\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename gnuchess.info
@include version.texi
@settitle GNU Chess @value{VERSION}

@c Define a new index for options.
@defcodeindex op
@c Combine everything into one index (arbitrarily chosen to be the
@c concept index).
@syncodeindex op cp
@c %**end of header

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

Copyright @copyright{} 2001--2011 Free Software Foundation, Inc.

@quotation
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''.
@end quotation
@end copying
@c If your manual is published on paper by the FSF, it should include
@c the standard FSF Front-Cover and Back-Cover Texts, as given in
@c maintain.texi.

@dircategory Basics
@direntry
* GNU Chess: (gnuchess).               Playing chess and related games.
@end direntry

@titlepage
@title GNU Chess
@subtitle for version @value{VERSION}, @value{UPDATED}
@author @url{http://www.gnu.org/software/chess}
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents


@ifnottex
@node Top
@top GNU Chess

This manual is for GNU Chess (version @value{VERSION}, @value{UPDATED}).

@insertcopying
@end ifnottex

@menu
* Overview::		General information.
* Contact info::	How to find help and to report bugs.
* Invocation::		Running @command{gnuchess}.
* Book::		Where and how to obtain a chess book usable by GNU Chess.
* Tests::		Some execution results.
* Chess notation::	Auxillary file formats (PGN, EPD).
* History::		A brief review of the long history of GNU Chess.
* Known problems::	Known problems
* Developers::		Notes for developers.
* GNU Free Documentation License:: Copying and sharing this documentation.
* Concept index::	Index of concepts.
@end menu


@node Overview
@chapter Overview

@cindex XBoard
@cindex overview

GNU Chess (@url{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.

@cindex author
@cindex Letouzey, Fabien
@cindex Stuart, Cracraft
The main author of GNU Chess version 6 is Fabien Letouzey,
The original author is Stuart Cracraft.


@node Contact info
@chapter Contact info

@cindex contact

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

	@email{bug-gnu-chess@@gnu.org}

Our official web page is:

        @uref{http://www.gnu.org/software/chess}

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

	@uref{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:

	@uref{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:

@itemize

@item
Version of the program.  The following command will print it:

@example
gnuchess --version
@end example

@item
How you started the program.  Whether running standalone or in 
combination with other programs such as XBoard.

@item
The actions you performed and the output or behaviour you observed.

@item
Output files if any.  In order to enable the writing to adapter.log,
set the following option in config file gnuchess.ini:

	@option{Log = true}

@item
Whether the problem is systematic (it always happens) or occasional.

@item
Any other information you may deem relevant.

@end itemize


@node Invocation
@chapter Running @command{gnuchess}

@menu
* Invoking gnuchess::
* Interactive game::
* Command list::
* Environment variables::
* Configuration file::
* Output files::
* XBoard chess engine::
* UCI chess engine::
* Internet::
@end menu


@node Invoking gnuchess
@section Invoking gnuchess

@cindex invoking
@cindex options
@cindex usage
@cindex help

The format for running the @command{gnuchess} program is:

@example
gnuchess @var{option} @dots{}
@end example

With no options, @command{gnuchess} starts in interactive mode and
it is ready to start a chess game.

@command{gnuchess} supports the following options:

@table @option
@item --help
@itemx -h
@opindex --help
@opindex -h
@cindex help
Print an informative help message on standard output and exit
successfully.

@item --version
@itemx -v
@opindex --version
@opindex -v
@cindex version
Print the version number and licensing information of Hello on
standard output and then exit successfully.

@item --quiet
@item --silent
@itemx -q
@opindex --quiet
@opindex --silent
@opindex -q
Make the program silent on startup.

@item --xboard
@itemx -x
@opindex --xboard
@opindex -x
@cindex xboard
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 @option{xboard} is accepted without leading dashes for backward
compatibility.

@item --post
@itemx -p
@opindex --post
@opindex -p
Start up showing thinking.  

Option @option{post} is accepted without leading dashes for backward
compatibility.

@item --easy
@itemx -e
@opindex --easy
@opindex -e
Disable thinking.  in opponent's time.  By default, the program runs in
hard mode, i.e.  it thinks opponent's time to think too.

@item --manual
@itemx -m
@opindex --manual
@opindex -m
Enable manual mode.

@item --uci
@itemx -u
@opindex --uci
@opindex -u
Enable UCI protocol (externally behave as UCI engine).

@item --memory size
@itemx -M size
@opindex --memory
@opindex -M
Specify memory usage in MB for hashtable.

@end table


@node Interactive game
@section Interactive game

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

@example
        Nf3
        g1f3
        O-O
        e1g1
@end example


@node Command list
@section Command list

@command{gnuchess} supports the following commands:

@table @command
@item ^C
@cindex ^C
Quit the program.

@item quit
@cindex quit
Quit the program.

@item exit
@cindex exit
In analysis mode this stops analysis, otherwise it quits the program.

@item help
@cindex help
Produces a help blurb corresponding to this list of commands.

@item usage
@cindex usage
Produce blurb on command line options.

(Same as @command{gnuchess --help})

@item book
@cindex 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)

@item version
@cindex version
Prints out the version of this program

(Same as @command{gnuchess --version})

@item pgnsave FILENAME
@cindex pgnsave
Saves the game so far to the file from memory

@item pgnload FILENAME
@cindex pgnload
Loads the game in the file into memory

@item force
@itemx manual
@cindex force
@cindex manual
Makes the program stop moving.  You may now enter moves
to reach some position in the future.

(Same as @command{gnuchess --manual})
   
@item white
@cindex white
Program plays black, set white to move.

Note: not implemented in this version.

@item black
@cindex 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.

@item go
@cindex go
Computer takes whichever side is on move and begins its
thinking immediately

@item easy
@cindex easy 
Disables thinking on opponent's time

(Same as @command{gnuchess --easy})

@item hard
@cindex hard
Enables thinking on opponent's time

@item post
@cindex 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 @command{gnuchess --post})

@item nopost
@cindex nopost
Turns off verbose thinking output

@item name NAME
@cindex 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.

@item result
@cindex result
Mostly used by Internet Chess server.

@item activate
@cindex 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.

@item rating COMPUTERRATING OPPONENTRATING
@cindex rating
Inputs the estimated rating for computer and for its opponent

@item new
@cindex new
Sets up new game (i.e.  positions in original positions)

@item time
@cindex time
Inputs time left in game for computer in hundredths of a second.
Mostly used by Internet Chess server.

@item otim
@cindex otim
Inputs time left in game for opponent in hundredths of a second.
Mostly used by Internet Chess server.

@item random
@cindex random
Randomizes play by perturbing the evaluation score slightly.
The degree of perturbation is adjustable.

Note: not implemented in this version.  Neither in v5

@item hash
@cindex hash
on - enables using the memory hash table to speed search

off - disables the memory hash table

@item memory N
@cindex memory
Sets the hash table to permit storage of N MB.

@item null
@cindex null
on - enables using the null move heuristic to speed search

off - disables using the null move heuristic

@item xboard
@cindex xboard
on - enables use of xboard/winboard

off - disables use of xboard/winboard

(Same as @command{gnuchess --xboard})

@item depth N
@cindex depth
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 

@item level MOVES MINUTES INCREMENT
@cindex level
Sets time control to be MOVES in MINUTES with each move giving
an INCREMENT (in seconds, i.e.  Fischer-style clock).

@item load
@itemx epdload
@cindex load
@cindex epdload
Loads a position in EPD format from disk into memory.

@item save
@itemx epdsave
@cindex save
@cindex epdsave
Saves game position into EPD format from memory to disk.

@item switch
@cindex switch
Switches side to move
 
Note: not implemented in this version.

@item solve FILENAME
@itemx solveepd FILENAME
@cindex solve
@cindex solveepd
Solves the positions in FILENAME

@item remove
@cindex remove
Backs up two moves in game history

@item undo
@cindex undo
Backs up one move in game history

@item show
@cindex 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.

@item test
@cindex 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.

@item analyze
@cindex 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.	

@end table


@node Environment variables
@section Environment variables

@cindex environment variable

If @env{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.


@node Configuration file
@section Configuration file

@cindex configuration file

@menu
* Structure::
* Options::
* Workarounds::
@end menu

@node Structure
@subsection Structure

A file called @file{gnuchess.ini} is used to define configuration options.
The file is applicable provided @option{--uci} is not used.
The file is looked in three places according to the following precedence:

@enumerate
@item
The directory where the program was started
@item
Environment variable@env{GNUCHESS_PKGDATADIR}
@item
The package data directory stated at configure time
@end enumerate

Sections are composed of @option{variable = value} lines.

Note: There can be spaces in variable names or values.  Do not use
quotes.

@option{[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.

@option{[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 @option{OwnBook = true} is
correct.  It will be replaced by the adapter with @option{setoption name OwnBook
value true} at engine startup.

Standard UCI options are @option{Hash}, @option{NalimovPath}, 
@option{NalimovCache} and @option{OwnBook}.  Hidden options like @option{Ponder}
or @option{UCI_xxx} are automatic and should not be put in the INI file.

@node Options
@subsection Options

These should be put in the @option{[Adapter]} section.

@table @code
@item Log
@cindex Log
Default: false

Whether the adapter should log all transactions with the interface and
the engine.  This should be necessary only to locate problems.


@item LogFile
@cindex LogFile
Default: @file{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.

@item Resign
@cindex 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).

@item  ResignMoves
@cindex  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.

@item ResignScore
@cindex ResignScore
Default: 600

This is the score in centipawns that will trigger resign "counting".

@item ShowPonder
@cindex ShowPonder
Default: true

Show search information during engine pondering.  Turning this off
might be better for interactive use in some interfaces.

@item KibitzMove
@cindex KibitzMove
Default: false

Whether to kibitz when playing a move.

@item KibitzPV
@cindex KibitzPV
Default: false

Whether to kibitz when the PV is changed (new iteration or new best move).

@item KibitzCommand
@cindex KibitzCommand
Default: tellall

xboard command to use for kibitzing, normally "tellall" for kibitzing
or "tellothers" for whispering.

@item KibitzDelay
@cindex 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.

@item Book
@cindex 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
@code{OwnBook} in the @option{[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.

@item BookFile
@cindex BookFile
Default: @file{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 @var{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.

@end table

These are UCI options for the @option{[Engine]} section:

@table @code
@item NullMove Pruning
@cindex 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.

@item NullMove Reduction
@cindex NullMove Reduction
(1-3 plies)

Default: 3

3 is rather aggressive, especially in the endgame.  It seems better
than always using 2 though.

@item Verification Search
@cindex 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.

@item Verification Reduction
@cindex 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".

@item History Pruning
@cindex 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.

@item History Threshold
@cindex 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.

@item Futility Pruning
@cindex 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.

@item Futility Margin
@cindex 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.

@item Delta Pruning
@cindex 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.

@item Delta Margin
@cindex Delta Margin
(centipawns)

Default: 50

Same behaviour as futility margin.  This one is probably safe.

@item Quiescence Check Plies
@cindex 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.

@item Evaluation options
@cindex 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?

@end table

The following options were used in PolyGlot v1.4, but are deprecated in 
GNU Chess:

@table @code
@item EngineName
@cindex 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.

@item EngineDir
@cindex 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.

@item EngineCommand
@cindex 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.

@end table

@node Workarounds
@subsection 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.

@table @code
@item UCIVersion
@cindex UCIVersion
Default: 2

The default value of 2 corresponds to UCI+.  Use 1 to select plain
UCI for engines that have problems with UCI+.

@item CanPonder
@cindex 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 @option{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.

@item SyncStop
@cindex 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.
@option{SyncStop} forces the adapter to wait for the (now useless) ponder search
to finish before launching the new search.

@item PromoteWorkAround
@cindex 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).

@end table

@node Output files
@section Output files

@cindex output
GNU Chess produces several output files:

@table @file

@item Adapter log file
@cindex Adapter log file
This file is written by the adapter.  The name of the file is
specified by the following option in @file{gnuchess.ini}:

@option{LogFile = FILENAME}

The file is produced if option @option{--uci} is not specified
and if the following variable is set in @file{gnuchess.ini}:

Typically, this file is named @file{adapter.log}, but any other
name will do the job.

@option{Log = true}

@item @file{log.nnn}
This file is written if command @command{name} was requested.  
The contents are the opponent's name and the game in 
coordinate algebraic notation.

@item @file{game.nnn}
This file is written if command @command{name} was requested.  
The contents are the opponent's name and the game in 
portable game notation (PGN).

@item @file{players.dat}
This file is written if command @command{name} was requested.  
The contents are the statistics of games.  This is the format of
each line:

@command{opponent-name wins loses draws}

@item @file{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 @command{DEBUG}.  Hence,
when installing the program, instead of

@example
./configure
make
make install
@end example

Use the following commands:

@example
./configure CFLAGS=-DDEBUG
make
make install
@end example

When analysing  a bug, this file could be very helpful.
Users are encouraged to provide it.

@end table


@node XBoard chess engine
@section XBoard chess engine

@cindex xboard
@cindex winboard
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:

@example
xboard -fcp 'gnuchess --xboard'

xboard -fd . -fcp './gnuchess --xboard'
@end example


@node UCI chess engine
@section UCI chess engine

@cindex uci
For GNU Chess to  behave as a pure UCI chess engine, execute
the following command:

@example
gnuchess --uci
@end example

In this mode, configuration file @file{gnuchess.ini} is ignored.


@node Internet
@section Internet

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

@uref{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:

@example
xboard -zp -ics -icshost freechess.org -icshelper timeseal -fcp 'gnuchess --xboard'
@end example

@node Book
@chapter Book

@cindex book
See options @option{Book} and @option{Bookfile} in Running GNU Chess - Configuration file - Options


@node Tests
@chapter Tests

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

@node Chess notation
@chapter Auxiliary file formats

@table @file
@item .bin
binary book format

@item .pgn
game listing like @command{1.  e4 e5 2.  Nf3} etc.

@item .epd
epd-style format using FEN notation.  See tests subdirectory for example.

@item log.nnn
record of an entire game from computer's viewpoint (thinking, etc.)

@item game.nnn
record of an entire game, similar to .pgn but auto-generated

@end table

The @file{.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 @file{log.nnn} and @file{game.nnn} files are written at the end of a game
when you use the @command{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.


@node History
@chapter History

@cindex history
@cindex authors
@cindex Cracraft, Stuart
@cindex Stanback, John
@cindex Kong-Sian, Chua
@cindex Letouzey, Fabien

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.


@node Known problems
@chapter Known problems

@menu
* Adapter::
@end menu

@node Adapter
@section Adapter

@cindex version 5
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.


@node Developers
@chapter Developers

@menu
* Background of v6::
* Guidelines followed to develop GNU Chess v6::
* Chess engine protocol adapter::
* Structure of the source code::
* C/C++ coexistence::
* Chess engine::
@end menu

@node Background of v6
@section Background of v6

@cindex version 5
@cindex Fruit
@cindex Letouzey, Fabien
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.

@node Guidelines followed to develop GNU Chess v6
@section Guidelines followed to develop GNU Chess v6

@cindex Fruit
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.  

@cindex Chess Engine Communication Protocol
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:

@uref{http://home.hccnet.nl/h.g.muller/engine-intf.html}

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

@uref{http://wbec-ridderkerk.nl/html/UCIProtocol.html}

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

@node Chess engine protocol adapter
@section Chess engine protocol adapter

@cindex PolyGlot
@cindex adapter
@cindex protocol
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.

@node Structure of the source code
@section Structure of the source code

@cindex modules
@cindex frontend
@cindex adapter
@cindex engine
We want to keep three loosely-coupled modules in GNU Chess v6:

@itemize
@item
frontend
@item
adapter
@item
engine
@end itemize

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:

@itemize
@item
frontend <--> adapter
@item
adapter  <--> engine
@end itemize

@cindex pipes
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 @file{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:

@table @file

@item src
@cindex src
@cindex @command{main}
Contains the @command{main} and a source file used to create the pipes that
communicate the three modules: frontend, adapter and engine.

@item src/frontend
@cindex src/frontend

Contains the frontend.  The code here is inherited from GNU Chess v5, with some
modifications mainly in @file{cmd.c} and a new file @file{engine.c} which 
addresses the message passing through modules.

@item src/adapter
@cindex src/adapter

Contains the chess protocol adapter, based on PolyGlot 1.4.  Minor changes, wrt
the baseline.

@item src/engine
@cindex src/engine

Contains the chess engine, based on Fruit 2.1.  Minor changes wrt the baseline.

@end table

@cindex binary
@cindex library
The sources in each @file{src} subdirectory are compiled as a static library.  
When link with the sources in top @file{src}, they result into the binary 
@command{gnuchess}.  The module libraries are named after their respective 
directory name:

@example
libfrontend.a
libadapter.a
libengine.a
@end example

@node C/C++ coexistence
@section C/C++ coexistence

@cindex C
@cindex C++
There is no problem in having both C and C++ in the same program.  However, 
there is no technical reason for this heterogeneity.  The only reason is the
different origins of the code.  It is desirable to get a more homogeneous
program in the future.

@cindex namespaces
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 compiled as C code.


@node Chess engine
@section Chess engine

@cindex chess engine

@menu
* Fruit overview::
* Board data structure::
* Search algorithm::
* Transposition table::
* Null move::
* Move ordering::
* Evaluation function::
* Speed::
@end menu

@node Fruit overview
@subsection 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).

@node Board data structure
@subsection 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.

@node Search algorithm
@subsection 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

@node Transposition table
@subsection 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.

@node Null move
@subsection 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.

@node Move ordering
@subsection Move ordering

The move ordering is rather basic:

@itemize

@item transposition-table move
@item captures sorted by MVV/LVA
@item promotions
@item killer moves (two per level, no counters)
@item history moves (piece-type/to-square table, with "aging").
@end itemize

@node Evaluation function
@subsection Evaluation function

The evaluation function includes:

@itemize

@item Material
@item Mobility
@item Drawish-material heuristics
@item Separated passed-pawn evaluation from the pawn hash table.
Interaction with pieces can be taken into account
@item Pawn-shelter penalty; with king placement this forms
some sort of a simplistic king-safety feature
@item Incremental move generation
@item Futility and delta pruning (not tested in conjunction with
history pruning and hence not activated by default)
@item Move ordering (bad captures are postponed)
@item History pruning (not tested seriously
yet enabled by default)
  
@end itemize

@node Speed
@subsection 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.


@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl.texi


@node Concept index
@unnumbered Concept index

@printindex cp

@bye
