Author: Dann Corbit
Date: 18:20:00 01/27/04
Go up one level in this thread
On January 27, 2004 at 20:50:25, Russell Reagan wrote:
>On January 27, 2004 at 15:42:27, Dann Corbit wrote:
>
>>>> /* ----------==== INTERNAL ITERATIVE DEEPENING ====----------- */
>>>>
>>>> /* If we're not doing a NULL move and we don't have a hash move and we're
>>>> * at least 3 ply away from the quiescence search, then try to get a good
>>>> * guess for the best move by doing a shallower search from here. */
>>>> if (depth >= 3 && !do_null) {
>>>> w = search(on_move, ply, depth-2, alpha, beta, do_null, 1);
>>>> /* Re-search properly if the previous search failed low, so that we
>>>> * know we're getting a good move, not just the move with the highest
>>>> * upper bound (which is essentially random and depends on the search
>>>> * order.) */
>>>> if (w <= alpha)
>>>> w = search(on_move, ply, depth-2, -32500, alpha+1, do_null, 1);
>>>> }
>
>>Because it is used for move ordering, and because the depth is 2 plies below the
>>current search depth, it is a sure win. The cost is 1/400th of a full search,
>>and the win is a good guess for the pv node.
>>
>>At least when I tested it in Beowulf it worked well. But I did not try it both
>>ways in Olithink.
>
>Hi Dann,
>
>I have a few questions about this code.
>
>The first IID search that you are doing (with alpha and beta as bounds) is
>considered an aspiration search, correct?
>
>If that is correct, then I have another question. I thought that if an
>aspiration search failed, then you couldn't reliably use the score that was
>returned as you are doing by re-searching with (-32500, alpha+1). Bruce writes
>about this problem on his webpage (scroll down to the part about search
>instability in aspiration searches). The code you posted looks equivalent to the
>code that he says will cause bad search instability.
>
>http://www.brucemo.com/compchess/programming/aspiration.htm
It's not the same thing. The aspiration search is in a loop. This only checks
the root at two plies less than the main search.
The entire purpose is to find a pretty good choice for the pv node if we don't
have a guess yet.
In other words:
No data in the hash table? Then compute a shallower answer.
It is two plies shallower than the main search. Even if your branching factor
is only 2, it is still 1/4 of the effort of a full search depth. If your
branching facter were 6, it would be 1/36 of the effort. And chances are good
we have already examined some side branches in the sub-tree, since we are
already deeper than the depth we are requesting.
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