Author: Ricardo Gibert
Date: 05:22:17 06/20/05
Go up one level in this thread
On June 20, 2005 at 05:31:17, Vasik Rajlich wrote:
...
>
>I don't understand why that would be the case. Can you explain?
To understand the reason, it is necessary to explain no progress pruning. Here
is an illustration of no progress pruning:
a - b - c - d - ... - e - f - g
\ \
h i
Let the letters a thru g represent successive moves in a line being analyzed. No
progress pruning says that if the position after move i instead of move f leads
to the same position as move instead d, then move i can be pruned from
consideration, because it could have been reached earlier.
Where this gets you in trouble is if move h turns out to be quite a good move.
The positions after moves from d to e will not reflect this fact, since move i
was pruned. The positions after moves from d to e will get stored in the HT with
the wrong score and if a subsequent line should get a HT hit that uses these
scores, the search will get fooled as to the true value of the positions after
the moves from d to e. This is not good.
One solution to this is to store the hash moves but not the hash scores for
positions dependent (d thru e) on a no progress prune. Another idea is to keep
the hash scores, but associate a reduced draft for the scores. Yet another idea
is to chuck no progress pruning as being more trouble than it is worth. The
latter idea is attractive, since it is not a big win.
>
>The interaction of no progress pruning with HT will cause you some problems
>because the pruning is path-dependent, while the positions in the HT are not.
>You'll store in the HT a position which was later pruned (actually reduced)
>because of lack of progress, and then you'll come to that same position via a
>different path where this pruning (actually reducing) shouldn't be done.
>
>The question not everybody agrees about is how of much of a problem this is.
>
>Vas
>
...
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