Chess engine

Interface protocol

The command-line interface of GNU Chess became the initial de facto standard, called the Chess Engine Communication Protocol and was 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 interfaces support both protocols, while others, such as WinBoard, support only one and depend on subsidiary interpreters 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 analyze and to acquire a better positional understanding. According to one survey,, the top engines have been increasing in strength by an average of 67 Elo per year since 1986.

A chess engine often uses a vast previously computed opening "book" to increase its playing strength for the first several moves up to possibly 20 moves or more in deeply analyzed lines.

Some chess engines maintain a database of chess positions, along with previously computed evaluations and best moves, in effect, a kind of "dictionary" of recurring chess positions. Since these positions are pre-computed, the engine merely plays one of the indicated moves in the database, thereby saving compute time, resulting in stronger more rapid play.

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 (speculative analysis while the opponent is thinking) as a method to increase their strength.

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. Most tournaments also allow any types of hardware, so only engine/hardware combinations are being compared.

Historically, commercial programs have been the strongest engines. 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.

Ratings

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.

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). Although very strong and open source, there are allegations from commercial software interests that they were derived from disassembled binary of Rybka. Due to the controversy, all these engines have been blacklisted from many tournaments and rating lists. Rybka in turn was accused of being based on Fruit, and in June 2011, the ICGA formally claimed Rybka was derived from Fruit and Crafty and banned Rybka from the International Computer Games Association World Computer Chess Championship, and revoked its previous victories (2007, 2008, 2009, and 2010) The ICGA was criticized for this decision by Dr. Søren Riis, a longstanding supporter of the Rybka program. Rybka is still included on several society ranking lists.

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. Then there are the BT2450 and BT2630 test suites by Hubert Bednorz and Fred Toennissen. These suites measure the tactical capability of the engine and have been used at least by REBEL. 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.

Strategic Test Suite (STS) by Swaminathan and Dann Corbit, tests chess engine's strategical strength.

The most difficult modern test suite is Nightmare II by Walter Eigenmann (2016). This collection contains 30 extremely complicated puzzles, compiled especially for chess engines.

Dedicated hardware

These chess playing systems include custom hardware or run on supercomputers.

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.

Historical

These chess programs run on obsolete hardware.

Kasparov versus the World (chess game played with computer assistance)

In 1999, Garry Kasparov played a chess game "Kasparov versus the World" over the Internet, hosted by the MSN Gaming Zone. Both sides used computer (chess engine) assistance. The "World Team" included participation of over 50,000 people from more than 75 countries, deciding their moves by plurality vote. The game lasted four months, with Kasparov playing "g7" on his 62nd move and announcing a forced checkmate in 28 moves found with the computer program Deep Junior. The World Team voters resigned on October 22. After the game Kasparov said "It is the greatest game in the history of chess. The sheer number of ideas, the complexity, and the contribution it has made to chess make it the most important game ever played."

Engines for chess variants

Some chess engines have been developed to play chess variants, adding the necessary code to simulate non-standard chess pieces, or to analyze play on non-standard boards. ChessV for example is capable of playing variants on a chessboard up to 12×8 in size, such as Capablanca chess (10×8 board).

For larger boards however, there are few chess engines that can play effectively, and indeed chess games played on an unbounded chessboard ("infinite chess") are virtually untouched by chess-playing software.

Other

There are hundreds of free and/or open source chess engines which conform to one of the above communication protocols. The top 50 strongest, freely available engines, according to the CCRL 40/40 rating list, are listed here.