of scenario would cause that? My algorithm is
length >= depth to replace.

Seems like triangular would be better and then use that to
prefill the hash table from the last iteration before the
next iteration would be best, as Bob Hyatt mentioned.

In looking at both pv's, they look fine and lead to positions
that are often equal in material without any gross material errors that
could be attributed to a misimplementation but there are definite
differences between the two methods for me.

Before I throw out the walk-the-hashtablepv, I want to be sure
that it is really no good. So I want to know exactly how a hash
table can be damaged such that its PV is corrupted. Under precisely
what circumstances can this happen?

I would have thought length >= depth would have prevented it as long
as there are no collisions. My hashkey is 64-bits and I am searching
<1M nodes per move for my 1 second searches so I'd expect no collisons.
Should I have some hook in there to check the collisions aren't happening
and are causing damage to the pv or is there another hash table side
effect that results in that damage?

Thanks all,


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