Thursday, 14 March 2013

FEN Strings

I've made some progress yesterday.  I have an elementary bitboard representation working and I can set and get the chess board using a FEN string.  A FEN String essentially represents the current board setup in a convenient notation.  Here is an example after white's initial e4:

rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1

Each string in between the slashes represents a rank in the chessboard, starting from the 8th rank. Lower case is black, upper case is white. Numbers represents empty squares between pieces.  For example '4P3' represents four empty squares, followed by a pawn, followed by three more empty squares, totaling eight.  The '/8/' therefore means (you've guessed it!) a complete empty rank.

The remaining string 'b KQkq e3 0 1' represents the side to move, the castling rights (e.g. 'K' means white can caste king-side, 'q' that black can caste queen-side, etc.), en-passant square and the move counters.  The en-passant square is the square directly behind the pawn that made the two-square move.  In this case, white move e4, which means that e3 is the en-passant square.  Black can capture the pawn if he had a pawn on either d4 or f4.  Regardless, the en-passant square is always indicated, whether it can be taken or not.  The move counts are the half-moves since a last capture or pawn move (for the 50-move draw rule), followed by the number of full moves.

I found myself constantly checking the validity of the FEN string during the parsing stage, which started to look really ugly.  So I turned to one of my best friends... regular expressions!

Regular expressions can get tricky...quickly, but is incredibly powerful to recognise patterns in strings.  Here is a short extract from Vicki that shows how a FEN string can be checked:

The 'verifyRank()' method simply verifies that the a rank adds up to eight. Doing that in a regular expression may get very hairy (actually I don't think it can be done). The code also verifies that there are 2 kings at least.  Of course, that can easily be done by the regular expression, but the string will get rather long. 

Tuesday, 12 March 2013

Move generation

The first step is move generation.  I have committed myself to Java and bitboards for Vicki 2.0.  The advantage of Java is that it is much more platform independent than C... well, mostly.  Bitboards is also a good move, given that it is theoretically faster than the postbox method and besides, it is something interesting to learn.

So the list of tasks for the next few days is as follows:

1. Set up data structures Done
2. Set and get board (FEN-based) Done
3. Simple textual display Done
4. Move generation for non-sliders (pawns, knights and kings) In progress...
5. Move generation for sliders (bishops, rooks and queens) Pending
6. Make move Pending
7. Unmake move Pending
8. Perft testing Pending

For now, the representation simply contains positions for each of the pieces.  The derived pieces will be added later.

Wednesday, 06 March 2013

Something stirring in the shadows...

I have only fond memories of Vicki...  The website is still up, but Vicki has not used a clock cycle in years.  The current release is several years old... written in C.

I am currently (as in already busy) rewriting Vicki.  Vicki 2.0 is a complete rewrite of the engine, reusing nothing but the name.

I am not looking at any existing computer chess code and I am avoiding any heavy bias to single papers and single units of theory - trying my own. The research in computer chess is all but exhausted.  So all that is left is to redesign the wheel for the fun of it and looking at existing code will spoil it for me... Vicki is not meant to "extend" the envelope of computer chess.  It's my Everest and South Pole.  

Vicki 2.0 is being written in Java.  Java's speed is comparable to C and C++ (in some people's opinions!) Also, I want something which is easily transferable between Linux and Windows platforms.  C does not really cut it for me any more.

Board representation will the elusive bitboard (or bitmap) technique.  It cannot be that hard... (famous last words!) and I cannot ignore it's advantages any longer.

I hope to post regularly and have something on the main website up and running soon.