Fruit is a chess engine developed by Fabien Letouzey. In the SSDF rating list released on November 24, 2006, Fruit version 2.2.1 had a rating of 2842. In the CEGT rating list released on January 24, 2007, Fruit version 2.2.1 had a rating of 2776.
Until Version 2.1 (Peach), Fruit was an open source engine. The source of the version 2.1 is still open (under GNU GPL) and contributed much to the development in computer chess in recent years. Some people still work on the old source code and have created variations from the original Fruit.
Technical details of Fruit 2.1
Fruit uses the classical Negascout (principal variation search) algorithm with iterative deepening to traverse the game tree. It also uses the null-move heuristic. The original version used a simplistic evaluation function with a robust search. Later versions have improved evaluation functions. The board representation is distinct — Fruit uses a 16x16 board.
Although Fabien Letouzey's development of Fruit stopped in 2007 with version 2.3.1, the earlier open source 2.1 version provided the basis for many other programs.
Toga II is a derivative created by Thomas Gaksch. It has more chess knowledge, multi-processor support, and perhaps a better search algorithm. It is based on Fruit 2.1 and is free. The strongest version is Toga II 3.0, released on 23 March 2013 by Jerry Donald. Experimental versions of Toga II running on computer clusters have competed in the World Computer Chess Championship (WCCC).
GambitFruit is another free derivative of Fruit 2.1, created by Ryan Benitez. It plays a more aggressive style and has more chess knowledge. GambitFruit also incorporates improvements from Toga II. Development of GambitFruit stopped in 2005.
In June 2011, a lengthy investigation by the International Computer Games Association (ICGA) determined Rybka was plagiarized from Fruit and Crafty. The author of Rybka, Vasik Rajlich, refused to address the allegations against Rybka with the ICGA, instead preferring a ex post facto public interview conducted by Nelson Hernandez on July 4, 2011. Rajlich had previously said: I went through the Fruit 2.1 source code forwards and backwards and took many things. 
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (April 2009)|
- Fruit Chess Engine by Fabien Letouzey
- Doggers, Peter. "Rybka disqualified and banned from World Computer Chess Championships". Chess Vibes. Retrieved 29 June 2011.
- Hernandez, Nelson. "Another interview with Vasik Rajlich".
- "Interview with Vasik Rajlich (Dec 2005)".