Author: Tord Romstad
Date: 02:28:55 09/01/04
Go up one level in this thread
On September 01, 2004 at 05:00:32, Chris Welty wrote:
>What is a Botvinnik-Markoff extension?
An extension which is triggered every time the null move fails for the same
reason twice in a row. Whenever the null move fails low (you may want to
do it only when it fails low by some margin), you set ThreatMove[Ply] to
the move that refuted the null move. The code for the actual extension
is something similar to this (should be located directly after the null
move search in your code):
if((null move failed low) &&
(ThreatMove[Ply]==ThreatMove[Ply-2] ||
(ThreatMove[Ply] and ThreatMove[Ply-2] captures the same piece)))
Extend
The idea is to discover horizon effect problems more quickly. WAC141 is
a great example of how the extension works. After 1. Qxf4 Bxf4 2. Rxh5,
the null move is refuted by 3. Rh8#. After the continuation 3... gxh5
4. Rxh5, the null move is again refuted by 4. Rh8#. Because the threat
move was the same at both nodes, the Botvinnik-Markoff extension is
triggered.
I add the BM extension on top of all other extensions. This is the only
case where I occasinally extend more than a full ply. In the position
after 1. Qxf4 Bxf4 2. Rxh5 gxh5 3. Rxh5, the mate threat extension and
the BM extension is triggered simultaneously, and I extend by 3/4+1/2
plies.
>Is there a limit to the amount of mate threat extensions and BM extensions that
>you do?
No. Many programs try to limit the extensions by dividing them by two
when Ply >= 2*IterationNumber, or something similar. I have tried schemes
like this many times, but it never had any positive effect for me. The only
limit I use is that I almost never extend by more than 1 ply at a single
node (the only exception is when the BM extension kicks in in addition to
some other extension, as explained above).
Tord
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