Rosetta Code

Generate random chess position

Revision as of 13:30, 1 April 2016 by rosettacode>Paulo Jorente

The purpose of this task is to generate a random chess position in FEN format. The position does not have to be realistic or even balanced, but it must comply to the following rules:

  • there is one and only one king of each color (one black king and one white king);
  • the kings must not be placed on adjacent squares;
  • there can not be any pawn in the promotion square (no white pawn in the eighth rank, and no black pawn in the first rank);
  • including the kings, up to 32 pieces of either color can be placed. There is no requirement for material balance between sides; The picking of pieces does not have to comply to a regular chess set : there can be five knights, twenty rooks, whatever... as long as the total number of pieces do not exceed thirty-two.
  • it is white's turn, it is assumed that both sides have lost castling rights and that there is no possibility for en passant (the FEN should thus end in w - - 0 1).
Generate random chess position is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

No requirement is made regarding the probability distribution of your method, but your program should be able to span a reasonably representative sample of all possible positions. For instance, programs that would always generate positions with say five pieces on the board, or with kings on a corner, would not be considered truly random.


C++

<lang cpp>

  1. include <ctime>
  2. include <iostream>
  3. include <string>
  4. include <algorithm>

class chessBoard { public: 

   void generateRNDBoard( int brds ) {
       int a, b, i; char c;
       for( int cc = 0; cc < brds; cc++ ) {
           memset( brd, 0, 64 );
           std::string pieces = "PPPPPPPPNNBBRRQKppppppppnnbbrrqk";
           random_shuffle( pieces.begin(), pieces.end() );

           while( pieces.length() ) {
               i = rand() % pieces.length(); c = pieces.at( i );
               while( true ) {
                   a = rand() % 8; b = rand() % 8;
                   if( brd[a][b] == 0 ) {
                       if( c == 'P' && !b || c == 'p' && b == 7 || 
                         ( ( c == 'K' || c == 'k' ) && search( c == 'k' ? 'K' : 'k', a, b ) ) ) continue;
                       break;
                   }
               }
               brd[a][b] = c;
               pieces = pieces.substr( 0, i ) + pieces.substr( i + 1 );
           }
           print();
       }
   }

private: 

   bool search( char c, int a, int b ) {
       for( int y = -1; y < 2; y++ ) {
           for( int x = -1; x < 2; x++ ) {
               if( !x && !y ) continue;
               if( a + x > -1 && a + x < 8 && b + y >-1 && b + y < 8 ) {
                   if( brd[a + x][b + y] == c ) return true;
               }
           }
       }
       return false;
   }
   void print() {
       int e = 0;
       for( int y = 0; y < 8; y++ ) {
           for( int x = 0; x < 8; x++ ) {
               if( brd[x][y] == 0 ) e++;
               else {
                   if( e > 0 ) { std::cout << e; e = 0; }
                   std::cout << brd[x][y];
               }
           }
           if( e > 0 ) { std::cout << e; e = 0; } 
           if( y < 7 ) std::cout << "/";
       }
       std::cout << " w - - 0 1\n\n";

       for( int y = 0; y < 8; y++ ) {
           for( int x = 0; x < 8; x++ ) {
               if( brd[x][y] == 0 ) std::cout << ".";
               else std::cout << brd[x][y];
           }
           std::cout << "\n";
       }

       std::cout << "\n\n";
   }
   char brd[8][8];

}; int main( int argc, char* argv[] ) { 

   srand( ( unsigned )time( 0 ) );
   chessBoard c;
   c.generateRNDBoard( 2 );
   return 0;

} </lang>

Output:
1R6/2bnQP1K/br1N1BP1/nPkp1P2/2p1P1P1/4Ppqp/p1r1ppp1/1PNR3B w - - 0 1
.R......
..bnQP.K
br.N.BP.
nPkp.P..
..p.P.P.
....Ppqp
p.r.ppp.
.PNR...B


1n1k2bp/1PppQpb1/N1p4p/1B2P1K1/1RB2P2/pPR1Np2/P1r1rP1P/P2q3n w - - 0 1
.n.k..bp
.PppQpb.
N.p....p
.B..P.K.
.RB..P..
pPR.Np..
P.r.rP.P
P..q...n

J

Implementation:

<lang J>getlayout=:3 :0 

 whilst. NB. first two positions are non-adjacent kings
   (0{pos) e. (1{pos)+(,-)1 7 8 9
 do.
   pos=: y?64
 end.

)

randboard=:3 :0 

 n=: ?30  NB. number of non-king pieces on board
 layout=: getlayout 2+n   NB. where they go
 white=: 0 1,?n#2         NB. which ones are white?
 pawns=: 0 0,?n#2             NB. where are the pawns?
 pawns=: pawns * 1- white*layout e.56+i.8
 pawns=: pawns * 1-(1-white)*layout e.i.8
 ptyp=: 'pkqbjnPKQBJN'{~(6*white)+1 1,(1-2}.pawns)*2+?n#4
 8 8$ptyp layout}64#'.'

)

NB. fen compress a line fen1=:3 :0 

 for_n.8-i.8 do.
   y=. y rplc (#&'.';":) n
 end.

)

NB. translate 8x8 board to fen notation NB. (just the task specific crippled fen) b2fen=:3 :0 

 (}.;|.<@(('/',fen1)"1) y),' w - - 0 1'

)

randfen=:b2fen@randboard</lang>

Example use:

<lang J> randfen q6J/1pb1p1p1/1Jq2k2/4p1Pp/1pP5/1K2Bp2/5p2/1P4P1 w - - 0 1 

  randfen

1Q3Q2/1Kn5/1PP5/2P5/2kNq1J1/7J/1P6/8 w - - 0 1 

  randfen

8/8/7K/8/8/8/k7/8 w - - 0 1 

  randfen

p4n2/1Q6/8/8/8/p4k1K/8/P5Qn w - - 0 1 

  randfen

bk1q3J/8/1N4K1/N3P3/3BQ3/j1P3P1/7P/5jq1 w - - 0 1 

  randfen

b1Q1Bb1J/j1N1PpK1/PpB1P1Pp/2pp2PN/3nnk2/3J4/P6P/1N2Qb1J w - - 0 1 </lang>

Perl 6

<lang perl6>sub pick-FEN { 

   # First we chose how many pieces to place
   my $n = (2..32).pick;

   # Then we pick $n squares
   my @n = (^64).pick($n);

   # We try to find suitable king positions on non-adjacent squares.
   # If we could not find any, we return recursively
   return pick-FEN() unless
   my @kings[2] = first -> [$a, $b] {
       $a !== $b && abs($a div 8 - $b div 8) | abs($a mod 8 - $b mod 8) > 1
   }, (@n X @n);

   # We make a list of pieces we can pick (apart from the kings)

my @pieces =

; # We make a list of two king symbols to pick randomly a black or white king my @k = <K k>.pick(*); return (gather for ^64 -> $sq { if $sq == @kings.any { take @k.shift } elsif $sq == @n.any { my $row = 7 - $sq div 8; take $row == 7 ?? @pieces.grep(none('P')).pick !! $row == 0 ?? @pieces.grep(none('p')).pick !! @pieces.pick; } else { take 'ø' } }).rotor(8)».join».subst(/ø+/,{ .chars }, :g).join('/') ~ ' w - - 0 1'; } say pick-FEN();</lang>

Output:
q2n1n2/1Qpk3Q/1r3bP1/1b1b4/2pRBR2/4P1bN/2R3K1/N1r2rPB w - - 0 1

REXX

This REXX version normally generates balanced number of pieces   (both sides have an equal number of total pieces,
but not necessarily of the same kind).

If the number of chessboard specified is negative, then the number of pieces for each side will be randonm.

This version also allows any number of chessboards to be displayed. <lang rexx>/*REXX pgm gens a chess position (random pieces & positions) in a FEN format.*/ parse arg seed CBs . /*obtain optional arguments from the CL*/ if seed\== & seed\="," then call random ,,seed /*RANDOM repeatability? */ if CBs == | CBs =',' then CBs=1 /*CBs: number of generated chessboards*/ 

                                      /* [↓]  maybe display any # of boards. */
     do boards=1  for abs(CBs)        /* [↓]  maybe display separator & title*/
     if abs(CBs)\==1  then do; say; say center(' board' boards" ",79,'▒');  end
     @.=.                             /*initialize the chessboard to be empty*/
             do p=1  for random(2,32) /*generate a random number of chessmen.*/
             if p<3  then call piece 'k'             /*a king of each color. */
                     else call piece substr('bnpqr', random(1, 5), 1)
             end  /*p*/                              /* [↑]  place a piece.  */
     call cb                          /*display the ChessBoard  and  its FEN.*/
     end          /*boards*/          /* [↑]  CB ≡  ─    ─                   */

exit /*stick a fork in it, we're all done. */ /*────────────────────────────────────────────────────────────────────────────*/ cb: fen=; do r=8 for 8 by -1; $= /*board rank (so far).*/ 

            do f=8  for 8  by -1;  $=$ || @.r.f;  end  /*f*/   /*append file.*/
          say $                                        /*display board rank. */
            do e=8  for 8  by -1;  $=changestr(copies(., e), $, e);  end  /*e*/
          fen=fen || $ || left('/', r\==1)  /*append  /  if not the 1st rank.*/
          end   /*r*/
   say                                      /*a blank line (after the board).*/
   say 'FEN='fen   "w - - 0 1"              /*show  Forsyth-Edwards Notation.*/
   return                                   /* [↑]  build/display chessboard.*/

/*────────────────────────────────────────────────────────────────────────────*/ piece: parse arg x; if p//2 then upper x; arg ux /*use white if odd P.*/ 

      if CBs<0 & p>2  then if random(1) then upper x    /*CBs>0? Use balanced*/

        do #=0  by 0;  r=random(1, 8);   f=random(1, 8) /*random rank & file.*/
        if @.r.f\==.   then iterate                     /*position occupied? */
        if (x=='p' & r==1)|(x=='P' & r==8) then iterate /*any promoting pawn?*/

        if ux=='K'  then do    rr=r-1 for 3             /*[↓]  neighbor≡king?*/
                            do ff=f-1 for 3;  z=@.rr.ff /*obtain the neighbor*/
                            upper z;  if z=='K'  then iterate #
                            end   /*rr*/                /*[↑]  neighbor≡king?*/
                         end      /*ff*/
        @.r.f=x;  return                                /*place random piece.*/
        end   /*#*/                                     /*#: not incremented.*/</lang>

Some older REXXes don't have a   changestr   BIF,   so one is included here:   ───►   CHANGESTR.REX.

output   showing five chess positions (starting with a specific position by seeding the   random   BIF with   96),
specifying the arguments (for Regina REXX under Windows):   96   5 

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ board 1 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
........
........
....K...
........
....k...
........
........
........

FEN=8/8/4K3/8/4k3/8/8/8 w - - 0 1

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ board 2 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
...Q..r.
r.kbn...
...pqn.p
..N.N.qQ
p....N.n
..N.....
...NB...
.R.Q.KBP

FEN=3Q2r1/r1kbn3/3pqn1p/2N1N1qQ/p4N1n/2N5/3NB3/1R1Q1KBP w - - 0 1

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ board 3 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
.....B.n
N...p.p.
.q..n...
.K..R...
.BP...P.
q....k..
........
R.N..n..

FEN=5B1n/N3p1p1/1q2n3/1K2R3/1BP3P1/q4k2/8/R1N2n2 w - - 0 1

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ board 4 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
......K.
pN..q...
...qN...
....n...
.....Q..
........
....k...
P.r....B

FEN=6K1/pN2q3/3qN3/4n3/5Q2/8/4k3/P1r4B w - - 0 1

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ board 5 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
qnqN...N
...B...N
n....P.Q
PP..b.Rn
KqB.....
pB.knp..
q.n.qB..
.......N

FEN=qnqN3N/3B3N/n4P1Q/PP2b1Rn/KqB5/pB1knp2/q1n1qB2/7N w - - 0 1

zkl

Translation of: perl6

<lang zkl>fcn pickFEN{ 

  # First we chose how many pieces to place: 2 to 32
  n := (0).random(2,33);

  # Then we pick $n squares: first n of shuffle (0,1,2,3...63)
  n = [0..63].walk().shuffle()[0,n]; 

  # We try to find suitable king positions on non-adjacent squares.
  # If we could not find any, we return recursively
  kings := Walker.cproduct(n,n).filter1(fcn([(a,b)]){  // Cartesian product
     a!=b and (a/8 - b/8).abs() or (a%8 - b%8).abs()>1
  }); # (a,b) on success, False on fail
  if(not kings) return(pickFEN());    // tail recursion

  # We make a list of pieces we can pick (apart from the kings)
  pieces,pnp,pnP := "p P n N b B r R q Q".split(" "), pieces-"p", pieces-"P";

  # We make a list of two king symbols to pick randomly a black or white king
  k := "K k".split(" ").shuffle();

  [0..63].apply('wrap(sq){  # look at each square
     if(kings.holds(sq)) k.pop();
     else if(n.holds(sq)){
        row,n,n2 := 7 - sq/8, (0).random(pieces.len()), (0).random(pnp.len());

if( row == 7) pnP[n2] // no white pawn in the promotion square else if(row == 0) pnp[n2] // no black pawn in the promotion square else pieces[n] // otherwise, any ole random piece 

     }
     else "#"  // empty square
  })
  .pump(List,T(Void.Read,7),"".append,subst)  // chunkize into groups of 8 chars (1 row)
  .concat("/") + " w - - 0 1"

} fcn subst(str){ // replace "#" with count of #s 

  re :=RegExp("#+");
  while(re.search(str,1)){ n,m:=re.matched[0]; str=String(str[0,n],m,str[n+m,*]) }
  str

}</lang> <lang zkl>do(5){ pickFEN().println() }</lang>

Output:
b3rQBr/n2b1Q1q/1R6/1BQRQ2n/RnB2r2/q3b1nQ/1BqB1bBQ/2q1Q3 w - - 0 1
b7/5qqB/qb3B2/8/3b4/Q1n3r1/3B2Q1/n6r w - - 0 1
r2R3N/Q1n5/1N6/1N1R1RBB/8/Rn1b1r2/3QbNN1/2n3rQ w - - 0 1
5b2/1R6/r4b1q/3Q4/8/8/8/5b2 w - - 0 1
8/1BQ5/8/1q6/1q6/3B1Rq1/5b2/3N3R w - - 0 1
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