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A chess engine is a computer program that can play the game of chess.

Interface

Most chess engines do not have their own graphical user interface (GUI) but are rather console applications that communicate with a GUI such as XBoard (Linux) and WinBoard (Windows) via a standard protocol.

Protocols

The command line interface of GNU Chess became the initial de facto standard, called the Chess Engine Communication Protocol and first supported by XBoard. When XBoard was ported to the Windows operating system as WinBoard this protocol was popularly renamed to 'WinBoard Protocol'. The WinBoard Protocol was itself upgraded and the two versions of the protocols are referred to as: 'WinBoard Protocol 1' (original version) and 'WinBoard Protocol 2' (newer version). There is another protocol, the Universal Chess Interface. Some engines support both major protocols, and each protocol has its supporters. The Winboard Protocol is more popular but many chess engine developers feel that the Universal Chess Interface is easier to implement. Some interface programs, such as Arena, support both protocols whereas others, such as WinBoard, support only one and depend on subsidiary interpretors, such as Polyglot, to translate.

Increasing strength

Chess engines increase in playing strength each year. This is partly due to the increase in processing power that enables calculations to be made to ever greater depths in a given time. In addition, programming techniques have improved enabling the engines to be more selective in the lines that they analyse and to acquire a better positional understanding.

Some chess engines use endgame tablebases to increase their playing strength during the endgame. An endgame tablebase is a database of all possible endgame positions with small groups of material. Each position is conclusively determined as a win, loss, or draw for the player whose turn it is to move, and the number of moves to the end with best play by both sides. Endgame tablebases in all cases identify the absolute best move in all positions included (identifying the move that wins fastest against perfect defense, or the move that loses slowest against optimal opposition). Such tablebases are available for all positions containing three to six pieces (counting the kings) and for some seven-piece combinations. When the maneuvering in an ending to achieve an irreversible improvement takes more moves than the horizon of calculation of a chess engine, an engine is not guaranteed to find the best move without the use of an endgame tablebase, and in many cases can fall foul of the fifty-move rule as a result.

Comparison between engines

Tournaments

The results of computer tournaments give one view of the relative strengths of chess engines. However, tournaments do not play a statistically significant number of games for accurate strength determination. In fact, the number of games that need to be played between fairly evenly matched engines, in order to achieve significance, runs into the thousands and is, therefore, impractical within the framework of a tournament [1]. Most tournaments also allow any types of hardware, so only engine/hardware combinations are being compared.

Historically, commercial programs have been the strongest engines. The 2008 WCCC was won by Rybka. To some extent, this is a self-fulfilling prophecy; if an amateur engine wins a tournament or otherwise performs well (for example, Zappa in 2005), then it is quickly commercialized. Titles gained in these tournaments garner much prestige for the winning programs, and are thus used for marketing purposes.

• World Computer Chess Championship
  • World Computer Speed Chess Championship
• Dutch open computer chess championship
• Internet Computer Chess Tournament (CCT)
• International Paderborn Computer Chess Championship
• North American Computer Chess Championship

Chess engine rating lists

Chess engine rating lists aim to provide statistically significant measures of relative engine strength. These lists play multiple games between engines on standard hardware platforms, so that processor differences are factored out. Some also standardize the opening books, in an attempt to measure the strength differences of the engines only. These lists not only provide a ranking, but also margins of error on the given ratings. Also rating lists typically play games continuously, publishing many updates per year, compared to tournaments which only take place annually.

There are a number of factors that vary among the chess engine rating lists:

• Time control. Longer time controls, such as 40 moves in 120 minutes, are better suited for determining tournament play strength, but also make testing more time-consuming.
• Hardware used. Faster hardware with more memory leads to stronger play.
• 64-bit (vs. 32-bit) hardware and operating systems favor bitboard-based programs.
• Multiprocessor vs. single processor hardware.
• Ponder settings (speculative analysis while the opponent is thinking).
• Transposition table sizes.
• Opening book settings.

These differences affect the results, and make direct comparisons between rating lists difficult.

As of October 2009, the top engines in the different lists are:

• Note that the listings in the above table only count the best entry for a given engine.

These ratings, although calculated by using the Elo system (or similar rating methods), have no direct relation to FIDE Elo ratings or to other chess federation ratings of human players. Except for some man versus machine games which the SSDF had organized many years ago (which were far from today's level), there is no calibration between any of these rating lists and player pools. Hence, the results which matter are the ranks and the differences between the ratings, not the absolute level of the numbers. Nevertheless, in view of recent man versus machine matches, it is generally undisputed that the top computer chess ratings should be at least in the range of top human performances, and probably significantly higher.

Test suites

Engines can be tested by measuring their performance on specific positions. Typical is the use of test suites, where for each given position there is one best move to find. These positions can be geared towards positional, tactical or endgame play. The Nolot test suite, for instance, focuses on deep sacrifices. ^[8] Then there are the BT2450 and BT2630 test suites by Hubert Bednorz and Fred Toennissen. These suites measure the tactical capability of the engine^[9] and have been used at least by REBEL.^[10] There is also a general test suite called Brilliancy by Dana Turnmire. The suite has been compiled mostly from How to Reassess Your Chess Workbook.^[11]

Engine categories

Ikarus and Ferret, both private engines, were past winners of the World Computer Speed Chess Championship.

Freely available chess engines

There are hundreds of freely available chess engines which conform to one of the above communication protocols. Many run on Windows or are open source. The top 50 strongest are listed here. Others may be found by examining the rating lists or external links.

Elo ratings taken from CCRL 40/40, computed on 24 July 2009.

Pedagogical engines

These open source chess programs were expressly written to teach the craft of chess programming.

• ChessBin.com C# Chess Engine.
• Chess Program in C#.
• FirstChess (in C).
• Gerbil
• Gray Matter (in C++).
• MSCP (in C)
• Mizar (in C).
• Viper (in C++).
• Xadreco Chess Engine (in C).
• ChEngine (in C#).
• Micro-Max (chess engine), a 133-line Chess Source by H.G.Muller (in C).

There are also copyrighted programs with available source:

• TSCP (in C).

Commercial programs

These chess programs are sold commercially. Most of these also include their own user interface.

• Chess Genius, by Richard Lang of Mephisto fame
• Chessmaster
• Chess Tiger
• Fritz (single processor), Deep Fritz (multi processor)
• Gandalf
• HIARCS
• Junior - (single- and multi-processor versions)
• The King - the engine of the commercial Chessmaster program
• Ktulu
• Loop (also the engine for Wii Chess)
• Naum versions 2.1 and later
• Onno
• Rebel - (see also ProDeo)
• Ruffian 2
• Rybka
• Shredder
• Deep Sjeng
• Smarthink
• Zappa

Dedicated hardware

These chess playing systems include custom hardware or run on supercomputers. All are historical; chess supercomputers have not competed in computer tournaments since Hydra played in 2006.

• Bebe, a strong bit-slice processor in the 1980s
• Belle
• Chess (Northwestern University)
• ChipTest
• Cray Blitz
• Deep Blue
• Deep Thought
• HiTech
• Hydra, predecessor was called Brutus

Commercial dedicated computers

In the 1980s and early 1990s, there was a competitive market for strong dedicated chess computers. Many form-factors were sold, from handheld peg-board computers to wooden auto-sensory boards with state-of-the-art processors. This market changed in the mid-90s when the economical embedded processors in dedicated chess computers could no longer compete with the fast processors in personal computers. Nowadays, most dedicated units sold are of beginner and intermediate strength.

• Chess Challenger, a line of chess computers sold by Fidelity Electronics from 1977[2] to 1992. These models won the first four World Microcomputer Chess Championships.
• ChessMachine, an ARM-based dedicated computer, which could run two engines:
  • The King, which later became the Chessmaster engine, was also used in the TASC R30 dedicated computer
  • Gideon, a version of Rebel, in 1992 became the first microcomputer to win the World Computer Chess Championship
• Mephisto, a line of chess computers sold by Hegener & Glaser. The units programmed by Richard Lang won six consecutive World Microcomputer Chess Championships. They bought out Fidelity in 1989.
• Novag sells a line of tactically strong computers, including the Constellation, Sapphire, and Star Diamond brands.
• Saitek sells mid-range units of intermediate strength. They bought out Hegener & Glaser and its Mephisto brand in 1994.
• Excalibur sells a line of intermediate strength units. Excalibur was started in 1992 by the son of the founder of Fidelity Electronics.
• Phoenix Chess Systems makes limited edition units based around StrongARM and XScale processors running modern engines

Historical programs

These chess programs run on obsolete hardware.

• 1K ZX Chess
• Kaissa
• Kotok-McCarthy
• Mac Hack
• Microchess was the first commercial game for a personal computer, developed first for the KIM-1 and later Commodore PET, Apple II, TRS-80 and others. Bobby Fischer played against MicroChess. [3]
• Sargon
• Socrates II
• Distant Armies, an Amiga program that played several versions of chess, including Chinese chess, several variants played on a circular board, and a large number of ancient variants. It did not play modern western chess; neither did it play Japanese shogi.

See also

• Chess Engines Grand Tournament
• Computer chess
• Internet chess server
• Chess Engine Communication Protocol
• Universal Chess Interface

Notes and references

External links

• Tim Mann's Chess Pages
• Top UCI Chess Engines with ratings and download links
• WB Chronology - another extensive list of free engines and links
• WBEC Ridderkerk - collection of free chess engine properties, latest updates, tournaments and chess programming links
• Free UCI Chess engines
• Chess Engine Rankings on the PlayChess server
• ChessComputers.org - site dedicated to electronic chess machines or dedicated chess computers
• Chess Engine's Opening Book for Arena 2.x.x GUI. - A general (learning) purpose Chess Engine's Book for Arena 2.x.x GUI.
• Chess Engine's Polyglot Opening Book for WinBoard GUI. - A general (learning) purpose Chess Engine's Polyglot Opening Book for WinBoard GUI.