Chess: Add En-passant

Author: https://github.com/petelliott
Commit: https://github.com/SerenityOS/serenity/commit/9d404727218
Pull-request: https://github.com/SerenityOS/serenity/pull/3099
Reviewed-by: https://github.com/stelar7

Games/Chess/Chess.cpp

@@ -154,32 +154,30 @@ bool Chess::is_legal_no_check(const Move& move, Colour colour) const
         return false;
 
     if (piece.type == Type::Pawn) {
-        // FIXME: Add en passant.
-        if (colour == Colour::White) {
-            if (move.to.rank == move.from.rank + 1 && move.to.file == move.from.file && get_piece(move.to).type == Type::None) {
+        int dir = (colour == Colour::White) ? +1 : -1;
+        unsigned start_rank = (colour == Colour::White) ? 1 : 6;
+        unsigned other_start_rank = (colour == Colour::White) ? 6 : 1;
+        unsigned en_passant_rank = (colour == Colour::White) ? 4 : 3;
+
+        if (move.to.rank == move.from.rank + dir && move.to.file == move.from.file && get_piece(move.to).type == Type::None) {
             // Regular pawn move.
             return true;
-            } else if (move.to.rank == move.from.rank + 1 && (move.to.file == move.from.file + 1 || move.to.file == move.from.file - 1)
-                && get_piece(move.to).colour == Colour::Black) {
+        } else if (move.to.rank == move.from.rank + dir && (move.to.file == move.from.file + 1 || move.to.file == move.from.file - 1)) {
+            Move en_passant_last_move = { { other_start_rank, move.to.file }, { en_passant_rank, move.to.file } };
+            if (get_piece(move.to).colour == opposing_colour(colour)) {
                 // Pawn capture.
                 return true;
-            } else if (move.from.rank == 1 && move.to.rank == move.from.rank + 2 && move.to.file == move.from.file && get_piece(move.to).type == Type::None) {
+            } else if (m_last_move.has_value() && move.from.rank == en_passant_rank && m_last_move.value() == en_passant_last_move
+                && get_piece(en_passant_last_move.to) == Piece(opposing_colour(colour), Type::Pawn)) {
+                // En passant.
+                return true;
+            }
+        } else if (move.from.rank == start_rank && move.to.rank == move.from.rank + (2 * dir) && move.to.file == move.from.file
+            && get_piece(move.to).type == Type::None && get_piece({ move.from.rank + dir, move.from.file }).type == Type::None) {
             // 2 square pawn move from initial position.
             return true;
         }
-        } else if (colour == Colour::Black) {
-            if (move.to.rank == move.from.rank - 1 && move.to.file == move.from.file && get_piece(move.to).type == Type::None) {
-                // Regular pawn move.
-                return true;
-            } else if (move.to.rank == move.from.rank - 1 && (move.to.file == move.from.file + 1 || move.to.file == move.from.file - 1)
-                && get_piece(move.to).colour == Colour::White) {
-                // Pawn capture.
-                return true;
-            } else if (move.from.rank == 6 && move.to.rank == move.from.rank - 2 && move.to.file == move.from.file && get_piece(move.to).type == Type::None) {
-                // 2 square pawn move from initial position.
-                return true;
-            }
-        }
+
         return false;
     } else if (piece.type == Type::Knight) {
         int rank_delta = abs(move.to.rank - move.from.rank);
@@ -308,6 +306,8 @@ bool Chess::apply_illegal_move(const Move& move, Colour colour)
 
     // FIXME: pawn promotion
 
+    m_last_move = move;
+
     if (move.from == Square("a1") || move.to == Square("a1") || move.from == Square("e1"))
         m_white_can_castle_queenside = false;
     if (move.from == Square("h1") || move.to == Square("h1") || move.from == Square("e1"))
@@ -347,6 +347,15 @@ bool Chess::apply_illegal_move(const Move& move, Colour colour)
         }
     }
 
+    if (get_piece(move.from).type == Type::Pawn && move.from.file != move.to.file && get_piece(move.to).type == Type::None) {
+        // En passant.
+        if (colour == Colour::White) {
+            set_piece({ move.to.rank - 1, move.to.file }, EmptyPiece);
+        } else {
+            set_piece({ move.to.rank + 1, move.to.file }, EmptyPiece);
+        }
+    }
+
     set_piece(move.to, get_piece(move.from));
     set_piece(move.from, EmptyPiece);
 

Games/Chess/Chess.h

@@ -27,6 +27,7 @@
 #pragma once
 
 #include <AK/IterationDecision.h>
+#include <AK/Optional.h>
 #include <AK/StringView.h>
 #include <AK/Traits.h>
 
@@ -92,6 +93,7 @@ public:
             , to(to)
         {
         }
+        bool operator==(const Move& other) const { return from == other.from && to == other.to; }
     };
 
     Chess();
@@ -103,6 +105,7 @@ public:
     bool in_check(Colour colour) const;
 
     bool apply_move(const Move&, Colour colour = Colour::None);
+    const Optional<Move>& last_move() const { return m_last_move; }
 
     enum class Result {
         CheckMate,
@@ -124,6 +127,7 @@ private:
 
     Piece m_board[8][8];
     Colour m_turn { Colour::White };
+    Optional<Move> m_last_move;
 
     bool m_white_can_castle_kingside { true };
     bool m_white_can_castle_queenside { true };

Games/Chess/ChessWidget.cpp

@@ -63,7 +63,7 @@ void ChessWidget::paint_event(GUI::PaintEvent& event)
 
         painter.fill_rect(tile_rect, ((sq.rank % 2) == (sq.file % 2)) ? board_theme().dark_square_color : board_theme().light_square_color);
 
-        if (m_last_move.has_value() && (m_last_move.value().to == sq || m_last_move.value().from == sq)) {
+        if (board().last_move().has_value() && (board().last_move().value().to == sq || board().last_move().value().from == sq)) {
             painter.fill_rect(tile_rect, m_move_highlight_color);
         }
 
@@ -115,8 +115,6 @@ void ChessWidget::mouseup_event(GUI::MouseEvent& event)
     auto target_square = mouse_to_square(event);
 
     if (board().apply_move({ m_moving_square, target_square })) {
-        m_last_move = Chess::Move(m_moving_square, target_square);
-
         if (board().game_result() != Chess::Result::NotFinished) {
             set_drag_enabled(false);
             update();
@@ -204,7 +202,6 @@ void ChessWidget::reset()
 {
     m_board = Chess();
     m_drag_enabled = true;
-    m_last_move = Optional<Chess::Move>();
     update();
 }
 

Games/Chess/ChessWidget.h

@@ -83,5 +83,4 @@ private:
     Gfx::IntPoint m_drag_point;
     bool m_dragging_piece { false };
     bool m_drag_enabled { true };
-    Optional<Chess::Move> m_last_move;
 };