README
GNU CHESS 5
by Stuart Cracraft <cracraft@gnu.org>
copyright (c) 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993
1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001
Modified Simon Waters <simon@wretched.demon.co.uk> 2001
Modified Simon Waters <simon@wretched.demon.co.uk> 2003
IMPORTANT: Please send all updates to Simon at the above address.
Table of Contents
Introduction
Who We Are
Data Structures
Move Generator
Search
Evaluation
Book
Hash Table
Auxillary File Formats (PGN, EPD)
Universal Board
Caveats
Compilers
Internet
Xboard/Winboard
Command List
INTRODUCTION
Welcome to the GNU CHESS 5 README. This is somewhat different than
a normal readme. You might consider this a manual. We've always found
multiple documents confusing, overlapping and sometimes contradictory
as far as software documentation goes. By putting it all together in
one place we hope to avoid these traditional inadequacies and be able
to maintain a single document. If you add documentation, add it to
this document only.
GNU Chess 5 is the new version of GNU Chess. The goal of creating this
new version was to eliminate perceived problems with past versions, the
major one being spaghetti-code that was extremely poorly documented, but
another being antiquated data structures and especially the ignominous
linked list. Another good reason was to have more standard file formats
for game positions and game listings.
WHO WE ARE
We are the GNU Chess developers and you may reach us at
bug-gnu-chess@gnu.org
We are indebted to our sponsor, the Free Software Foundation
whose web page is:
http://www.gnu.org
and which also serves as our software depository for new versions of
GNU and GNU Chess.
We also have a Usenet bulletin board, gnu.chess. Feel free to post and
support. Please become a developer and contribute your time and coding skill
to GNU Chess. Make a donation of your time and money.
But, as developers we like to develop our own ideas. Thus, if you have
an idea check to see that no one else is working on it (posting on the
above bulletin board or sending an email should be sufficient to find
if someone is working on the idea and if you can collaborate with
them.)
We don't like messages asking us to implement features. Everybody
has a list a mile long. Instead, contribute by writing code or pointing
out very clearly a bug. To report a bug, tell us the version number
of the program ("./gnuchess --version").
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.
DATA STRUCTURES
The primary data structure of GNU Chess is the bitboard. A bitboard
is a 64-bit GNU C long long. It represents characteristics of a position.
For example, 12 bitboards are sufficient to describe the whereabouts
of all the pieces for both sides, i.e.
BitBoard board.b[2][6];
So for example with a knight equal to 2 and white equal to 0 all the
knights are located by the reference
#define white 0
#define knight 2
... board.b[white][knight] ...
Testing whether a particular square has a knight on it could be done
with
if (BitBoard[B1] & board.b[white][knight]) { ... }
Another set of move arrays is helpful for calculating the simple moves
of a knight or a king
MoveArray[knight or king][sq]
This returns a bitmap of the squares from which a knight or king
could move from the square sq. Squares are based at 0 for a1 (White's
queen's rook 1) and numbered left to right up until 63 for h8 (Black's
king's rook 1).
Another basic data structure is the board. It is defined in common.h
as the Board typedef:
typedef struct
{
BitBoard b[2][7]; /* Pieces by side (0 - white, 1 black
by piece (1 - pawn ... 6 - king */
BitBoard friend[2]; /* Friendly (this side's) pieces */
BitBoard blocker; /* Enemy pieces */
BitBoard blockerr90;
BitBoard blockerr45;
BitBoard blockerr315;
short ep; /* Location of en passant square */
short flag; /* Relevant flags relating to castle privs */
short side; /* Color of side on move 0 - white 1 - black */
short material[2]; /* Total material by side not inc. king */
short pmaterial[2]; /* Total pawn material by side not inc. king */
short castled[2]; /* True (1) if side is castled */
short king[2]; /* Location of king 0 - a1 .. 63 - h8 */
} Board;
Basic data structure typedefs are defined in common.h and allocated in
main.c for the most part. Please read and understand those files. The
best way to understand data structures is to add new evaluation terms.
MOVE GENERATOR
This is a rotated bit-board method which is considered state-of-the-art
currently.
SEARCH
Based on Professor Tony Marsland's modification to alpha-beta minimax,
called Principal Variation Search (PVS), this algorithm performs credibly.
EVALUATION
Evaluation in this version is quite a bit different than before.
Earlier versions used piece/square tables with some end-leaf
evaluation (but primary pc/sq tables). These are tables filled with
values regarding the importance of having pieces on particular squares.
It was filled once, at the beginning of the search.
The drawback of pc/sq tables is that the information is typically of
less and less importance the deeper a program searches because the
board changes so much. With computers getting faster and faster, deeper
and deeper searches are possible and so the pc/sq tables can provide
misleading direction to the program, resulting in anti-positional moves.
More recently there has been a return by some to what we espouse here:
full end-leaf evaluation. Further, we use bitboards (64-bit quantities)
to represents characteristics of the board. This harkens back, ironically
to the early days of computer chess when giant number-crunching machines
back in the 60's used bitmaps to describe positions.
Bitboards in this version of GNU are defined using the "BitBoard" typedef
defined in common.h. main.c contains most of the bitboards and these
are accessed and used throughout the program and particularly by
the move generator and the evaluator.
The evaluator in eval.c consists of a series of scoring routines like
ScoreP (), ScoreN (), ScoreB (), corresponding to the piece being
scored. The routine evaluates all pieces of that type (P - pawn,
N - knight, B - bishop, etc.) on the current board and returns a
score.
Typically a loop is used of the form
short sq; /* Location of the piece of this type */
short s; /* Score value for all pieces
BitBoard b; /* Stores the bitboard representing location of the piece */
s = 0; /* Score starts out as zero */
b = board.b[side][knight];
while (b) {
sq = leadz(b);
CLEARBIT (b, sq);
if (piece on sq has some property)
s += SOME_BONUS_OR_PENALTY; /* defined in eval.h */
}
return(s);
As you can see, this routine locates each piece in the 64-bit map
where the corresponding square of the 64 is set to 1 meaning a piece
is there. Hence for example in the opening position, board.b[white][bishop]
would have the 3rd and 7th low-order bits set corresponding to the original
locations of bishops in a game on C1 and F1. Likewise the values
BitPosArray[C1] and BitPosArray[F1] can be used to return 64-bit
quantities for checking specific matches as in
if (BitPosArray[A1] & board.b[side][bishop])
s += SOME_VERY_NEGATIVE_PENALTY_FOR_BISHOP_IN_A1_CORNER
Writing evaluation code comes very naturally using these methods. Try
to avoid too many specific square checks as those are expensive. Ideas
as shown in the CTL() routine can be used to check for piece placement
on specific squares being advantageous or disadvantageous.
Primary evaluation is done with Evaluate(). Certain specifics are
calculated which are deemed very important such as king evaluation
and pawn evaluation. Then a "lazy evaluation" scenario is checked
for which can save time. Otherwise other pieces are also evaluated.
Very important for evaluation is the ability to see what board you
are evaluating. Typically this should be sufficient when you add
the new term:
/* ... new logic ... */
{
s += SOME_NEW_BONUS (define in eval.h)
printf("The condition is triggered:\n");
ShowBoard ();
getchar();
}
This lets you see the board at the point the condition is triggered
which adds the bonus or penalty to the evaluation score.
BOOK
The opening book is implemented as a simple binary file consisting of
a set of sequential records of struct hashtype as defined in the module
book.c. This data structure is simply two part, a 64-bit HashType (see
common.h) and a 32-bit score.
The binary book stored in book.dat is compiled from the file book.pgn
using the command "book add book.pgn" into a sequential set of binary
records in the format as described above. book.pgn is simply a set of
game records in portable game notation format. A set of master games
may be used or specific openings programmed this way for a user-changeable
opening book.
HASH TABLE
The hash table is simply an area of memory where information about
positions is stored. In this version of the program there are two
types of hash tables: general and pawn.
The general hash table size is controlled by the variable HASHSLOTS
as defined in common.h. Likewise the pawn hash table size is controlled
by the variable PAWNSLOTS in common.h.
The number of hashtable slots can be controlled from the command
line (Type "gnuchess -help" for details), or via the interactive
hashsize command.
Typically middle-game searches are sped up by 25%-50% by the general
hash table and by much more in endgames where there are few pieces
(because so many of the positions turn out to be cached already in
the hash table.)
Pawn evaluation is traditionally expensive because there are so many
things to evaluate. The pawn hash table remembers all the different
pawn structures in the search. Typically pawn structure evaluation
(without reference to pieces) may be calculated by simple table
lookup this way 90-99% of the time. Hence, any amount of pawn logic
that is pure-pawn and not related to pieces may be added without guilt.
On the other hand, pawn structure that relates to pieces must be
recalculated for every position. See ScoreP() in eval.c
AUXILLARY FILE FORMATS
.dat - binary book format, simply a 64-bit position hash and a 32-bit score
.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 .dat file format is a simple binary format for the compiled book
which is read by the program when it is using book. See the section BOOK
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.
COMPILERS
We like GNU C in all its various forms. For Unix and Linux, use GNU C.
For Microsoft Windows platforms we compile and test using Cygwin,
which is a port of many GNU packages, including GCC.
Cygwin may require specific run time DLL's to provide the interface,
these should be provided with any executable you receive.
Whilst GCC is the supported compiler, a key goal is portability. If
you experience problems compiling GNU Chess with a modern C compiler
please let the developers know.
INTERNET
GNU CHESS 5 has been tested substantially on the Free Internet Chess
Servers (freechess.org) with Xboard (See Zippy documentation in the
Xboard/Winboard distribution http://www.tim-mann.org/).
GNU Chess 5.06 and later should also operate with icsDrone. Testing
5.06 with icsDrone 1.5.0 showed no immediate issues.
XBOARD/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.
COMMAND LIST
^C
Typically the interrupt key stops a search in progress,
makes the move last considered best and returns to the
command prompt
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
add - compiles book.dat 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
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.
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).
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.
(Also "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.
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 (NOT IMPLEMENTED)
Mostly used by Internet Chess server.
random (NOT IMPLEMENTED)
Randomizes play by perturbing the evaluation score slightly.
The degree of perturbation is adjustable.
hash
on - enables using the memory hash table to speed search
off - disables the memory hash table
hashsize N
Sets the hash table to permit storage of N positions
N is rounded down to nearest power of 2.
(Also "gnuchess --hashsize=N")
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
(Also "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
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
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
test
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
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.
