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Fairy-Stockfish

Overview

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Fairy-Stockfish is a chess variant engine derived from Stockfish designed for the support of fairy chess variants and easy extensibility with more games. It can play various regional, historical, and modern chess variants as well as games with user-defined rules. For compatibility with graphical user interfaces it supports the UCI, UCCI, USI, UCI-cyclone, and CECP/XBoard protocols.

The goal of the project is to create an engine supporting a large variety of chess-like games, equipped with the powerful search of Stockfish. Despite its generality the playing strength is on a very high level in almost all supported variants. Due to its multi-protocol support Fairy-Stockfish works with almost any chess variant GUI.

Installation

You can download the Windows executable or Linux binary from the latest release or compile the program from source. The program comes without a graphical user interface, so you perhaps want to use it together with a compatible GUI, or play against it online at pychess, lishogi, or lichess. Read more about how to use Fairy-Stockfish in the wiki.

If you want to preview the functionality of Fairy-Stockfish before downloading, you can try it out on the Fairy-Stockfish playground in the browser.

Optional NNUE evaluation parameter files to improve playing strength for many variants are in the list of NNUE networks. For the regional variants Xiangqi, Janggi, and Makruk dedicated releases with built-in NNUE networks are available. See the wiki for more details on NNUE.

Contributing

If you like this project, please support its development via patreon or paypal, by contributing CPU time to the framework for testing of code improvements, or by contributing to the code or documentation. An introduction to the code base can be found in the wiki.

Supported games

The games currently supported besides chess are listed below. Fairy-Stockfish can also play user-defined variants loaded via a variant configuration file, see the file src/variants.ini and the wiki.

Regional and historical games

Chess variants

Shogi variants

Related games

Help

See the Fairy-Stockfish Wiki for more info, or if the required information is not available, open an issue or join our discord server.

Bindings

Besides the C++ engine, this project also includes bindings for other programming languages in order to be able to use it as a library for chess variants. They support move, SAN, and FEN generation, as well as checking of game end conditions for all variants supported by Fairy-Stockfish. Since the bindings are using the C++ code, they are very performant compared to libraries directly written in the respective target language.

Python

The python binding pyffish contributed by @gbtami is implemented in pyffish.cpp. It is e.g. used in the backend for the pychess server.

Javascript

The javascript binding ffish.js contributed by @QueensGambit is implemented in ffishjs.cpp. The compilation/binding to javascript is done using emscripten, see the readme.

Ports

WebAssembly

For in-browser use a port of Fairy-Stockfish to WebAssembly is available at npm. It is e.g. used for local analysis on pychess.org. Also see the Fairy-Stockfish WASM demo available at https://fairy-stockfish-nnue-wasm.vercel.app/.

Stockfish

Overview

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Stockfish is a free, powerful UCI chess engine derived from Glaurung 2.1. Stockfish is not a complete chess program and requires a UCI-compatible graphical user interface (GUI) (e.g. XBoard with PolyGlot, Scid, Cute Chess, eboard, Arena, Sigma Chess, Shredder, Chess Partner or Fritz) in order to be used comfortably. Read the documentation for your GUI of choice for information about how to use Stockfish with it.

The Stockfish engine features two evaluation functions for chess, the classical evaluation based on handcrafted terms, and the NNUE evaluation based on efficiently updatable neural networks. The classical evaluation runs efficiently on almost all CPU architectures, while the NNUE evaluation benefits from the vector intrinsics available on most CPUs (sse2, avx2, neon, or similar).

Files

This distribution of Stockfish consists of the following files:

The UCI protocol and available options

The Universal Chess Interface (UCI) is a standard protocol used to communicate with a chess engine, and is the recommended way to do so for typical graphical user interfaces (GUI) or chess tools. Stockfish implements the majority of it options as described in the UCI protocol.

Developers can see the default values for UCI options available in Stockfish by typing ./stockfish uci in a terminal, but the majority of users will typically see them and change them via a chess GUI. This is a list of available UCI options in Stockfish:

For developers the following non-standard commands might be of interest, mainly useful for debugging:

A note on classical evaluation versus NNUE evaluation

Both approaches assign a value to a position that is used in alpha-beta (PVS) search to find the best move. The classical evaluation computes this value as a function of various chess concepts, handcrafted by experts, tested and tuned using fishtest. The NNUE evaluation computes this value with a neural network based on basic inputs (e.g. piece positions only). The network is optimized and trained on the evaluations of millions of positions at moderate search depth.

The NNUE evaluation was first introduced in shogi, and ported to Stockfish afterward. It can be evaluated efficiently on CPUs, and exploits the fact that only parts of the neural network need to be updated after a typical chess move. The nodchip repository provides additional tools to train and develop the NNUE networks. On CPUs supporting modern vector instructions (avx2 and similar), the NNUE evaluation results in much stronger playing strength, even if the nodes per second computed by the engine is somewhat lower (roughly 80% of nps is typical).

Notes:

  1. the NNUE evaluation depends on the Stockfish binary and the network parameter file (see the EvalFile UCI option). Not every parameter file is compatible with a given Stockfish binary, but the default value of the EvalFile UCI option is the name of a network that is guaranteed to be compatible with that binary.

  2. to use the NNUE evaluation, the additional data file with neural network parameters needs to be available. Normally, this file is already embedded in the binary or it can be downloaded. The filename for the default (recommended) net can be found as the default value of the EvalFile UCI option, with the format nn-[SHA256 first 12 digits].nnue (for instance, nn-c157e0a5755b.nnue). This file can be downloaded from

https://tests.stockfishchess.org/api/nn/[filename]

replacing [filename] as needed.

What to expect from the Syzygy tablebases?

If the engine is searching a position that is not in the tablebases (e.g. a position with 8 pieces), it will access the tablebases during the search. If the engine reports a very large score (typically 153.xx), this means it has found a winning line into a tablebase position.

If the engine is given a position to search that is in the tablebases, it will use the tablebases at the beginning of the search to preselect all good moves, i.e. all moves that preserve the win or preserve the draw while taking into account the 50-move rule. It will then perform a search only on those moves. The engine will not move immediately, unless there is only a single good move. The engine likely will not report a mate score, even if the position is known to be won.

It is therefore clear that this behaviour is not identical to what one might be used to with Nalimov tablebases. There are technical reasons for this difference, the main technical reason being that Nalimov tablebases use the DTM metric (distance-to-mate), while the Syzygy tablebases use a variation of the DTZ metric (distance-to-zero, zero meaning any move that resets the 50-move counter). This special metric is one of the reasons that the Syzygy tablebases are more compact than Nalimov tablebases, while still storing all information needed for optimal play and in addition being able to take into account the 50-move rule.

Large Pages

Stockfish supports large pages on Linux and Windows. Large pages make the hash access more efficient, improving the engine speed, especially on large hash sizes. Typical increases are 5..10% in terms of nodes per second, but speed increases up to 30% have been measured. The support is automatic. Stockfish attempts to use large pages when available and will fall back to regular memory allocation when this is not the case.

Support on Linux

Large page support on Linux is obtained by the Linux kernel transparent huge pages functionality. Typically, transparent huge pages are already enabled, and no configuration is needed.

Support on Windows

The use of large pages requires "Lock Pages in Memory" privilege. See Enable the Lock Pages in Memory Option (Windows) on how to enable this privilege, then run RAMMap to double-check that large pages are used. We suggest that you reboot your computer after you have enabled large pages, because long Windows sessions suffer from memory fragmentation, which may prevent Stockfish from getting large pages: a fresh session is better in this regard.

Compiling Stockfish yourself from the sources

Stockfish has support for 32 or 64-bit CPUs, certain hardware instructions, big-endian machines such as Power PC, and other platforms.

On Unix-like systems, it should be easy to compile Stockfish directly from the source code with the included Makefile in the folder src. In general it is recommended to run make help to see a list of make targets with corresponding descriptions.

    cd src
    make help
    make net
    make build ARCH=x86-64-modern

When not using the Makefile to compile (for instance, with Microsoft MSVC) you need to manually set/unset some switches in the compiler command line; see file types.h for a quick reference.

When reporting an issue or a bug, please tell us which Stockfish version and which compiler you used to create your executable. This information can be found by typing the following command in a console:

    ./stockfish compiler

Understanding the code base and participating in the project

Stockfish's improvement over the last decade has been a great community effort. There are a few ways to help contribute to its growth.

Donating hardware

Improving Stockfish requires a massive amount of testing. You can donate your hardware resources by installing the Fishtest Worker and view the current tests on Fishtest.

Improving the code

If you want to help improve the code, there are several valuable resources:

Terms of use

Stockfish is free, and distributed under the GNU General Public License version 3 (GPL v3). Essentially, this means you are free to do almost exactly what you want with the program, including distributing it among your friends, making it available for download from your website, selling it (either by itself or as part of some bigger software package), or using it as the starting point for a software project of your own.

The only real limitation is that whenever you distribute Stockfish in some way, you MUST always include the full source code, or a pointer to where the source code can be found, to generate the exact binary you are distributing. If you make any changes to the source code, these changes must also be made available under the GPL.

For full details, read the copy of the GPL v3 found in the file named Copying.txt.