GNU Chess
1 Overview
2 Contact info
3 Running 'gnuchess'
3.1 Invoking gnuchess
3.2 Interactive game
3.3 Command list
3.4 Environment variables
3.5 Configuration file
3.5.1 Structure
3.5.2 Options
3.5.3 Workarounds
3.6 Output files
3.7 XBoard chess engine
3.8 UCI chess engine
3.9 Internet
4 Book
5 Tests
6 Auxiliary file formats
7 History
8 Known problems
8.1 Adapter
9 Developers
9.1 Background of v6
9.2 Guidelines followed to develop GNU Chess v6
9.3 Chess engine protocol adapter
9.4 Structure of the source code
9.5 C/C++ coexistence
9.6 Chess engine
9.6.1 Fruit overview
9.6.2 Board data structure
9.6.3 Search algorithm
9.6.4 Transposition table
9.6.5 Null move
9.6.6 Move ordering
9.6.7 Evaluation function
9.6.8 Speed
10 Translations
Appendix A GNU Free Documentation License
Concept index
GNU Chess
*********
This manual is for GNU Chess (version 6.2.8, 24 March 2021).
This manual is for GNU Chess (version 6.2.8, 24 March 2021), which is
a complete chess program, frequently used as a chess engine.
Copyright (C) 2001-2021 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, with no Front-Cover Texts,
and with no Back-Cover Texts. A copy of the license is included in
the section entitled "GNU Free Documentation License".
1 Overview
**********
GNU Chess () is a computer program
for playing chess. It can be used to interactively play chess on a text
terminal, but it is more often used in conjunction with a GUI program
such as GNU XBoard.
Because it is protected by the GNU General Public License, users are
free (in perpetuity) to share and change it.
The main author of GNU Chess version 6 is Fabien Letouzey, The
original author is Stuart Cracraft.
2 Contact info
**************
We are the GNU Chess developers and you may reach us at:
Our official web page is:
We are indebted to our sponsor, the Free Software Foundation whose
web page is:
and which also serves as our software depository for new versions of
GNU and GNU Chess.
You can download the latest version from GNU's FTP site at:
The code is provided for the purpose of encouraging you to do the
programming. If you lack the programming skills to do so, try dabbling
in it. You might surprise yourself.
If you want to report a possible bug in GNU Chess, please send a
message to the e-mail address indicated above, providing precise
information about the conditions that led to the possible bug. As a
general guideline, you can kindly include the follow information:
* Version of the program. The following command will print it:
gnuchess --version
* How you started the program. Whether running standalone or in
combination with other programs such as XBoard.
* The actions you performed and the output or behaviour you observed.
* Output files if any. In order to enable the writing to
adapter.log, set the following option in config file gnuchess.ini:
'Log = true'
* Whether the problem is systematic (it always happens) or
occasional.
* Any other information you may deem relevant.
3 Running 'gnuchess'
********************
3.1 Invoking gnuchess
=====================
The format for running the 'gnuchess' program is:
gnuchess OPTION ...
With no options, 'gnuchess' starts in interactive mode and it is
ready to start a chess game.
'gnuchess' supports the following options:
'--help'
'-h'
Print an informative help message on standard output and exit
successfully.
'--version'
'-v'
Print the version number and licensing information of Hello on
standard output and then exit successfully.
'--quiet'
'--silent'
'-q'
Make the program silent on startup.
'--xboard'
'-x'
Start the program in xboard mode, i.e. as an xboard engine. This
is typically used for using the program as backend of other chess
GUI such as XBoard.
Option 'xboard' is accepted without leading dashes for backward
compatibility.
'--post'
'-p'
Start up showing thinking.
Option 'post' is accepted without leading dashes for backward
compatibility.
'--easy'
'-e'
Disable thinking in opponent's time. By default, the program runs
in hard mode, i.e. it uses opponent's time to think too.
'--manual'
'-m'
Enable manual mode.
'--uci'
'-u'
Enable UCI protocol (externally behave as UCI engine).
'--memory size'
'-M size'
Specify memory usage in MB for hashtable.
'--addbook filename'
'-a filename'
Compile book.bin from pgn book 'filename' and quits.
'--graphic'
'-g'
Enable graphic mode based on Unicode chess symbols.
3.2 Interactive game
====================
Moves are accepted either in standard algebraic notation (SAN) or in
coordinate algebraic notation. Examples:
Nf3
g1f3
O-O
e1g1
3.3 Command list
================
'gnuchess' supports the following commands:
'^C'
Quit the program.
'quit'
Quit the program.
'exit'
In analysis mode this stops analysis, otherwise it quits the
program.
'help'
Produces a help blurb corresponding to this list of commands.
'usage'
Produce blurb on command line options.
(Same as 'gnuchess --help')
'book'
Handle the book. Requires a subcommand:
add - compiles book.bin from book.pgn
on - enables use of book
off - disables use of book
best - play best move from book
worst - play worst move from book
random - play any move from book
prefer (default) - choose a good move from book (Method subject to
variation)
'version'
Prints out the version of this program
(Same as 'gnuchess --version')
'pgnsave FILENAME'
Saves the game so far to the file from memory
'pgnload FILENAME'
Loads the game in the file into memory (cf. pgnreplay)
'pgnreplay FILENAME'
Loads the game in the file into memory, and enables commands first,
last, next, previous. This allows replaying a saved game step by
step. (cf. pgnload)
'first'
Go to start position of pgn loaded game with pgnreplay.
'last'
Go to last position of pgn loaded game with pgnreplay.
'next'
Advances one move in pgn loaded game with pgnreplay.
'n'
Advances one move in pgn loaded game with pgnreplay.
'previous'
Back one move in pgn loaded game with pgnreplay.
'p'
Back one move in pgn loaded game with pgnreplay.
'force'
'manual'
Makes the program stop moving. You may now enter moves to reach
some position in the future.
(Same as 'gnuchess --manual')
'white'
Program plays black, set white to move.
Note: not implemented in this version.
'black'
Program plays white, set black to move.
(White and black commands are mainly for icsDrone and will cause
the current en-passant capture square to be forgotten).
Note: not implemented in this version.
'go'
Computer takes whichever side is on move and begins its thinking
immediately
'easy'
Disables thinking on opponent's time
(Same as 'gnuchess --easy')
'hard'
Enables thinking on opponent's time
'post'
Arranges for verbose thinking output showing variation, score,
time, depth, etc.
If pondering (see hard) is on, the program will output it's
thinking whilst the opponent is thinking.
(Same as 'gnuchess --post')
'nopost'
Turns off verbose thinking output
'name NAME'
Lets you input your name. Also writes the log.nnn and a
corresponding game.nnn file. For details please see auxillary file
format sections.
'result'
Mostly used by Internet Chess server.
'activate'
This command reactivates a game that has been terminated
automatically due to checkmate or no more time on the clock.
However, it does not alter those conditions. You would have to
undo a move or two or add time to the clock with level or time in
that case.
Note: not implemented in this version.
'rating COMPUTERRATING OPPONENTRATING'
Inputs the estimated rating for computer and for its opponent
'new'
Sets up new game (i.e. positions in original positions)
'time'
Inputs time left in game for computer in hundredths of a second.
Mostly used by Internet Chess server.
'otim'
Inputs time left in game for opponent in hundredths of a second.
Mostly used by Internet Chess server.
'random'
Randomizes play by perturbing the evaluation score slightly. The
degree of perturbation is adjustable.
Note: not implemented in this version. Neither in v5
'hash'
on - enables using the memory hash table to speed search
off - disables the memory hash table
'memory N'
Sets the hash table to permit storage of N MB.
'null'
on - enables using the null move heuristic to speed search
off - disables using the null move heuristic
'xboard'
on - enables use of xboard/winboard
off - disables use of xboard/winboard
(Same as 'gnuchess --xboard')
'depth N'
Sets the program to look N ply (half-moves) deep for every search
it performs. If there is a checkmate or other condition that does
not allow that depth, then it will not be
'level MOVES MINUTES INCREMENT'
Sets time control to be MOVES in MINUTES with each move giving an
INCREMENT (in seconds, i.e. Fischer-style clock).
'load'
'epdload'
Loads a position in EPD format from disk into memory.
'save'
'epdsave'
Saves game position into EPD format from memory to disk.
'switch'
Switches side to move
Note: not implemented in this version.
'solve FILENAME'
'solveepd FILENAME'
Solves the positions in FILENAME
'remove'
Backs up two moves in game history
'undo'
Backs up one move in game history
'show'
Requires a subcommand:
board - displays the current board
time - displays the time settings
moves - shows all moves using one call to routine
escape - shows moves that escape from check using one call to
routine
noncapture - shows non-capture moves
capture - shows capture moves
eval [or score] - shows the evaluation per piece and overall
game - shows moves in game history
pin - shows pinned pieces
Note: 'show eval' and 'show pin' not implemented in this version.
'test'
Requires a subcommand:
movelist - reads in an epd file and shows legal moves for its
entries
capture - reads in an epd file and shows legal captures for its
entries
movegenspeed - tests speed of move generator
capturespeed - tests speed of capture move generator
eval - reads in an epd file and shows evaluation for its entries
evalspeed tests speed of the evaluator
Note: not implemented in this version.
'analyze'
Switches program into analysis mode, this is primarily intended for
communicating analysis to an external interface using the Xboard
chess engine protocol. It enables "force", "post", and "hard", at
the same time, whilst altering the output format of post to conform
with the engine protocol.
'graphic'
Enables graphic mode. In this mode, the board is displayed using
the Unicode chess symbols (Codepoints from U+2654 to U+265F). The
graphic mode requires a terminal with Unicode support.
(Same as 'gnuchess --graphic')
'nographic'
Disables graphic mode. The board is displayed using the good-old
classic view based on plain ASCII.
3.4 Environment variables
=========================
If 'GNUCHESS_PKGDATADIR' is defined, it will be taken as the path for
the config file and for the book, in case the files are not found in the
current directory. If it is not defined, they will be taken from the
package data directory, in case the files are not found in the current
directory.
3.5 Configuration file
======================
3.5.1 Structure
---------------
A file called 'gnuchess.ini' is used to define configuration options.
The file is applicable provided '--uci' is not used. The file is looked
in three places according to the following precedence:
1. The directory where the program was started
2. Environment variable'GNUCHESS_PKGDATADIR'
3. The package data directory stated at configure time
Sections are composed of 'variable = value' lines.
Note: There can be spaces in variable names or values. Do not use
quotes.
'[Adapter]' section
This section is used by the adapter only. The engine is unaware of
these options. The list of available options is detailed below in this
document.
'[Engine]' section
This section contains engine UCI options. The PolyGlot-based adapter
does not understand them, but sends the information to the engine at
startup (converted to UCI form). You can add any UCI option that makes
sense to the engine (not just the common options about hash-table size
and tablebases).
Note: use INI syntax, not UCI. For example 'OwnBook = true' is
correct. It will be replaced by the adapter with 'setoption name
OwnBook value true' at engine startup.
Standard UCI options are 'Hash', 'NalimovPath', 'NalimovCache' and
'OwnBook'. Hidden options like 'Ponder' or 'UCI_xxx' are automatic and
should not be put in the INI file.
3.5.2 Options
-------------
These should be put in the '[Adapter]' section.
'Log'
Default: false
Whether the adapter should log all transactions with the interface
and the engine. This should be necessary only to locate problems.
'LogFile'
Default: 'adapter.log'
The name of the log file. Note that it is put where the program
was launched from, not into the engine directory.
WARNING: Log files are not cleared between sessions, and can become
very large. It is safe to remove them though.
'Resign'
Default: false
Set this to "true" if you want the adapter to resign on behalf of
the engine.
NOTE: Some engines display buggy scores from time to time although
the best move is correct. Use this option only if you know what
you are doing (e.g. you always check the final position of games).
'ResignMoves'
Default: 3
Number of consecutive moves with "resign" score (see below) before
the adapter resigns for the engine. Positions with only one legal
move are ignored.
'ResignScore'
Default: 600
This is the score in centipawns that will trigger resign
"counting".
'ShowPonder'
Default: true
Show search information during engine pondering. Turning this off
might be better for interactive use in some interfaces.
'KibitzMove'
Default: false
Whether to kibitz when playing a move.
'KibitzPV'
Default: false
Whether to kibitz when the PV is changed (new iteration or new best
move).
'KibitzCommand'
Default: tellall
xboard command to use for kibitzing, normally "tellall" for
kibitzing or "tellothers" for whispering.
'KibitzDelay'
Default: 5
How many seconds to wait before starting kibitzing. This has an
affect only if "KibitzPV" is selected, move kibitzes are always
sent regardless of the delay.
'Book'
Default: false
Indicates whether the adapter should use a book. This has no
effect on the engine own book (which can be controlled with the UCI
option 'OwnBook' in the '[Engine]' section). In particular, it is
possible to use both a PolyGlot book and an engine book. In that
case, the engine book will be used whenever PolyGlot is out of
book. Remember that PolyGlot is unaware of whether the engine is
itself using a book or not.
'BookFile'
Default: 'book.bin'
The name of the (binary) book file. Note that PolyGlot will look
for it in the directory it was launched from, not in the engine
directory. Of course, full path can be used in which case the
current directory does not matter.
If the file is not found in the current directory, it will be
looked for in GNUCHESS_PKGDATADIR if the variable is defined, or in
the package data directory otherwise.
Note that there is no option to control book usage. All parameters
are fixed when compiling a PGN file into a binary book (see below).
This is purposeful and is not likely to change.
Using a book does not require any additional memory, this can be
important for memory-limited tournaments.
These are UCI options for the '[Engine]' section:
'NullMove Pruning'
(Always/Fail High/Never)
Default: Fail High
"Always" actually means the usual conditions (not in check, etc
...). "Fail High" adds the condition that the static evaluation
fails high. Never use "Never" (ever)! OK you can use "Never" to
test a Zugzwang problem.
I expect that this option has little effect (assuming the first two
choices only). It was only added because most engines do not use
the fail-high condition.
'NullMove Reduction'
(1-3 plies)
Default: 3
3 is rather aggressive, especially in the endgame. It seems better
than always using 2 though.
'Verification Search'
(Always/Endgame/Never)
Default: Endgame
This tries to solve some Zugzwang-related problems. It is expected
to hardly have any effect in games. The default value should be
sufficient for most-common Zugzwang situations.
'Verification Reduction'
(1-6 plies)
Default: 5
5 guarantees that the cost of verification search is negligible in
most cases. Of course it means Zugzwang problems need a lot of
depth to get solved, if ever! With such a reduction, verification
search is similar to Vincent Diepeveen's "double null move".
'History Pruning'
(true/false)
Default: true
A bit dodgy, but fun to experiment with. It should help in blitz,
but it's possible it actually hurts play in longer games.
'History Threshold'
(percentage)
Default: 60%
This is the thing, as it affects the search tree! Lower values are
safer, and higher values more aggressive. THIS VALUE HAS NOT BEEN
TUNED! There is a good chance Fruit's strength can be improved by
changing this option.
'Futility Pruning'
(true/false)
Default: false
Very common but controversial. Makes the engine a tiny bit better
at tactics but slightly weaker positionally. It might be
beneficial by a very small amount, but has not been tested in
conjunction with history pruning yet.
'Futility Margin'
(centipawns)
Default: 100
This value is somewhat aggressive. It could lead to problems in
the endgame. Larger values prune less but will lead to fewer
positional errors.
'Delta Pruning'
(true/false)
Default: false
Similar to futility pruning. Probably safer because it is used
mainly during the middlegame. Has not been tested with history
pruning either.
'Delta Margin'
(centipawns)
Default: 50
Same behaviour as futility margin. This one is probably safe.
'Quiescence Check Plies'
(0-2 plies)
Default: 1
Fruit tries safe (SEE >= 0) checks at the first plies of the
quiescence search. 0 means no checks at all (as in most older
engines). 1 is the same as previous versions of Fruit. 2 is
probably not worth the extra cost. It could be interesting when
solving mate problems though.
'Evaluation options'
(percentage)
Default: 100%
These options are evaluation-feature multipliers. You can modify
Fruit's playing style to an extent or make Fruit weaker for
instance by setting "Material" to a low value.
"Material" is obvious. It also includes the bishop-pair bonus.
"Piece Activity": piece placement and mobility. "King Safety":
mixed features related to the king during early phases "Pawn
Structure": all pawn-only features (not passed pawns). "Passed
Pawns": ... can you guess?
The following options were used in PolyGlot v1.4, but are deprecated
in GNU Chess:
'EngineName'
Default: GNU Chess
This is the name that will appear in the xboard interface. It is
cosmetic only. You can use different names for tweaked versions of
the same engine.
If no "Engine Name" is given, the UCI name will be used.
'EngineDir'
Default: .
Full path of the directory where the engine is installed. You can
use "." (without the quotes) if you know that PolyGlot will be
launched in the engine directory or the engine is in the "path" and
does not need any data file.
'EngineCommand'
Put here the name of the engine executable file. You can also add
command-line arguments. Path searching is used and the current
directory will be "EngineDir".
NOTE: Unix users are recommended to prepend "./"; this is required
on some secure systems.
3.5.3 Workarounds
-----------------
Work arounds are identical to options except that they should be used
only when necessary. Their purpose is to try to hide problems with
various software (not just engines). The default value is always
correct for bug-free software.
These workarounds are unlikely to be used or meaningful for GNU
Chess.
'UCIVersion'
Default: 2
The default value of 2 corresponds to UCI+. Use 1 to select plain
UCI for engines that have problems with UCI+.
'CanPonder'
Default: false
The adapter now conforms to the documented UCI behaviour: the
engine will be allowed to ponder only if it (the engine) declares
the 'Ponder' UCI option. However some engines which can actually
ponder do not declare the option. This work around lets The
adapter know that they can ponder.
'SyncStop'
Default: false
When a ponder miss occurs, the adapter interrupts the engine and
immediately launches a new search. While there should be no
problem with this, some engines seem confused and corrupt their
search board. 'SyncStop' forces the adapter to wait for the (now
useless) ponder search to finish before launching the new search.
'PromoteWorkAround'
Default: false
Some engines do not specify a promotion piece, e.g. they send
"e7e8" instead of the correct "e7e8q". This work around enables
the incorrect form (and of course promotes into a queen).
3.6 Output files
================
GNU Chess produces several output files:
'Adapter log file'
This file is written by the adapter. The name of the file is
specified by the following option in 'gnuchess.ini':
'LogFile = FILENAME'
The file is produced if option '--uci' is not specified and if the
following variable is set in 'gnuchess.ini':
Typically, this file is named 'adapter.log', but any other name
will do the job.
'Log = true'
'log.nnn'
This file is written if command 'name' was requested. The contents
are the opponent's name and the game in coordinate algebraic
notation.
'game.nnn'
This file is written if command 'name' was requested. The contents
are the opponent's name and the game in portable game notation
(PGN).
'players.dat'
This file is written if command 'name' was requested. The contents
are the statistics of games. This is the format of each line:
'opponent-name wins loses draws'
'gnuchess.debug'
This file contains internal information that is useful for
debugging purposes. For this file to be written, it is necessary
to define preprocessor directive 'DEBUG'. Hence, when installing
the program, instead of
./configure
make
make install
Use the following commands:
./configure CPPFLAGS=-DDEBUG
make
make install
When analysing a bug, this file could be very helpful. Users are
encouraged to provide it.
3.7 XBoard chess engine
=======================
Running the program with the "-xboard" command line parameter sets it to
produce output acceptable to and accept input suitable for XBoard, the
graphical display front-end with mouse interface.
For historical reasons the option "xboard" does not need to be
preceeded by "-"; however we would encourage the new syntax.
How to run XBoard with GNU Chess as backend (first chess engine):
xboard -fcp 'gnuchess --xboard'
xboard -fd . -fcp './gnuchess --xboard'
How to run XBoard with GNU Chess playing against itself:
./xboard -fcp './gnuchess -x' -fd . -scp './gnuchess -x' -sd . -pieceImageDirectory 'svg' -matchMode T -tc 1
3.8 UCI chess engine
====================
For GNU Chess to behave as a pure UCI chess engine, execute the
following command:
gnuchess --uci
In this mode, configuration file 'gnuchess.ini' is ignored.
3.9 Internet
============
For GNU Chess to run in Internet, Zippy is required. See Zippy
documentation in the XBoard/WinrBoard distribution:
this is an example of how to run GNU Chess on FICS using XBoard as
frontend and Zippy as connector:
xboard -zp -ics -icshost freechess.org -icshelper timeseal -fcp 'gnuchess --xboard'
4 Book
******
See options 'Book' and 'Bookfile' in Running GNU Chess - Configuration
file - Options
5 Tests
*******
GNU Chess 6 has been tested on the Free Internet Chess Server
() with XBoard.
6 Auxiliary file formats
************************
'.bin'
binary book format
'.pgn'
game listing like '1. e4 e5 2. Nf3' etc.
'.epd'
epd-style format using FEN notation. See tests subdirectory for
example.
'log.nnn'
record of an entire game from computer's viewpoint (thinking, etc.)
'game.nnn'
record of an entire game, similar to .pgn but auto-generated
The '.bin' file format is a simple binary format for the compiled
book which is read by the program when it is using book. See the book
section for more detail.
EPD and PGN require little introduction. These are the uniformly
accepted standards for position recording and game recording.
Note that 'log.nnn' and 'game.nnn' files are written at the end of a
game when you use the 'name' command to give the computer your name. It
is highly recommended to do this since the resulting two files that
match in a monotonically-increasing extension numbered suffix may be
used for reporting bugs and keeping track of your games.
7 History
*********
The first version of GNU Chess was written by Stuart Cracraft back in
1984. Versions from 2 to 4 were written by John Stanback. Version 5
was written by Chua Kong-Sian. Version 6 was written by Fabien
Letouzey.
8 Known problems
****************
8.1 Adapter
===========
These are known problems of PolyGlot v1.4 as described by Fabien
Letouzey.
The addition of Chess960 support lead to a change in internal-move
representation for castling. This slightly affected the opening-book
format. We recommend that you recompile books with this version.
Several users reported engines losing on time. The playing
conditions always mixed playing on an Internet server with pondering.
It is not yet clear what the source of the problem is, but there
seems to be a forever incompatibility between the xboard and UCI
protocol regarding a complex pondering/remaining-time relation.
9 Developers
************
9.1 Background of v6
====================
Since version 5.07 of the program was released in 2003, GNU Chess has
basically remained unchanged. Then Fabien Letouzey developped Fruit,
which proved to be a stronger chess engine and shaked the computer chess
world. At some point in time, he kindly assigned copyright to FSF,
which allowed us to make Fruit 2.1 (the latest free version) the base
for GNU Chess v6.
Therefore, it can be fairly said that Fabien Letouzey is remarkably
the main contributor to GNU Chess v6.
9.2 Guidelines followed to develop GNU Chess v6
===============================================
The idea was to use Fruit 2.1 as the base for GNU Chess v6, keeping
external interfaces backwards compatible. This is particularly
important since GNU Chess has been world-wide used for long.
GNU Chess can be used in two modes: interactively using the
command-line interface on a text console, and used as backend engine
from a graphical frontend. Both cases rely on almost identical grammar.
The former is described in GNU Chess help.
When GNU Chess is used as a pure chess engine, for instance, as
backend for XBoard, it uses the Chess Engine Communication Protocol (aka
XBoard protocol), which is described here:
Since Fruit uses the Universal Chess Interface (UCI), keeping the
former interface was the main challenge of GNU Chess v6. UCI is
described here:
GNU Chess can also run as a UCI chess engine, if the '--uci'
command-line option is specified.
9.3 Chess engine protocol adapter
=================================
UCI is very different from the Chess Engine Communication Protocol.
PolyGlot is a UCI-to-xboard adapter developed by Fabien. It connects a
UCI chess engine to an xboard interface such as WinBoard. UCI2WB is
another such adapter (for Windows).
Standalone PolyGlot can be used, along with Fruit, as chess engine
for chess frontends, suchs as XBoard. In that case, PolyGlot and Fruit
run as two independent single-threaded processes. PolyGlot starts
first, and it forks Fruit. Both processes get communicated by by means
of pipes: PolyGlot captures Fruit's standrad input and output.
PolyGlot tries to solve known problems with other adapters. For
instance, it detects and reports draws by fifty-move rule, repetition,
etc.
PolyGlot 1.4 has been adapted and incorporated to GNU Chess v6 as
chess engine protocol adapter. It connects Fruit-based GNU Chess engine
to the good old GNU Chess frontend.
9.4 Structure of the source code
================================
We want to keep three loosely-coupled modules in GNU Chess v6:
* frontend
* adapter
* engine
The main program contains the frontend and starts two addtional
threads, one for the adapter, and another one for the engine. The three
components comprise a chain, thus there are two links:
* frontend <-> adapter
* adapter <-> engine
The links are based on pipes. There is no need for additional
synchronization mechanisms such as mutex. The changes in PolyGlot and
Fruit are minimal, since they were already using the same mechanism to
communicate with each other.
Another technical problem was the fact that GNU Chess v5 was written
in C, whereas PolyGlot and Fruit are written in C++. In GNU Chess v6
the main program, the adapter and the engine are in C++, but the
frontend remains in C.
Source code was placed under a single 'src' in GNU Chess v5, as
usual. Three additional directories have been created for GNU Chess v6,
so the code is organized in four directories:
'src'
Contains the 'main' and a source file used to create the pipes that
communicate the three modules: frontend, adapter and engine.
'src/frontend'
Contains the frontend. The code here is inherited from GNU Chess
v5, with some modifications mainly in 'cmd.c' and a new file
'engine.c' which addresses the message passing through modules.
'src/adapter'
Contains the chess protocol adapter, based on PolyGlot 1.4. Minor
changes, wrt the baseline.
'src/engine'
Contains the chess engine, based on Fruit 2.1. Minor changes wrt
the baseline.
The sources in each 'src' subdirectory are compiled as a static
library. When link with the sources in top 'src', they result into the
binary 'gnuchess'. The module libraries are named after their
respective directory name:
libfrontend.a
libadapter.a
libengine.a
9.5 C/C++ coexistence
=====================
The source code of the frontend comes mainly from version 5.07, which
was written in C. There is no problem in having both C and C++ in the
same program, and so have they lived together in all versions from 6.0.0
to 6.1.0. However, in version 6.1.1 and subsequent versions the
frontend is compiled in C++. This change has been introduced to avoid
compilation warnings in output.cc (former output.c) as a result of the
introduction of Unicode literals.
PolyGlot and Fruit have a common origin, which means that there is a
lot of shared code. The point is that the shared code is very similar
but not identical, which would make hard an eventual unification. In
order to avoid massive name clash, all the code has been wrapped in
namespaces, one for the adapter, one for the engine. The frontend
remains in the default namespace.
9.6 Chess engine
================
9.6.1 Fruit overview
--------------------
Fruit was designed to help with the study of game-tree search
algorithms, when applied to chess. It is now released as a chess
engine, which is a somewhat different category of programs. Therefore
the source code contains entire files and also functions that are either
not used by the engine, or could be replaced with a much simpler
(although somewhat less efficient) equivalent.
As a chess engine, Fruit combines a "robust" search algorithm with a
"minimalist" evaluation function. The latter is not a design choice,
and will hopefully change in the future.
The following description is only a very incomplete description.
Please consult the source code for an absolute definition.
The search algorithm was designed to accommodate with heavy
forward-pruning eccentricities (such as search inconsistencies).
9.6.2 Board data structure
--------------------------
Fruit uses the 16x12 board. Although this structure is not very
popular, it can be seen as simply combining 10x12 (mailbox) with 16x8
(0x88).
0x88 was picked in Fruit because of the small memory requirements of
vector calculations (much smaller tables). It is possible that Fruit
uses bitboards for pawns in the future.
9.6.3 Search algorithm
----------------------
The main search algorithm is a classical PVS with iterative deepening.
Search enhancements such as a transposition table and null-move pruning
are also used (see below).
A few details in the PVS implementation are not-so-standard and are
there to supposedly enhance the stability of the search (like reducing
the consequences of search inconsistencies). For example the re-search
window after a scout fail high of score "value" (with value > alpha) is
[alpha,beta], not [value,beta]. As another example, I only allow null
move when the static evaluation fails high (i.e. eval() >= beta).
Whether these features improve the strength of the engine is an open
question.
The main search function is full_search() in search_full.cpp
9.6.4 Transposition table
-------------------------
Fruit uses 4 probes and replaces the shallowest entry. Time stamping is
used so that entries from previous searches are considered available for
overwriting.
Enhanced Transposition Cutoff (ETC) is also used 4 plies (and more)
away from the horizon.
9.6.5 Null move
---------------
Fruit uses R=3 recursive null move, even in the endgame.
In Fruit, a precondition to using null move is that the static eval
fails high. One of the consequences of this is that no two null moves
can be played in a row (this is because the evaluation is symmetrical).
This is a usual condition but notice that in Fruit the null-move
condition is "pure" (independent of move paths). The fail-high
condition was selected for other reasons however.
Also, a verification search is launched in the endgame.
9.6.6 Move ordering
-------------------
The move ordering is rather basic:
* transposition-table move
* captures sorted by MVV/LVA
* promotions
* killer moves (two per level, no counters)
* history moves (piece-type/to-square table, with "aging").
9.6.7 Evaluation function
-------------------------
The evaluation function includes:
* Material
* Mobility
* Drawish-material heuristics
* Separated passed-pawn evaluation from the pawn hash table.
Interaction with pieces can be taken into account
* Pawn-shelter penalty; with king placement this forms some sort of a
simplistic king-safety feature
* Incremental move generation
* Futility and delta pruning (not tested in conjunction with history
pruning and hence not activated by default)
* Move ordering (bad captures are postponed)
* History pruning (not tested seriously yet enabled by default)
9.6.8 Speed
-----------
Fruit is not fast (in nodes per second) given the little it is
calculating. Some "optimisations" could be undone in order to make the
code shorter and more flexible.
10 Translations
***************
English is the native language of GNU Chess, as usual in the GNU
Project. Starting on version 6.1.0, GNU Chess features
internationalization support based on GNU 'gettext'. The Translation
Project contributes message strings translated into other languages.
See the Translation Project page for GNU Chess
(http://translationproject.org/domain/gnuchess.html) for the currently
available translations.
During the installation of the operating system, the language to be
used is selected. This selection is made system-wide. An individual
user can select his or her own language by setting the 'locale' through
environment variables. Typically, a command like this (in 'bash') will
set German as the language to be used:
export LANGUAGE=de_DE
See details on this at
Appendix A GNU Free Documentation License
*****************************************
Version 1.3, 3 November 2008
Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
Everyone is permitted to copy and distribute verbatim copies
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0. PREAMBLE
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ADDENDUM: How to use this License for your documents
====================================================
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Concept index
*************
* Menu:
* --addbook: Invoking gnuchess. (line 187)
* --easy: Invoking gnuchess. (line 170)
* --graphic: Invoking gnuchess. (line 191)
* --help: Invoking gnuchess. (line 139)
* --manual: Invoking gnuchess. (line 175)
* --memory: Invoking gnuchess. (line 183)
* --post: Invoking gnuchess. (line 163)
* --quiet: Invoking gnuchess. (line 150)
* --silent: Invoking gnuchess. (line 150)
* --uci: Invoking gnuchess. (line 179)
* --version: Invoking gnuchess. (line 144)
* --xboard: Invoking gnuchess. (line 154)
* -a: Invoking gnuchess. (line 187)
* -e: Invoking gnuchess. (line 170)
* -g: Invoking gnuchess. (line 191)
* -h: Invoking gnuchess. (line 139)
* -m: Invoking gnuchess. (line 175)
* -M: Invoking gnuchess. (line 183)
* -p: Invoking gnuchess. (line 163)
* -q: Invoking gnuchess. (line 150)
* -u: Invoking gnuchess. (line 179)
* -v: Invoking gnuchess. (line 144)
* -x: Invoking gnuchess. (line 154)
* ^C: Command list. (line 210)
* activate: Command list. (line 332)
* adapter: Chess engine protocol adapter.
(line 1035)
* adapter <1>: Structure of the source code.
(line 1057)
* Adapter log file: Output files. (line 826)
* analyze: Command list. (line 457)
* author: Overview. (line 68)
* authors: History. (line 964)
* binary: Structure of the source code.
(line 1104)
* black: Command list. (line 292)
* book: Command list. (line 228)
* Book: Options. (line 598)
* book <1>: Book. (line 920)
* BookFile: Options. (line 609)
* C: C/C++ coexistence. (line 1116)
* C++: C/C++ coexistence. (line 1116)
* CanPonder: Workarounds. (line 796)
* chess engine: Chess engine. (line 1134)
* Chess Engine Communication Protocol: Guidelines followed to develop GNU Chess v6.
(line 1017)
* configuration file: Configuration file. (line 486)
* contact: Contact info. (line 74)
* Cracraft, Stuart: History. (line 964)
* Delta Margin: Options. (line 717)
* Delta Pruning: Options. (line 708)
* depth: Command list. (line 382)
* easy: Command list. (line 304)
* engine: Structure of the source code.
(line 1057)
* EngineCommand: Options. (line 770)
* EngineDir: Options. (line 762)
* EngineName: Options. (line 753)
* environment variable: Environment variables.
(line 477)
* epdload: Command list. (line 392)
* epdsave: Command list. (line 396)
* Evaluation options: Options. (line 735)
* exit: Command list. (line 216)
* first: Command list. (line 262)
* force: Command list. (line 281)
* frontend: Structure of the source code.
(line 1057)
* Fruit: Background of v6. (line 995)
* Fruit <1>: Guidelines followed to develop GNU Chess v6.
(line 1008)
* Futility Margin: Options. (line 699)
* Futility Pruning: Options. (line 689)
* go: Command list. (line 300)
* graphic: Command list. (line 464)
* hard: Command list. (line 309)
* hash: Command list. (line 362)
* help: Invoking gnuchess. (line 128)
* help <1>: Invoking gnuchess. (line 139)
* help <2>: Command list. (line 220)
* history: History. (line 964)
* History Pruning: Options. (line 671)
* History Threshold: Options. (line 679)
* invoking: Invoking gnuchess. (line 128)
* KibitzCommand: Options. (line 585)
* KibitzDelay: Options. (line 591)
* KibitzMove: Options. (line 574)
* KibitzPV: Options. (line 579)
* Kong-Sian, Chua: History. (line 964)
* last: Command list. (line 265)
* Letouzey, Fabien: Overview. (line 68)
* Letouzey, Fabien <1>: History. (line 964)
* Letouzey, Fabien <2>: Background of v6. (line 995)
* level: Command list. (line 387)
* library: Structure of the source code.
(line 1104)
* load: Command list. (line 392)
* Log: Options. (line 530)
* LogFile: Options. (line 536)
* main: Structure of the source code.
(line 1085)
* manual: Command list. (line 281)
* memory: Command list. (line 367)
* modules: Structure of the source code.
(line 1057)
* n: Command list. (line 271)
* name: Command list. (line 324)
* namespaces: C/C++ coexistence. (line 1124)
* new: Command list. (line 345)
* next: Command list. (line 268)
* nographic: Command list. (line 471)
* nopost: Command list. (line 321)
* null: Command list. (line 370)
* NullMove Pruning: Options. (line 630)
* NullMove Reduction: Options. (line 644)
* options: Invoking gnuchess. (line 128)
* otim: Command list. (line 352)
* output: Output files. (line 823)
* overview: Overview. (line 60)
* p: Command list. (line 277)
* pgnload: Command list. (line 254)
* pgnreplay: Command list. (line 257)
* pgnsave: Command list. (line 251)
* pipes: Structure of the source code.
(line 1070)
* PolyGlot: Chess engine protocol adapter.
(line 1035)
* post: Command list. (line 312)
* previous: Command list. (line 274)
* PromoteWorkAround: Workarounds. (line 814)
* protocol: Chess engine protocol adapter.
(line 1035)
* Quiescence Check Plies: Options. (line 724)
* quit: Command list. (line 213)
* random: Command list. (line 356)
* rating: Command list. (line 342)
* remove: Command list. (line 408)
* Resign: Options. (line 545)
* ResignMoves: Options. (line 555)
* ResignScore: Options. (line 562)
* result: Command list. (line 329)
* save: Command list. (line 396)
* show: Command list. (line 414)
* ShowPonder: Options. (line 568)
* solve: Command list. (line 405)
* solveepd: Command list. (line 405)
* src: Structure of the source code.
(line 1085)
* src/adapter: Structure of the source code.
(line 1095)
* src/engine: Structure of the source code.
(line 1100)
* src/frontend: Structure of the source code.
(line 1089)
* Stanback, John: History. (line 964)
* Stuart, Cracraft: Overview. (line 68)
* switch: Command list. (line 399)
* SyncStop: Workarounds. (line 805)
* test: Command list. (line 438)
* time: Command list. (line 348)
* uci: UCI chess engine. (line 897)
* UCI: Guidelines followed to develop GNU Chess v6.
(line 1023)
* UCIVersion: Workarounds. (line 790)
* undo: Command list. (line 411)
* Universal Chess Interface: Guidelines followed to develop GNU Chess v6.
(line 1023)
* usage: Invoking gnuchess. (line 128)
* usage <1>: Command list. (line 223)
* Verification Reduction: Options. (line 661)
* Verification Search: Options. (line 652)
* version: Invoking gnuchess. (line 144)
* version <1>: Command list. (line 246)
* version 5: Adapter. (line 975)
* version 5 <1>: Background of v6. (line 995)
* white: Command list. (line 287)
* winboard: XBoard chess engine.
(line 877)
* XBoard: Overview. (line 60)
* xboard: Invoking gnuchess. (line 154)
* xboard <1>: Command list. (line 375)
* xboard <2>: XBoard chess engine.
(line 877)