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.
At the World Computer Chess Championship in Reykjavík in 2005, Fruit 2.2 scored 8.5 out of 11, finishing in second place behind Zappa.
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.
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Fruit uses the classical Negascout (PVS) 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.
As of July 23, 2007, Fruit became freeware. The latest version Fruit 2.3 and Fruit 2.3.1 are free to download on superchessengine.com. Fruit 2.3.1 was one of the top 3 free UCI chess engines.
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.[1] It is based on Fruit 2.1 and is free. The strongest versions are 1.4 beta5c and 1.4.1SE. Experimental versions of Toga II running on computer clusters have competed in the World Computer Chess Championship (WCCC).
In 2008, forks of Toga II started to appear, like Grapefruit and Cyclone.
GambitFruit is another free derivative of Fruit 2.1, created by Ryan Benitez. It plays a more aggressive style and has more chess knowledge.[1] GambitFruit also incorporates improvements from Toga II.[1] Development of GambitFruit stopped in 2005.
In June 2011, the strong chess engine Rybka was disqualified from the WCCC, after a lengthy investigation found evidence of plagiarism of Fruit and Crafty.[2] The author of Rybka has since declined to comment.
An up-to-date ranking of Toga II and Fruit derivatives is found at the CCRL.