Chesslin

by Baudsurfer / Red Sector Inc.

This is a size-coding exercise to code a playable chess engine in 256 bytes.

This proof-of-concept is very experimental and bears several shortcomings when comparing with any other real FIDE existing chess game engine - you have been warned.

It plays like a fish as AI is reduced to a half-ply max solely.  It also has no end-game detection, pawns move only a single square, it cannot castle or do promotions - let alone en-passant - and takes about a hundred seconds to play.  It also only works on Microsoft Windows XP SP3.  Like the minimalist Edlin line editor, Cheesslin focuses on a single console line.  Whites start at the bottom of the virtual chess board, but SAN notation order is inverse ranks:

A B C D E F G H 1 r n b q k b n r 2 p p p p p p p p 3 4 5 6 7 P P P P P P P P 8 R N B Q K B N R

So in order to test Chesslin, one can uudecode the below binaries to input first algebraic notation h7 h6 characters: starting the game by moving the white pawn on H file from seventh rank to sixth rank.

A longer example string sequence of gameplay is: h7 h6 h2 h3 g8 f6 h3 h4 f6 g4 h4 h5 g4 h2 g1 h3 h2 f1 h3 g5

Remember, if your keyboard input is not legal chess, then Chesslin will silently expect you to enter again a conforming four ASCII character string just to proceed.  Thus, if only a single faulty character was entered, you will need to fill in with three more "dummy" characters before retyping a desired algebraic notation because validation only occurs every four characters exactly.

All bugs are ofc mine.

; "You don't need eyes to see You need vision." - Faithless _ ; Special greets to : Impure ASCII 1940 and Divine Stylers! | | ; Greets : Alco Bon^2 BReWErS CODEX Flush Lineout Mandarine .--' `--. ; Onslaught Paranoimia Quartex Rebels Razor1911 RiOT Titan. `--. .--' ; _ _ _ _ _ ___| |___ ; ______)\___ )\_________)\_______________)\_________)\ / \ ;/________ __\\ __________ _________ ____ /____ ____ / \ / ; ______)\\__ \____ ___)\_____ _)\ /(_____)\ /(____ \ / ; _/ _ _/ / _ \_\____ _ \_\ \/ _/\ \/ _/ \ / ; \ \ \___\__ \ / \) __\___ __ /_\___ __ /_____ __> <__ ; \ \_/ / \ / \/ /__\)_ _/__\)_ / (___ ___) ; \ / /____/\ /\ /\ / /\/ X /\/ / | | ; /_________/ \__/ \_______/_\____ ___/ \________/ ::::;| | ;: ___)\ __)\____________ | |;: : ;.-------------------------------, \ \\_\ \_____ ____/ gRK | | ;\Red Sector Incorporated presents\ \ \_(__)_ _)\ ___ ( ) ; \Chesslin minimalist chess engine\ \ \ (__) \_/ /_ _/ \_ ; \A 256 bytes DOS tiny intro XPSP3\ \ \ \ _ / \ _> <_ ; \For 2600 Hacker Quarterly _ 2016\ \ \/ \ \ \_(___________) ;;;;,\Coded by Baudsurfer\RSi \\ &FU \ \ /\ \ \_____X___________> ; `------------------------' `----' \_/ \_____\___/ w equ word ; 16-bit prettifying helper,Chesslin v1.0 in 2016 d equ dword ; 32-bit prettifying helper,fasm assembler syntax org 100h ; binary ip execution seg start address above psp pusha ; para down stack and avoid input buff collisions rep stosb ; prepare board empty squares assumes ax=0 cx=255 cwd ; set Black=0=top active player turn, White=8=bot xchg ax,di ; shorter mov di,ax prepares writing segment base mov cl,20h ; 32 initialization decoding bit rotations in all a:mov eax,52364325h ; back-rank "rnbqkbnr" nibble-encoded 32b pattern rol eax,cl ; rotate next Black chess piece value in lsnibble and al,15 ; isolate a Black chess piece value from lsnibble stosb ; left-to-right write Black back-rank major piece mov [di+0eh],si ; left-to-right write Black pawns assumes si=100h mov [di+5eh],bp ; left-to-right write White pawns assumes bp=9xxh or al,8 ; transforms Black back-rank major piece to White mov [di+6fh],al ; left-to-right write White back-rank major piece sub cl,3 ; fixes back-rank pattern nibble rotation counter loop a ; file-by-file ranks init loops 20h/(3+1)=8 times b:mov si,0fffbh ; point source index to algebraic notation buffer push si ; shorter save of algebraic notation buffer start mov cx,4 ; print dword ascii algebraic notation buffer str c:lodsb ; get one of four usr/cpu bytes from ascii buffer int 29h ; dos api fast console out display char al=[di++] loop c ; continue until ascii file-first pair chars left xor dl,8 ; alternate active player turn Black=0 or White=8 pop di ; shorter restore algebraic notation buffer start jnz h ; if active player turn is White then do keyboard fldz ; else Black active player turn fpu load +0.0 cst fbstp [di-6] ; and store back 80-bit packed bcd decimal number e:mov si,0fff5h ; zeroed this,best score 0fff5h and coords 0fff7h lodsw ; move lsb=potential capture vs. msb=best capture cmp al,ah ; compare this capture value against best capture jc f ; prune calculations if capture already lower val call n ; else verify the attack potential chess legality jc f ; capture higher value but move was illegal chess mov [di-7],al ; successful calculation thus store newer highest fild d [di] ; successful calculation thus load current coords fistp d [si] ; successful calculation thus store highest coord f:inc d [di] ; resume exploring exhaustive [0;0ffffh] interval jnz e ; including subset ["1a1a";"8h8h"] until finished mov cl,2 ; convert int32 to two file-first algebraic words g:lodsw ; get first int16 msw/lsw algebraic notation word aam 16 ; integer to expanded zero-based file/rank nibble add ax,2960h ; translate file/rank to ascii chess board origin stosw ; write pair=half of the ascii move buffer string loop g ; get next int16 msw/lsw words algebraic notation jmp k ; and proceed examining ascii move buffer strings h:mov si,di ; di points to 0fffbh for both input and verifify i:mov di,si ; resets every input to algebraic notation buffer mov cl,4 ; one file-first algebraic notation is four bytes j:cbw ; zero accumulator msb to set funct get keystroke int 16h ; al=dos bios keyboard services api blocking read stosb ; src file=fffb;rank=fffc dst file=fffd;rank=fffe loop j ; all file-first algebraic ascii quartet inputed? call n ; else verify algebraic ascii move is legal chess jc i ; if not then proceed to ask user input move anew k:call l ; converts algebraic notation buffer ascii source push w b ; redirect second fall-through return to printout l:lodsw ; algebraic notation buffer ascii source then dst sub ax,3161h ; convert to zero-based alphanumerical 3161h="a1" aad 16 ; convert to x88 board representation (al+=ah*16) mov di,ax ; add x88 chess board representation memory start test cl,cl ; verify caller's asked mode is passive or active jnz m ; call asked mode mutex is passive so skip writes xchg [di],ch ; call asked mode mutex is active so write board! m:and al,88h ; test if inside main chess board x88 bitmask use ret ; return to standard callers or printout redirect n:pusha ; save reg vals in: si=fff7h/fffbh di=fffbh/ffffh mov si,0fffbh ; point source index to current ascii move buffer mov cl,8 ; set passive mode count mutex for only verifying call x ; convert buffer ascii src pair to x88 memory add jz u ; source is non-conforming : illegal empty square xor dl,al ; sets move conformitiy using active player color test dl,cl ; test move conformity using active player colour jnz u ; source is non-conforming : opponent turn colour mov bx,di ; else if source conforming then save piece addr. mov dh,al ; else if source conforming then save piece value call x ; convert buffer ascii dest to x88 memory address jz o ; if move nature not an attack skip over captures xor dl,al ; sets move conformitiy using active player color test dl,cl ; test move conformity using active player colour jnz u ; destination is non-conforming : same turn color o:sub di,bx ; source & destination conforming so obtain delta mov [0fff5h],al ; save piece value as non-transactional potential mov al,dh ; restore previous saved move source piece nature and al,7 ; normalize gray piece nature colorless isolation test al,1 ; determine source piece's parity interval length jz p ; piece face=piece nature=piece value=piece score mov cl,4 ; override halfing default interval len if parity p:cmp al,1 ; test if moving piece is a special handling pawn mov bx,y ; piece memory address off-by-one index ret fixed xlatb ; move piece original start offset memory address xchg ax,di ; offset becomes accumulator becomes displacement jnz s ; leave if move source piece not special handling test dh,8 ; else adjust move source pawn color displacement jnz q ; no White pawn displacement sub-interval fixings scasd ; displacement interval offset+=4 for black pawns q:test ch,ch ; verify if pawn is attacking an opponent piece ? mov cx,2 ; loop index clears msb placeholder also sets lsb jnz s ; if non-empty square : pawn attacking diagonally dec cx ; else decrease parity interval size special case r:scasw ; displacement interval start+=2 prunes attacking s:add di,bx ; set displacement interval scanning start offset repnz scasb ; verify move exists in displacement sub-interval jz v ; ZF set legal src piece displacement delta found jmp u ; illegal src piece displacement: delta not found t:pop ax ; bail shotcircuits nested dataflow function call u:stc ; carry mutex persists indicating move is illegal v:popa ; persistant CF mutex is indicator to legal chess ret ; restore move mode mutex cl=passive or cl=active x:call l ; verify this move legal within inside main board jnz t ; exits for illegal move piece outside main board cmpxchg [di],al ; discriminate from special case zero return vals y:db 195,21,7,19,15,15,15 ; p[1]PF4,n[2]PF8,b[3]PF4,q[4]PF8,r[5]PF4,k[6]PF8 z:db -33,-31,-18,-14,14 ; prev label is ret+1 parity displacement offsets db 18,31,33,-16,16,-1,1 ; z array is displacement overlap interval values db 15,17,-15,-17,-16 ; knight rook+8 bishop+12 pawns White+12 Black+18 db -32,15,17,16 ; queen and king moves are rook+bishop+pawn moves

Code: chesslin.asm