Improve the is_fen_valid function

[johndoknjas]

Some discussion of this function in PR #124. If possible, would be good to make this function more accurate, although this may be challenging. The function works by checking if the fen syntax is valid, and also by giving a temporary SF executable the FEN and seeing if it crashes. This seems to work well for most use cases but it's not completely accurate.

See the discussion in #124 for a test case that the function is inconsistent with (it's a position with both kings next to each other: 8/8/8/3k4/3K4/8/8/8 b - - 0 1). If some improvements can be made for the function, this test case should ideally pass for any version of Stockfish being used.

[kieferro]

(@johndoknjas I know you're aware of this, but I'm also commenting on it here so it's transparent to other people who find this issue).

Hello, this project is no longer maintained in this repo but on this fork. (For more information about this please look here).
Since this issue has no comments, it should be quite easy to copy the content and title into a new issue over there. (For more specific information on that, look here).
This makes it easier for us to keep track of things, and it's more convenient because we then have the issues right where they are being worked on. So please create a new issue in the new repo and then close this one. Thank you!

[ghost]

bool Position::is_valid_fen(const std::string &fen) {
   std::istringstream iss(fen);
   std::string board, side, castleRights, ep;

   if (!iss) return false;

   iss >> board;

   if (!iss) return false;

   iss >> side;

   if (!iss) {
      castleRights = "-";
      ep = "-";
   } else {
      iss >> castleRights;
      if (iss)
         iss >> ep;
      else
         ep = "-";
   }

   // Let's check that all components of the supposed FEN are OK.
   if (side != "w" && side != "b") return false;
   if (castleRights != "-" && castleRights != "K" && castleRights != "Kk"
       && castleRights != "Kkq" && castleRights != "Kq" && castleRights !="KQ"
       && castleRights != "KQk" && castleRights != "KQq" && castleRights != "KQkq"
       && castleRights != "k" && castleRights != "q" && castleRights != "kq"
       && castleRights != "Q" && castleRights != "Qk" && castleRights != "Qq"
       && castleRights != "Qkq")
      return false;
   if (ep != "-") {
      if (ep.length() != 2) return false;
      if (!(ep[0] >= 'a' && ep[0] <= 'h')) return false;
      if (!((side == "w" && ep[1] == '6') || (side == "b" && ep[1] == '3')))
         return false;
   }

   // The tricky part: The board.
   // Seven slashes?
   if (std::count(board.begin(), board.end(), '/') != 7) return false;
   // Only legal characters?
   for (int i = 0; i < board.length(); i++)
      if (!(board[i] == '/' || (board[i] >= '1' && board[i] <= '8')
            || piece_type_is_ok(piece_type_from_char(board[i]))))
         return false;
   // Exactly one king per side?
   if (std::count(board.begin(), board.end(), 'K') != 1) return false;
   if (std::count(board.begin(), board.end(), 'k') != 1) return false;
   // Other piece counts reasonable?
   size_t wp = std::count(board.begin(), board.end(), 'P'),
      bp = std::count(board.begin(), board.end(), 'p'),
      wn = std::count(board.begin(), board.end(), 'N'),
      bn = std::count(board.begin(), board.end(), 'n'),
      wb = std::count(board.begin(), board.end(), 'B'),
      bb = std::count(board.begin(), board.end(), 'b'),
      wr = std::count(board.begin(), board.end(), 'R'),
      br = std::count(board.begin(), board.end(), 'r'),
      wq = std::count(board.begin(), board.end(), 'Q'),
      bq = std::count(board.begin(), board.end(), 'q');
   if (wp > 8 || bp > 8 || wn > 10 || bn > 10 || wb > 10 || bb > 10
       || wr > 10 || br > 10 || wq > 9 || bq > 10
       || wp + wn + wb + wr + wq > 15 || bp + bn + bb + br + bq > 15)
      return false;

   // OK, looks close enough to a legal position. Let's try to parse
   // the FEN and see!
   Position p;
   p.from_fen(board + " " + side + " " + castleRights + " " + ep);
   return p.is_ok(true);
}