     [file version.texi not included]

GNU Chess
*********

This manual is for GNU Chess (version VERSION, UPDATED).

This manual is for GNU Chess (version VERSION, UPDATED), which is a complete chess program, frequently used as a chess engine.

Copyright © 2001--2020 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''.

Overview
********

GNU Chess (http://www.gnu.org/software/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.

Contact info
************

We are the GNU Chess developers and you may reach us at:

bug-gnu-chess@gnu.org

Our official web page is:

http://www.gnu.org/software/chess

We are indebted to our sponsor, the Free Software Foundation whose web page is:

http://www.fsf.org

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:

ftp://ftp.gnu.org/gnu/chess/

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.

Running gnuchess
****************

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.

Interactive game
================

Moves are accepted either in standard algebraic notation (SAN) or in coordinate algebraic notation.  Examples:

             Nf3
             g1f3
             O-O
             e1g1

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.

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.

Configuration file
==================

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 variableGNUCHESS_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.

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.

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).

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.

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 and WinBoard, the graphical display front-ends 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 chess backend:

     xboard -fcp 'gnuchess --xboard'

     xboard -fd . -fcp './gnuchess --xboard'

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.

Internet
========

For GNU Chess to run in Internet, Zippy is required. See Zippy documentation in the XBoard/WinrBoard distribution:

http://www.tim-mann.org/

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'

Book
****

See options Book and Bookfile in Running GNU Chess - Configuration file - Options

Tests
*****

GNU Chess 6 has been tested on the Free Internet Chess Server (http://www.freechess.org) with XBoard.

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.

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.

Known problems
**************

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.

Developers
**********

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.

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:

http://home.hccnet.nl/h.g.muller/engine-intf.html

Since Fruit uses the Universal Chess Interface (UCI), keeping the former interface was the main challenge of GNU Chess v6.  UCI is described here:

http://wbec-ridderkerk.nl/html/UCIProtocol.html

GNU Chess can also run as a UCI chess engine, if the --uci command-line option is specified.

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.

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

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.

Chess engine
============

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).

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.

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

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.

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.

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").

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)

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.

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 http://www.gnu.org/software/gettext/manual/gettext.html#Users

GNU Free Documentation License
******************************

     [file fdl.texi not included]