Chess: Added ability to copy board state as FEN

You can now copy the board state as Forsyth-Edwards Notation. You can
then paste this into other chess programs/games, or into ours when
it gets implemented.

Games/Chess/ChessWidget.cpp

@@ -292,6 +292,11 @@ void ChessWidget::maybe_input_engine_move()
     });
 }
 
+String ChessWidget::get_fen() const
+{
+    return m_board.to_fen();
+}
+
 bool ChessWidget::export_pgn(const StringView& export_path) const
 {
     auto file_or_error = Core::File::open(export_path, Core::File::WriteOnly);

Games/Chess/ChessWidget.h

@@ -62,6 +62,7 @@ public:
     void set_drag_enabled(bool e) { m_drag_enabled = e; }
     RefPtr<Gfx::Bitmap> get_piece_graphic(const Chess::Piece& piece) const;
 
+    String get_fen() const;
     bool export_pgn(const StringView& export_path) const;
 
     void resign();

Games/Chess/main.cpp

@@ -30,6 +30,7 @@
 #include <LibGUI/AboutDialog.h>
 #include <LibGUI/ActionGroup.h>
 #include <LibGUI/Application.h>
+#include <LibGUI/Clipboard.h>
 #include <LibGUI/FilePicker.h>
 #include <LibGUI/Icon.h>
 #include <LibGUI/Menu.h>
@@ -121,6 +122,10 @@ int main(int argc, char** argv)
 
         dbgln("Exported PGN file to {}", export_path.value());
     }));
+    app_menu.add_action(GUI::Action::create("Copy FEN", { Mod_Ctrl, Key_C }, [&](auto&) {
+        GUI::Clipboard::the().set_data(widget.get_fen().bytes());
+        GUI::MessageBox::show(window, "Board state copied to clipboard as FEN.", "Copy FEN", GUI::MessageBox::Type::Information);
+    }));
     app_menu.add_separator();
 
     app_menu.add_action(GUI::Action::create("New game", { Mod_None, Key_F2 }, [&](auto&) {

Libraries/LibChess/Chess.cpp

@@ -205,6 +205,73 @@ Board::Board()
     set_piece(Square("h8"), { Colour::Black, Type::Rook });
 }
 
+String Board::to_fen() const
+{
+    StringBuilder builder;
+
+    // 1. Piece placement
+    int empty = 0;
+    for (unsigned rank = 0; rank < 8; rank++) {
+        for (unsigned file = 0; file < 8; file++) {
+            const Piece p(get_piece({ 7 - rank, file }));
+            if (p.type == Type::None) {
+                empty++;
+                continue;
+            }
+            if (empty > 0) {
+                builder.append(String::number(empty));
+                empty = 0;
+            }
+            String piece = char_for_piece(p.type);
+            if (piece == "")
+                piece = "P";
+
+            builder.append(p.colour == Colour::Black ? piece.to_lowercase() : piece);
+        }
+        if (empty > 0) {
+            builder.append(String::number(empty));
+            empty = 0;
+        }
+        if (rank < 7)
+            builder.append("/");
+    }
+
+    // 2. Active color
+    ASSERT(m_turn != Colour::None);
+    builder.append(m_turn == Colour::White ? " w " : " b ");
+
+    // 3. Castling availability
+    builder.append(m_white_can_castle_kingside ? "K" : "");
+    builder.append(m_white_can_castle_queenside ? "Q" : "");
+    builder.append(m_black_can_castle_kingside ? "k" : "");
+    builder.append(m_black_can_castle_queenside ? "q" : "");
+    builder.append(" ");
+
+    // 4. En passant target square
+    if (!m_last_move.has_value())
+        builder.append("-");
+    else if (m_last_move.value().piece.type == Type::Pawn) {
+        if (m_last_move.value().from.rank == 1 && m_last_move.value().to.rank == 3)
+            builder.append(Square(m_last_move.value().to.rank - 1, m_last_move.value().to.file).to_algebraic());
+        else if (m_last_move.value().from.rank == 6 && m_last_move.value().to.rank == 4)
+            builder.append(Square(m_last_move.value().to.rank + 1, m_last_move.value().to.file).to_algebraic());
+        else
+            builder.append("-");
+    } else {
+        builder.append("-");
+    }
+    builder.append(" ");
+
+    // 5. Halfmove clock
+    builder.append(String::number(min(m_moves_since_capture, m_moves_since_pawn_advance)));
+    builder.append(" ");
+
+    // 6. Fullmove number
+    builder.append(String::number(1 + m_moves.size() / 2));
+
+    return builder.to_string();
+}
+
 Piece Board::get_piece(const Square& square) const
 {
     ASSERT(square.rank < 8);
@@ -474,6 +541,7 @@ bool Board::apply_illegal_move(const Move& move, Colour colour)
 
     m_last_move = move;
     m_moves_since_capture++;
+    m_moves_since_pawn_advance++;
 
     if (move.from == Square("a1") || move.to == Square("a1") || move.from == Square("e1"))
         m_white_can_castle_queenside = false;
@@ -514,6 +582,9 @@ bool Board::apply_illegal_move(const Move& move, Colour colour)
         }
     }
 
+    if (move.piece.type == Type::Pawn)
+        m_moves_since_pawn_advance = 0;
+
     if (get_piece(move.to).colour != Colour::None) {
         const_cast<Move&>(move).is_capture = true;
         m_moves_since_capture = 0;

Libraries/LibChess/Chess.h

@@ -139,6 +139,8 @@ public:
     bool apply_move(const Move&, Colour colour = Colour::None);
     const Optional<Move>& last_move() const { return m_last_move; }
 
+    String to_fen() const;
+
     enum class Result {
         CheckMate,
         StaleMate,
@@ -180,6 +182,7 @@ private:
     Colour m_resigned { Colour::None };
     Optional<Move> m_last_move;
     int m_moves_since_capture { 0 };
+    int m_moves_since_pawn_advance { 0 };
 
     bool m_white_can_castle_kingside { true };
     bool m_white_can_castle_queenside { true };