Computer Chess Club Archives




Subject: Re: not using nullmove? [generalized null move]

Author: Robert Hyatt

Date: 07:41:15 02/14/04

Go up one level in this thread

On February 14, 2004 at 01:05:46, Dann Corbit wrote:

>I think the interesting thing about null move is that it is easy to figure out
>if a move is really bad -- which is the main reason why it causes a benefit.  A
>tempo advantage is easily calculated by just switching sides.
>But the idea can be generalized nicely.
>Suppose that you have a seven ply search performed and it is time to do an 8 ply
>search.  Suppose (further) that from the root node only you never analyze the
>root itself but instead analyze all of the child nodes of the root and save the
>Now, you have some sort of estimate for the "goodness" of each move. Things
>might get reversed on the next ply, but it isn't terribly likely.  After all, we
>look at crappy moves and search them less deeply with the null move heuristic.
>Now, suppose that we have 10 moves available:
>move 0 has an evaluation of +100 centipawns
>move 1 has an evaluation of +80 centipawns
>move 2 has an evaluation of +77 centipawns
>move 3 has an evaluation of +60 centipawns
>move 4 has an evaluation of +53 centipawns
>move 5 has an evaluation of +19 centipawns
>move 6 has an evaluation of +5 centipawns
>move 7 has an evaluation of -10 centipawns
>move 8 has an evaluation of -110 centipawns
>move 9 has an evaluation of -910 centipawns
>Will we (knowing this) want to search them all the same?
>Why not use a logarithmic scale based on the difference between the best
>possible move and the move under consideration?
>Another advantage:
>Zugzwang sorts itself out.
>We can also add refinements like increasing the shear with mobility increase
>(greater reductions if I have 90 choices than if I have 12).
>Using estimates from a database also works nicely.  Unfortunately, hash table
>estimates have not worked out well for me (for attempts using other than the

There is one other major flaw.  You just made the search _many_
 times slower.


The math:

Doing an 8 ply search.  Assume a branching factor of 38.

the 8 ply search will search

N1 = 38^4 + 38^4 + 1 leaf nodes.

the 38 x 7 ply search will search

N2 = 38 * (38^4 + 38^3 + 1) nodes

N1 = 4,170,273

N2 = 81,320,342

N2 / N1 = 19.5 times slower.  Not a good thing.

This is the penalty you pay if you want the ply=2 scores.  Alpha/beta only gives
the best move / ply=1 score...  Additional information has huge additional cost.

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