readme

一款运行在linux上的象棋游戏。用GTK/GNOME环境下用GLADE开发。

README
GNU CHESS 5
by Stuart Cracraft <[e-mail]>
copyright (c) 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993
1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001
Modified Simon Waters <[e-mail]> 2001
Modified Simon Waters <[e-mail]> 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 

	[e-mail]

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