A chess engine is a computer program that can play the game of chess.
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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), WinBoard (Windows) or Arena (Windows) via a standard protocol.
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.
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.
Many engines use permanent brain as a method to increase their strength.
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 [2]. Most tournaments also allow any types of hardware, so only engine/hardware combinations are being compared.
Historically, commercial programs have been the strongest engines. 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.
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:
These differences affect the results, and make direct comparisons between rating lists difficult.
| Rating list | Time control (moves/minutes) |
Year started |
Last updated | Engine/platform entries |
Games played |
Top three engines | Rating |
|---|---|---|---|---|---|---|---|
| CCRL[1] | 40/40[2] Ponder OFF |
2005 | November 30, 2011 | 234 | 363,201 | Houdini 2.0 x64 4CPU Rybka 4.1 x64 4CPU Critter 1.2 x64 4CPU |
3335 3264 3259 |
| CEGT[3] | 40/20[4] Ponder OFF |
2006 | December 4, 2011 | 980 | 553,886 | Houdini 1.5a x64 6CPU Deep Rybka 4.1 x64 4CPU Critter 1.2 x64 4CPU |
3290 3242 3234 |
| IPON[5] | 5m+3s ~16min/game Ponder ON |
2006 | December 30, 2011 | 95 | 161,200 | Houdini 2.0 STD SSE4.2 x64 1CPU Critter 1.4 SSE4.2 x64 1CPU Komodo 4 SSE4.2 x64 1CPU |
3016 2977 2975 |
| SWCR[6] | 40/10 Ponder ON |
2009 | December 1, 2011 | 66 | 150,247 | Houdini 2.0c x64 1CPU Rybka 4 x64 Exp. 42 1CPU Komodo 3.0 x64 1CPU |
3022 2967 2962 |
| SSDF[7] | 40/120 Ponder ON |
1984 | May 11, 2011 | 311 | 120,010 | Deep Rybka 4 x64 4CPU Naum 4.2 x64 4CPU Deep Shredder 12 x64 4CPU |
3216 3155 3115 |
| WBEC[8] | 40/40 Ponder ON |
2001 | May 15, 2011 | 226 (Historically: 850+[9]) |
100,749 | Rybka 4 x64 2CPU Stockfish 2.0.1 x64 2CPU Thinker 5.5.4A1 x64 2CPU |
3124 3121 3114 |
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. Also, each list calibrates their Elo via a different method. Therefore no Elo comparisons can be made between the lists. Nevertheless, in view of recent man versus machine matches, it is generally undisputed that top computer chess engines should be rated at least in the range of top human performances, and probably significantly higher.
Missing from many rating lists are IPPOLIT and its derivatives (e.g. Fire [3]). Although very strong and open source, there are allegations from commercial software interests that they were derived from disassembled binary of Rybka.[10] Due to the controversy, all these engines have been blacklisted from many tournaments and rating lists. Although Rybka has been accused of being based on Fruit, it is not blacklisted from computer chess tournaments or rating lists.[11] In June 2011, Rybka was found guilty of being derived from Fruit and Crafty and Rybka has been banned from the ICGA (International Computer Games Association) World Computer Chess Championship, and its previous victories (2007, 2008, 2009, and 2010) has been revoked.[12] As a consequence of this some of the rating lists will stop including Rybka in their lists.
Also missing from some rating lists is Houdini, a very strong free engine by Robert Houdart that appeared in the middle of 2010. Houdini 1.5a has taken the top spot in the rating lists that include it.
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.[13] Then there are the BT2450 and BT2630 test suites by Hubert Bednorz and Fred Toennissen. These suites measure the tactical capability of the engine[14] and have been used at least by REBEL.[15] 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.[16]
Strategic Test Suite (STS) by Swaminathan and Dann Corbit, tests chess engine's strategical strength.[17]
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, freely available engines are listed here.[18] Others may be found by examining the rating lists or external links.
| Engine (strongest version) | Author (Country) | Elo |
|---|---|---|
| Alaric v707 | Peter Fendrich (Sweden) | 2763 |
| Alfil v11 | Enrique Sanchez (Spain) | 2700 |
| Bison v9.11 | Ivan Bonkin (Russia) | 2825 |
| Booot v5.1.0 | Alex Morozov (Ukraine) | 2951 |
| Bright v0.4a | Allard Siemelink (Netherlands) | 3003 (4CPU) |
| BugChess2 v1.7 | Francois and Jean-Philippe Karr (France) | 2800 (32-bit) |
| Chronos v1.9.9 | Guillermo Filia (Argentina) | 2849 (64-bit) |
| Colossus 2008b | Martin Bryant (England) | 2745 |
| Crafty v23.3 | Robert Hyatt (US) | 2950 (64-bit 4CPU) |
| Critter v1.4 | Richard Vida (Slovakia) | 3305 (64-bit 4CPU) |
| Cyclone v3.4 | Fabien Letouzey (France), Thomas Gaksch, Norman Schmidt | 2971 (2CPU) |
| Cyrano v0.6b17 | Harald Johnsen (France) | 2748 |
| Daydreamer v1.75 | Aaron Becker (USA) | 2783 (32-bit) |
| Delfi v5.4 | Fabio Cavicchio (Italy) | 2825 (2CPU) |
| Deuterium 10.01.27.213 | Ferdinand Mosca (Philippines) | 2770 |
| Doch v1.2 | Don Dayley (USA) | 2991 (64-bit) |
| E.T. Chess 13.01.08 | Eric Triki (France) | 2749 |
| Fire v1.31 | Norman Schmidt (USA) | 3260 |
| Frenzee Feb08 | Sune Fischer (Denmark) | 2799 (32-bit) |
| Fruit v2.3.1 | Fabien Letouzey (France), Ryan Benitez (USA) | 2887 |
| Glaurung v2.2 | Tord Romstad (Norway) | 3003 (64-bit 4CPU) |
| Grapefruit v1.0 | Fabien Letouzey (France), Thomas Gaksch (Germany), Vadim Demichev | 2977 (32-bit 2CPU) |
| Gull v1.2 | Vadim Demichev (Russia) | 3038 (64-bit) |
| Hamsters v0.7.1 | Alessandro Scotti (Italy) | 2723 |
| Hannibal v1.0a | Edsel Apostol (Philippines), Sam Hamilton (USA) | 2941 (64-bit) |
| Houdini v1.5a | Robert Houdart (Belgium) | 3309 |
| IvanHoe v9.47b | Yakov Petrovich Golyadkin, Igor Igorovich Igoronov, Robert Pescatore, Yusuf Ralf Weisskopf, Ivan Skavinsky Skavar | 3270 |
| Jonny v4.00 | Johannes Zwanzger (Germany) | 2955 (4CPU) |
| Komodo v3.0 | Don Dailey | 3280 (64-bit) |
| List v5.12 (last free version) |
Fritz Reul (Germany) | 2720 |
| Loop 13.6 (Loop 2007) (last free version) |
Fritz Reul (Germany) | 2943 (64-bit 4CPU) |
| Movei v00.8.438 (10 10 10) | Uri Blass (Israel) | 2773 |
| Naraku v1.4 | Marco Meloni (Italy) | 2820 |
| Naum v2.0 (last free version) |
Aleksandar Naumov (Serbia/Canada) | 2802 (64-bit) |
| Pharaon v3.5.1 | Franck Zibi (France) | 2744 (2CPU) |
| Pro Deo v1.2 (freeware version of the commercial program Rebel) |
Ed Schröder (Netherlands) | 2720 |
| Protector v1.3.6 | Raimund Heid (Germany) | 3070 (64-bit) |
| Rybka v.2.2n2 (last free version) |
Vasik Rajlich (Czech Republic) | 3126 (64-bit 4CPU) |
| Scorpio v2.7 | Daniel Shawul (Ethiopia) | 2851 (32-bit) |
| Slow Chess Blitz WV2.1 | Jonathan Kreuzer (US) | 2741 |
| Spark v1.0 | Allard Siemelink (Netherlands) | 3097 (64-bit 4CPU) |
| Spike v1.4 Leiden | Volker Böhm and Ralf Schäfer (Germany) | 3149 |
| Stockfish v2.1.1 | Tord Romstad (Norway), Marco Costalba (Italy) & Joona Kiiski (Finland) | 3259 (64-bit 4CPU) |
| TheMadPrune v1.1.25 | Fabien Letouzey (France), WH Lowery Jnr (USA) | 2978 (2CPU) |
| Thinker v5.4C Inert | Lance Perkins (Canada) | 3029 (64-bit) |
| Toga II v1.4.1SE | Derived from Fruit v2.1 by Fabien Letouzey (France), Thomas Gaksch (Germany) | 3007 (4CPU) |
| Tornado v4.4 | Engin Ustun (Germany) | 2840 (32-bit) |
| Twisted Logic 20100131x | Edsel Apostol (Philippines) | 2874 (64-bit) |
| Umko v1.0 | Borko Boskovic (Slovenia) | 2906 (32-bit) |
| Wildcat v8 | Igor Korshunov (Belarus) | 2731 |
| Zappa v1.1 (latest free version) |
Anthony Cozzie (US) | 2712 (64-bit) |
These open source chess programs were expressly written to teach the craft of chess programming.
There is also source-available proprietary software.
These small programs are written for hobby or to win programming contests, currently able to play all legal chess moves although with limited text interface.
These chess programs are sold commercially. Most of these also include their own user interface.
| Name | Author | Country |
|---|---|---|
| ApiChess | Max Himam | France |
| Azraël | Christopher Conkie | United Kingdom |
| Carnivor | Michael Sherwin | USA |
| Cheetah | Ralf Schäfer | Germany |
| Chepla | Mikael Bäckman | Sweden |
| Chimp | Andy Duplain | United Kingdom |
| Chiron | Ubaldo Andrea Farina | Italy |
| Cipollino | Giancarlo delli Colli | Italy |
| Cogito | Joerg Schaefer | Germany |
| Cowrie Chess | Chan Rasjid | Singapore |
| Czolgista | Tomasz Kazimierski | Poland |
| Diep | Vincent Diepeveen | The Netherlands |
| Dr. Theopolis | Corby Nichols | USA |
| EdlChess | Stephan Edlich | Germany |
| EGM | Pawel Kobylarz | Poland |
| Eichhörnchen | Wieland Belka | Germany |
| Ferret past winner of the World Computer Speed Chess Championship |
Bruce Moreland | USA |
| Flywheel | Don Cross | USA |
| Freccia | Stefano Gemma | Italy |
| Grok | Peter Kappler | USA |
| HansDamf | Gerd Isenberg | Germany |
| Hector for Chess | Csaba Jergler | Hungary |
| Ikarus past winner of the World Computer Speed Chess Championship |
||
| Kallisto | Bart Weststrate | Netherlands |
| LearningLemming | Sam Hamilton | USA |
| MeneChess | Shaun Howe | United Kingdom |
| Moneypenny | Matt Shoemaker | USA |
| NaltaP312 | Yves Catineau | France |
| Nightmare | Joost Buijs | Netherlands |
| Now | Mark Lefler | USA |
| Nullmover | Michael Langeveld | Netherlands |
| Olympus | Joshua Shriver | USA |
| Pandix | Gyula Horváth | Hungary |
| Pebble | Adam Goodwin | USA |
| Philidor | Christian Barreteau and Bruno Lucas | France |
| Purple Haze | Vincent Ollivier | France |
| Sibyl | Milikas Anastasios | Greece |
| Sillycon | Lasse Hansen | Norway |
| Spandrel | Robert Purves | New Zealand |
| Symbolic | Steven Edwards | USA |
| Telepath | Charles Roberson | USA |
| Tinker | Brian Richardson | USA |
| TwilightChess | Tony Paletta | France |
| Tzunami | Ivo Tops | Netherlands |
| Vlad Tepes | Henk Fennema | Netherlands |
| WaDuuttie | Maarten Claessens | Netherlands |
| Waster | Geoff Westwood | United Kingdom |
| Weid | Jaap Weidemann | South Africa |
| XiniX | Tony van Roon-Werten | Netherlands |
| Z | Manuel Díaz | Spain |
| Zeta | Srdja Matovic | Montenegro |
| Ziggurat | David Norris | United States |
| Zilch | Mike Leany | USA |
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.
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.
These chess programs run on obsolete hardware.