Check+Extensions

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 * Check Extensions** have two distinct forms: one of them extends when giving check, the other - when //evading// it. In each case, typical depth to extend is **one** ply. The reason behind check extension is that we are in a forcing sequence, so that it is desirable to know its outcome with more certainty, and the number of replies to check is limited, so we do not have to be afraid of a search explosion. Also, not extending checks may easily lead to the horizon effect, delaying the threat so far that the program cannot see it. However, some programmers don't extend checks (and captures) with negative SEE or even reduce them. Robert Hyatt claimed a significant gain in Crafty by doing so.

If a program does not consider checks in the quiescence search, then we should take care that it does not enter it while in check. This is also a form of check extension. In its most straightforward form, check extension is implemented in TSCP.

=See also=
 * Checks in Quiescence Search
 * One Reply Extensions
 * Singular Extensions

=Forum Posts=

2000 ...

 * [|Check Extension] by Martin Bauer, CCC, January 03, 2003
 * [|What's best low BF or good WAC result?] by Albert Bertilsson, CCC, March 18, 2003 » Win at Chess
 * [|Buggy check extensions, back to square one =(.] by Albert Bertilsson, CCC, March 20, 2003
 * [|check extention explosion] by Aivaras Juzvikas, CCC, April 03, 2004 » Search Explosion
 * [|Checks in QSearch] by Dan Honeycutt, Winboard Programming Forum, November 23, 2004 » Checks in Quiescence Search

2005 ...

 * [|hashing, check extensions and depth] by Mike Adams, CCC, May 27, 2007
 * [|Revisiting Check Extensions] by Mark Lefler, CCC, June 07, 2007
 * [|checks in q-search] by Robert Hyatt, CCC, September 02, 2008
 * [|check extension] by Andrew Short, CCC, August 07, 2008
 * [|check extensions] by Robert Hyatt, CCC, October 28, 2008
 * [|Check extension] by Harm Geert Muller, CCC, April 09, 2009

2010 ...

 * [|Problem with exploding tree because of extensions] by Oliver Brausch, CCC, January 05, 2010
 * [|Problems when implementing checks in qsearch] by Luca Hemmerich, CCC, February 03, 2010 » Checks in Quiescence Search
 * [|Spite checks] by Harm Geert Muller, CCC, May 13, 2014
 * [|Pointless delays] by Harm Geert Muller, CCC, July 25, 2014

2015 ...

 * [|Check-extension in QS] by Harm Geert Muller, CCC, April 03, 2015 » Checks in Quiescence Search
 * [|Is a Check Extension Really a Win?] by Steve Maughan, CCC, May 15, 2015
 * [|Spite checks, again] by Harm Geert Muller, CCC, October 23, 2016
 * [|Starting with check extensions] by Luis Babboni, CCC, February 28, 2017
 * [|Check Extensions] by David Cimbalista, CCC, March 02, 2017
 * [|Check extension vs LMR] by Harm Geert Muller, CCC, April 04, 2017 » LMR
 * [|Depth extensions and effect on transposition queries] by Kenneth Jones, CCC, April 16, 2018 » Transposition Table

=External Links=
 * Extension Techniques in REBEL (checks) from [|Programmer Corner] by Ed Schröder
 * [|Check extension] from Bruce Moreland's [|Programming Topics]

=References= =What links here?= include page="Check Extensions" component="backlinks" limit="180"
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