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Rybka is a computer chess engine designed by International Master Vasik Rajlich. As of February 2011, Rybka is one of the top-rated engines on chess engine rating lists and has won many computer chess tournaments.
Although Rybka won four consecutive World Computer Chess Championships from 2007 to 2010, it was stripped of these titles after an International Computer Games Association panel concluded in June 2011 that Rybka plagiarized code from both the Crafty and the Fruit chess engines. The finding is disputed. On October 2013, Edward Schroder (Netherlands) published a two-part series on Chess Vibes, totally exonerating Rybka before the FIDE takes his ethics case before their commission in Tallinn (Estonia).
The word rybka, pronounced [ˈrɪpka], means little fish in Czech (and in many other Slavic languages) and Vasik Rajlich was once asked in an interview by Alexander Schmidt, "Did you choose the name Rybka because your program always slipped out of your hands like a little fish?" He replied, "As for the name Rybka – I am sorry but this will remain my private secret." It is interesting to point out that the programmer's wife, Iweta Rajlich (Radziewicz), was born under an astrological sign of Pisces. Iweta Radziewicz was born in Poland and is still considered one of the strongest female chess players. The word rybka means exactly the same in the Polish and Czech languages. It is also known that Iweta was the main tester for the program.
The fact that Fischer, fisher, translates into several Slavic languages as rybak should not go unnoticed.
Rybka 2.2n2 (which is available as a free download) has been claimed to be rated at 3110 Elo on four CPUs, and Rybka 3 is supposedly about 100 Elo stronger than 2.2n2.
On March 21, 2010, the latest SSDF rating list was released, with Deep Rybka 3 in first place in the rankings with a rating of 3227.
Though Rybka is no longer allowed to participate in computer tournaments, its rating as of the regularly updated IPON lists ranks Deep Rybka 4.1 as #5 in the computer chess world, with an ELO of 2,952.
Rybka is a closed-source program, but still some details have been revealed: Rybka uses a bitboard representation, and is an alpha-beta searcher with a relatively large aspiration window. It uses very aggressive pruning, leading to imbalanced search trees. The details of the evaluation function are unknown, but since version 2.3.1 it has included work by GM Larry Kaufman on material imbalances, much of which was worked out in a series of papers in the 1990s.
Several members of the Rybka team are strong chess players: Vasik Rajlich, the main author of Rybka is an International Master (IM). GM Larry Kaufman is the 2008 Senior Chess World Champion, and from version 2.3 through version 3 was in primary charge of the evaluation function. Iweta Rajlich, Vasik Rajlich's wife and the main Rybka tester is a Women's GM (WGM) and IM. Jeroen Noomen (who used to work on Rebel) and Dagh Nielsen were the authors of its opening book – the latter is one of the world's top freestyle chess players. Both are now less active, and Jiri Dufek is in charge of the book.
Vasik Rajlich started working on his chess program at the beginning of 2003. The first Rybka beta was released on December 2, 2005.
In January 2004, Rybka participated in the 6th Programmers Computer Chess Tournament (CCT6) event, placing 53rd out of 54 competers, losing 5 games, drawing 3, and beating the last-place finisher who had 0 points (Tohno). 
In April 2004, Rybka participated in Chess War V conducted by Olivier Deville, finishing 23rd in the D Division.
In April 2004, Rybka participated in the Swiss System Season 3 by Claude Dubois, scoring 6 wins, 6 losses and 6 draws in the Top 200 to finish 71st.
Rybka participated in Chess War VI finishing 42nd in the D Division. Rybka participated in Chess War VII finishing 48th in the C Division. Rybka participated in AEGT round 3, scoring 89 wins, 28 losses and 15 draws.
Note: it is now alleged that the above Rybka versions participated illegally by containing code from Crafty. See Crafty controversy below.
In December 2005, Rybka participated in the 15th International Paderborn Computer Chess Championship. Rybka won the tournament with a score of 5½ points out of 7, ahead of other engines such as Gandalf, Zappa, Spike, Shredder and Fruit.
On CCT8 in February 2006, Rybka won with a score of 8 out of 9, going undefeated. In the April 2006 PAL/CSS Freestyle main tournament, an unaided Rybka 1.1 took first place. In the final tournament, Rybka 1.1 finished in second and third place, behind Hydra. In the 6th Leiden ICT in May 2006, Rybka won with a score of 8½ out of 9, ahead of Sjeng, Gandalf and Shredder. At the 14th World Computer Chess Championship in Turin, Italy in May 2006, Rybka, playing under the name Rajlich, finished second, tied with Shredder, after Junior, the winning 2006 World champion. In the June 2006 PAL/CSS Freestyle main tournament, the Rybka team, playing under the handle Rajlich, tied for first place with Intagrand. In the final, the Rybka team took clear first place, a point ahead of the field. All 8 qualifiers for to the final were Rybka users. In the 2006 Dutch open computer chess championship, Rybka 2.2 finished in first place with a perfect score of 9 out of 9. In December 2006, Rybka participated in the 16th International Paderborn Computer Chess Championship. Rybka won the tournament with a score of 6½ points out of 7.
In February 2007, Rybka participated in the CCT9 and won with 6/7. In the 7th Leiden ICT in May 2007, Rybka won with a score of 7½ out of 9, ahead of Zappa and HIARCS. Rybka won the 15th World Computer Chess Championship in June 2007 with a score of 10 out of 11. The Rybka team, playing under the handle Rajlich, won the June 2007 PAL/CSS Freestyle final with a score of 6/9. Later that year it won again the Dutch open computer chess championship, scoring 8/9.
In January 2008, Rybka tied for first place in CCT10 with 5.5/7. In October 2008, Rybka won the 16th World Computer Chess Championship, held in Beijing, China, scoring 8/9. A month later Rybka won the 27th Open Dutch Computer Chess Championship, held in Leiden, scoring a perfect 9/9.
In March 2009, Rybka won CCT11 with 7.5/9 and the 17th World Computer Chess Championship, held in Pamplona, Spain, with a score of 8/9.
Odds matches versus grandmasters
- Rybka 4 was released May 26, 2010. Vasik Rajlich has given the following information at the Rybka forum:
- Rybka 4 is a normal UCI engine, without copy protection.
- There are separate single-processor and multi-processor versions.
- Full chess analysis packages which include Rybka 4 will be made by ChessBase (www.chessbase.com) and Convekta/ChessOK (www.chessok.com).
- Plain Rybka 4 UCI without GUI for download only is available from RybkaChess (www.rybkachess.com).
- All of these versions of Rybka 4 will be identical and can be used in any UCI-compliant GUI.
- Native Linux support was confirmed years ago without a specific date, but looks unlikely. Release plan for platforms other than Windows was expected to come after Rybka 3.0 is finished. Rybka does, however, run under Wine, and 64-bit Rybka runs under both Linux and Mac OS Xusing an experimental, Rybka-specific translation layer called microwine.
- Support for Pocket PCs and similar mobile devices were announced in 2007. But products for Pocket PCs and similar mobile devices were never released.
Crafty chess engine controversy
Fruit GPL chess engine controversy
WCCC disqualification and banning
Rejection of the ICGA decision by the CSVN
Those who were in favour of the sanctions were severely questioned by (e.g.) Miguel A. Ballicora, George Speight and Søren Riis. Their opposition did make an impression on us, because these people can rely upon a vast expertise in the field of chess programming, law and mathematical logic. When finally dutchman Ed Schrøder, former world computer chess champion, joined the aforementioned critics of ICGA, we no longer seemed to have a choice.
David Levy interview
You say that the reason Vasik Rajlich had been under attack for years is that "it had been known, by some people," that he was guilty. Many them have stood to profit from his downfall. Why was it necessary to nominate exactly those people to the jury panel that would pass judgement over him. It meant that the entire investigation and sentencing would in no way represent due process, but become an ad hoc procedure put together by the ICGA, one which one must assume could come to no other conclusion than the one it reached. Are you satisfied that justice was done?
... We did not nominate people to the panel. We asked for people who have an interest and relevant expertise to volunteer to join the panel. ... At most, three active competitors of Rajlich voted. None of those three panel members took an active role in the discussions so they did not influence the other members. Even if we had excluded the votes of these three, who make up fewer than one-fifth of the voting members, all the remaining voters agreed that Rajlich was guilty of violating the ICGA's Tournament Rule 2...