3 [![Build Status](https://travis-ci.org/official-stockfish/Stockfish.svg?branch=master)](https://travis-ci.org/official-stockfish/Stockfish)
4 [![Build Status](https://ci.appveyor.com/api/projects/status/github/official-stockfish/Stockfish?branch=master&svg=true)](https://ci.appveyor.com/project/mcostalba/stockfish/branch/master)
6 [Stockfish](https://stockfishchess.org) is a free, powerful UCI chess engine
7 derived from Glaurung 2.1. It is not a complete chess program and requires a
8 UCI-compatible GUI (e.g. XBoard with PolyGlot, Scid, Cute Chess, eboard, Arena,
9 Sigma Chess, Shredder, Chess Partner or Fritz) in order to be used comfortably.
10 Read the documentation for your GUI of choice for information about how to use
16 This distribution of Stockfish consists of the following files:
18 * Readme.md, the file you are currently reading.
20 * Copying.txt, a text file containing the GNU General Public License version 3.
22 * src, a subdirectory containing the full source code, including a Makefile
23 that can be used to compile Stockfish on Unix-like systems.
28 Currently, Stockfish has the following UCI options:
31 Write all communication to and from the engine into a text file.
34 A positive value for contempt favors middle game positions and avoids draws.
36 * #### Analysis Contempt
37 By default, contempt is set to prefer the side to move. Set this option to "White"
38 or "Black" to analyse with contempt for that side, or "Off" to disable contempt.
41 The number of CPU threads used for searching a position. For best performance, set
42 this equal to the number of CPU cores available.
45 The size of the hash table in MB. It is recommended to set Hash after setting Threads.
51 Let Stockfish ponder its next move while the opponent is thinking.
54 Output the N best lines (principal variations, PVs) when searching.
55 Leave at 1 for best performance.
58 Lower the Skill Level in order to make Stockfish play weaker (see also UCI_LimitStrength).
59 Internally, MultiPV is enabled, and with a certain probability depending on the Skill Level a
60 weaker move will be played.
62 * #### UCI_LimitStrength
63 Enable weaker play aiming for an Elo rating as set by UCI_Elo. This option overrides Skill Level.
66 If enabled by UCI_LimitStrength, aim for an engine strength of the given Elo.
67 This Elo rating has been calibrated at a time control of 60s+0.6s and anchored to CCRL 40/4.
70 If enabled, show approximate WDL statistics as part of the engine output.
71 These WDL numbers model expected game outcomes for a given evaluation and
72 game ply for engine self-play at fishtest LTC conditions (60+0.6s per game).
75 Assume a time delay of x ms due to network and GUI overheads. This is useful to
76 avoid losses on time in those cases.
78 * #### Minimum Thinking Time
79 Search for at least x ms per move.
82 Lower values will make Stockfish take less time in games, higher values will
86 Tells the engine to use nodes searched instead of wall time to account for
87 elapsed time. Useful for engine testing.
90 An option handled by your GUI. If true, Stockfish will play Chess960.
92 * #### UCI_AnalyseMode
93 An option handled by your GUI.
96 Path to the folders/directories storing the Syzygy tablebase files. Multiple
97 directories are to be separated by ";" on Windows and by ":" on Unix-based
98 operating systems. Do not use spaces around the ";" or ":".
100 Example: `C:\tablebases\wdl345;C:\tablebases\wdl6;D:\tablebases\dtz345;D:\tablebases\dtz6`
102 It is recommended to store .rtbw files on an SSD. There is no loss in storing
103 the .rtbz files on a regular HD. It is recommended to verify all md5 checksums
104 of the downloaded tablebase files (`md5sum -c checksum.md5`) as corruption will
105 lead to engine crashes.
107 * #### SyzygyProbeDepth
108 Minimum remaining search depth for which a position is probed. Set this option
109 to a higher value to probe less agressively if you experience too much slowdown
110 (in terms of nps) due to TB probing.
112 * #### Syzygy50MoveRule
113 Disable to let fifty-move rule draws detected by Syzygy tablebase probes count
114 as wins or losses. This is useful for ICCF correspondence games.
116 * #### SyzygyProbeLimit
117 Limit Syzygy tablebase probing to positions with at most this many pieces left
118 (including kings and pawns).
121 ## What to expect from Syzygybases?
123 If the engine is searching a position that is not in the tablebases (e.g.
124 a position with 8 pieces), it will access the tablebases during the search.
125 If the engine reports a very large score (typically 153.xx), this means
126 that it has found a winning line into a tablebase position.
128 If the engine is given a position to search that is in the tablebases, it
129 will use the tablebases at the beginning of the search to preselect all
130 good moves, i.e. all moves that preserve the win or preserve the draw while
131 taking into account the 50-move rule.
132 It will then perform a search only on those moves. **The engine will not move
133 immediately**, unless there is only a single good move. **The engine likely
134 will not report a mate score even if the position is known to be won.**
136 It is therefore clear that this behaviour is not identical to what one might
137 be used to with Nalimov tablebases. There are technical reasons for this
138 difference, the main technical reason being that Nalimov tablebases use the
139 DTM metric (distance-to-mate), while Syzygybases use a variation of the
140 DTZ metric (distance-to-zero, zero meaning any move that resets the 50-move
141 counter). This special metric is one of the reasons that Syzygybases are
142 more compact than Nalimov tablebases, while still storing all information
143 needed for optimal play and in addition being able to take into account
148 Stockfish supports large pages on Linux and Windows. Large pages make
149 the hash access more efficient, improving the engine speed, especially
150 on large hash sizes. Typical increases are 5..10% in terms of nps, but
151 speed increases up to 30% have been measured. The support is
152 automatic. Stockfish attempts to use large pages when available and
153 will fall back to regular memory allocation when this is not the case.
157 Large page support on Linux is obtained by the Linux kernel
158 transparent huge pages functionality. Typically, transparent huge pages
159 are already enabled and no configuration is needed.
161 ### Support on Windows
163 The use of large pages requires "Lock Pages in Memory" privilege. See
164 [Enable the Lock Pages in Memory Option (Windows)](https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/enable-the-lock-pages-in-memory-option-windows)
165 on how to enable this privilege. Logout/login may be needed
166 afterwards. Due to memory fragmentation, it may not always be
167 possible to allocate large pages even when enabled. A reboot
168 might alleviate this problem. To determine whether large pages
169 are in use, see the engine log.
171 ## Compiling Stockfish yourself from the sources
173 Stockfish has support for 32 or 64-bit CPUs, certain hardware
174 instructions, big-endian machines such as Power PC, and other platforms.
176 On Unix-like systems, it should be easy to compile Stockfish
177 directly from the source code with the included Makefile in the folder
178 `src`. In general it is recommended to run `make help` to see a list of make
179 targets with corresponding descriptions.
184 make build ARCH=x86-64-modern
187 When not using the Makefile to compile (for instance with Microsoft MSVC) you
188 need to manually set/unset some switches in the compiler command line; see
189 file *types.h* for a quick reference.
191 When reporting an issue or a bug, please tell us which version and
192 compiler you used to create your executable. These informations can
193 be found by typing the following commands in a console:
200 ## Understanding the code base and participating in the project
202 Stockfish's improvement over the last couple of years has been a great
203 community effort. There are a few ways to help contribute to its growth.
205 ### Donating hardware
207 Improving Stockfish requires a massive amount of testing. You can donate
208 your hardware resources by installing the [Fishtest Worker](https://github.com/glinscott/fishtest/wiki/Running-the-worker:-overview)
209 and view the current tests on [Fishtest](https://tests.stockfishchess.org/tests).
211 ### Improving the code
213 If you want to help improve the code, there are several valuable resources:
215 * [In this wiki,](https://www.chessprogramming.org) many techniques used in
216 Stockfish are explained with a lot of background information.
218 * [The section on Stockfish](https://www.chessprogramming.org/Stockfish)
219 describes many features and techniques used by Stockfish. However, it is
220 generic rather than being focused on Stockfish's precise implementation.
221 Nevertheless, a helpful resource.
223 * The latest source can always be found on [GitHub](https://github.com/official-stockfish/Stockfish).
224 Discussions about Stockfish take place in the [FishCooking](https://groups.google.com/forum/#!forum/fishcooking)
225 group and engine testing is done on [Fishtest](https://tests.stockfishchess.org/tests).
226 If you want to help improve Stockfish, please read this [guideline](https://github.com/glinscott/fishtest/wiki/Creating-my-first-test)
227 first, where the basics of Stockfish development are explained.
232 Stockfish is free, and distributed under the **GNU General Public License version 3**
233 (GPL v3). Essentially, this means that you are free to do almost exactly
234 what you want with the program, including distributing it among your
235 friends, making it available for download from your web site, selling
236 it (either by itself or as part of some bigger software package), or
237 using it as the starting point for a software project of your own.
239 The only real limitation is that whenever you distribute Stockfish in
240 some way, you must always include the full source code, or a pointer
241 to where the source code can be found. If you make any changes to the
242 source code, these changes must also be made available under the GPL.
244 For full details, read the copy of the GPL v3 found in the file named