Generate random chess position

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.

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);

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.

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