Author: Vasik Rajlich
Date: 03:01:02 09/01/04
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On September 01, 2004 at 05:28:55, Tord Romstad wrote: >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 Tord, do you have any statistics for how often this extension triggers? I'd guess that it's more of a positional extension than a tactical extension. Highly tactical positions should generally have varying refutations of a null move, while in a quieter position null moves could generally be refuted by the same quiet move. (Let's say something general like Nc3-d5.) Also - is this the same as Botvinnik's original formulation? I thought that null move was obscure back then. Vas
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