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GNU Chess is a computer program for playing chess. GNU Chess is one of the oldest computer chess programs for Unix-based computers and one of the earliest available with full source code. GNU Chess has been ported to many other platforms. The GNU Chess project is one of the older parts of the GNU package of software, having started in 1984 after its author read Richard Stallman's GNU Manifesto and having collaborated with Stallman, pre-GNU.
The first version of GNU Chess was written by Stuart Cracraft. Versions from 2 to 4 were written by John Stanback. Version 5 was written by Chua Kong-Sian. Dozens of developers have enhanced GNU Chess. On modern computer chess architecture, GNU Chess plays at master strength. More importantly, its structural style is simple enough to be readable. The goal of GNU Chess is to serve as a basis for research. GNU Chess has been used in numerous research contexts.
GNU Chess is free software, licensed under the terms of the GNU General Public License, and is maintained by collaborating developers. It is often used in conjunction with a GUI program such as XBoard. Initial versions of XBoard's Chess Engine Communication Protocol were based on GNU Chess's command line interface.
In 1998–1999 GNU Chess underwent a transition to version 5. Version 5 was essentially a complete rewrite from scratch of GNU Chess to eliminate spaghetti code and replace antiquated data structures with more advanced computer chess implementation techniques. These included bitboards, a search algorithm called Principal Variation Search (PVS) (a variation by Professor Tony Marsland of alpha-beta minimax), and full end-leaf evaluation.
GNU Chess uses a number of additional techniques to improve its performance, such as an opening book (generated by studying master games) to help it start well and hash tables storing previously-analyzed positions to prevent wasting time re-analyzing already analyzed positions.
GNU Chess has remained essentially unchanged for several years. Many other solid (and stronger) chess programs with full source code are now available, such as Crafty and many others.