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a chess engine by [[Robert Houdart]], which appeared in 2010 as closed source engine, free for non-commercial use. In January 2011, Houdini '''1.5''' was leading [[Ingo Bauer|Ingo Bauer's]] [[IPON]] [[Engine Rating Lists|rating list]] <ref>[http://www.inwoba.de/News2009_2011.7z IPON-Rating-List Archive 2009/2010/2011] by [[Ingo Bauer]]</ref> . Houdini '''2''', released in September 2011, is commercial and bundled with a number of [[ChessOK]] products, such as [[Aquarium]], [[Chess Assistant]] <ref>[http://chessok.com/?p=25808 Houdini 2 Aquarium products family has been released!], [[ChessOK]], October 07, 2011</ref>, and since December 2011, [[Chess King]] <ref>[http://chessok.com/?p=26441 Chess King Released], [[ChessOK]], December 14, 2011</ref>. Houdini 1.5a remains available as a free download <ref>[http://www.talkchess.com/forum/viewtopic.php?topic_view=threads&p=421920&t=40245 Re: Houdini 2 is released] by [[Robert Houdart]], [[CCC]], September 02, 2011</ref>. Houdini '''3''', released in October 2012, has further improved by more than 50 Elo points <ref>[http://www.inwoba.de/index.html IPON-Rating-List, 2012.10.17] by [[Ingo Bauer]]</ref> . According to its author, Houdini 3 applies a accelerated [[Principal Variation Search|principal variation search]], also dubbed "Smart" [[Fail-High]]. If a different move becomes [[Best Move|best]] at very high [[Depth|search depths]] at the [[Root|root]], re-searches to determine its [[Exact Score|exact score]] and the new [[Principal Variation|principal variation]] are done faster due to a reduced search depth, presumably in conjunction with [[Extensions|extensions]] along the PV <ref>[http://www.talkchess.com/forum/viewtopic.php?topic_view=threads&p=488422&t=45624 Re: Houdini 3 reducing the depth feature] by [[Robert Houdart]], [[CCC]], October 19, 2012</ref> . Since November 2012, Houdini 3 is available as [[ChessBase]] engine <ref>[http://chessbase-shop.com/en/products/houdini_3_standard_multiprocessor_version Houdini 3 Standard multiprocessor version], [[ChessBase|ChessBase Shop]]</ref> . Houdini '''4''', released in November 2013, was about 50 Elo stronger than its predecessor, and features 6-men [[Syzygy Bases|Syzygy bases]].

Robert Houdart claims his engine is original <ref>[http://www.open-chess.org/viewtopic.php?f=3&t=534&start=20#p4798 Re: Why Is Houdini Not Entered In The WCCC??] by [[Robert Houdart]], [[Computer Chess Forums|Open Chess Forum]], August 04, 2010</ref>, and admits taking ideas from the open source programs [[Ippolit|Ippolit]]/[[RobboLito|Robbolito]], [[Stockfish]] and [[Crafty]] <ref>[http://www.cruxis.com/chess/houdini.htm Houdini Chess Engine - Acknowledgements]</ref> . Other programmers and forum members raised suspicions that Robert took ideas from the controversial Ippolit/Robbolito source code too literally as initial base of his program <ref>[http://www.open-chess.org/viewtopic.php?f=3&t=534&start=40#p5084 Re: Why Is Houdini Not Entered In The WCCC??] by [[Franklin Titus|kingliveson]], [[Computer Chess Forums|Open Chess Forum]], August 09, 2010</ref> <ref>[http://www.open-chess.org/viewtopic.php?f=3&t=534&start=40#p5089 Re: Why Is Houdini Not Entered In The WCCC??] by [[Robert Houdart]], [[Computer Chess Forums|Open Chess Forum]], August 10, 2010</ref> <ref>[http://www.open-chess.org/viewtopic.php?f=3&t=1647&start=10#p14724 Re: Computer Chess Biggest Liar] by [[Franklin Titus|kingliveson]], [[Computer Chess Forums|OpenChess Forum]], October 11, 2011</ref> <ref>[http://rybkaforum.net/cgi-bin/rybkaforum/topic_show.pl?pid=427014#pid427014 Re: Interesting poll] by [[Ed Schroder|Rebel]], [[Computer Chess Forums|Rybka Forum]], July 15, 2012</ref> .

* [[Diogo R. Ferreira]] ('''2013'''). ''The Impact of the Search Depth on Chess Playing Strength''. [[ICGA Journal#36_2|ICGA Journal, Vol. 36, No. 2]]» [[Depth]], [[Depth#DiminishingReturns|Diminishing Returns]], [[Match Statistics]], [[Playing Strength]] <ref>[https://www.hiarcs.net/forums/viewtopic.php?t=10004 Ply versus ELO] by Greg, [[Computer Chess Forums|HIARCS Forum]], May 30, 2020 » [[Diogo R. Ferreira#Impact|Diogo R. Ferreira - Impact of the Search Depth ...]]</ref>

* [http://www.talkchess.com/forum3/viewtopic.php?f=2&t=65783 Whatever happened to Houdini for Android?] by AdminX[[Ted Summers]], [[CCC]], November 21, 2017

* [http://www.talkchess.com/forum3/viewtopic.php?f=2&t=67097 Houdini 6 - Initial position until depth 50] by [[Andreas Strangmüller]], [[CCC]], April 13, 2018 »[[Initial Position]]==2020 ...==* [http://www.talkchess.com/forum3/viewtopic.php?f=10&t=73433 Houdini is a Stockfish Derivative (and started life as a Robbolito derivative)] by cucumber, [[CCC]] (Engine Origins requires registration), March 22, 2020* [https://groups.google.com/d/msg/fishcooking/DygaIdBvJm0/cjtMosvdBQAJ Houdini is a clone of Stockfish 8] by c9publice...@gmail.com, [[Computer Chess Forums|FishCooking]], March 26, 2020* [http://www.talkchess.com/forum3/viewtopic.php?f=2&t=73630 An Old Engine] by [[Michael Byrne|MikeB]], [[CCC]], April 11, 2020* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=75040&start=44 Re: Is cloning a hobby?] by [[Andrew Grant]], [[CCC]], September 17, 2020