Author: Tony Werten
Date: 11:01:24 12/28/02
Go up one level in this thread
On December 28, 2002 at 12:12:01, Russell Reagan wrote:
>I am a little confused about how to determine when to re-search after you have
>tried an aspiration search. I was looking at Bruce's PVS example on his website,
>and it seems counter-intuitive (not saying it's wrong, just confusing to me).
>
>if (fFoundPv) {
> val = -AlphaBeta(depth - 1, -alpha - 1, -alpha);
> if ((val > alpha) && (val < beta)) // Check for failure.
> val = -AlphaBeta(depth - 1, -beta, -alpha);
> } else
> val = -AlphaBeta(depth - 1, -beta, -alpha);
>}
>
>What this says to me is that if the aspiration search returns a score that lies
>within the bounds of (alpha,beta), the asipration search failed, and we need to
>re-search. As I understand it, you only need to re-search when you discover that
>the node is not a PV node (as you assumed). If the score is within the bounds of
>(alpha,beta), doesn't mean that it's a PV node? So why the re-search? It seems
>more natural to me that you would research if the aspiration search returned a
>score less than alpha (which means we have an alpha node, and need to re-search)
>or a score greater than beta (which means we have a beta node, and we're done).
>
>Where is the mistake in my thinking?
Youre mixing up things. There are 2 kinds of researching.
1) After a fail high or fail low ( that is: at the root ), happens at a non pv.
2) "Internal", at a pv node.
1) Is when the rootmove has score x, and you assume the rest is not better. The
assumption was wrong and another move appears to be better.
2) Happens when you assume the root move (or pv) has a score between x and y.
In this pvline there are more moves at every ply then only pv moves. You assume
that these moves are less then pvscore so you only check for that (being worse).
If a move is better than score it means it is somewhere between x and y (
probably between score and y ) and you have to check if this is true.
Tony
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