feat: basic En Passant implementation

3 files changed, 71 insertions, 7 deletions

diff --git a/src/board.rs b/src/board.rs
index 186e569..58e4037 100644
--- a/src/board.rs
+++ b/src/board.rs
@@ -185,7 +185,7 @@ impl Board {
                     };
                     for source in serialize_bitboard(*piece & able_to_double_push_mask) {
                         for target in serialize_bitboard(self.attacks.pawn_double_pushes[color as usize][source as usize] & empty) {
-                            moves.push(Move { source, target, kind: MoveKind::Quiet });
+                            moves.push(Move { source, target, kind: MoveKind::DoublePush });
                         };
                     }
                 }
@@ -254,7 +254,7 @@ impl Board {
         let move_target_bb = mov.target.to_bitboard();
 
         // Remove existing piece (if any) from target square
-        let captured_piece = match self.pieces
+        let mut captured_piece = match self.pieces
             .iter()
             .enumerate()
             .find(|(piece_type, bitboard)| *bitboard & mov.target.to_bitboard() > 0)
@@ -266,6 +266,26 @@ impl Board {
                 None => None,
             };
 
+        // En Passant captures diffirently
+        captured_piece = match mov.kind {
+            MoveKind::EnPassant => {
+                debug_assert!(captured_piece.is_none(), "No capture should be found at this point");
+                let captured_bb = Square::from(mov.source.rank() * 8 + mov.target.file()).to_bitboard();
+                match self.pieces
+                    .iter()
+                    .enumerate()
+                    .find(|(piece_type, bitboard)| *bitboard & captured_bb > 0)
+                {
+                    Some((pawn_type, _)) => {
+                        self.pieces[pawn_type] ^= captured_bb;
+                        Some(PieceType::from(pawn_type))
+                    }
+                    None => panic!("Pawn captured by En Passant was not found"),
+                }
+            },
+            _ => captured_piece,
+        };
+
         // Move a piece from source square to target
         match self.pieces
             .iter()
@@ -282,13 +302,27 @@ impl Board {
                 None => panic!("Move is malformed: source piece not found"),
             };
 
+        // Double push should set En Passant target square
+        match mov.kind {
+            MoveKind::DoublePush => {
+                self.ep_target = match mov.source.rank() {
+                    1 => Some(mov.source.nort_one()),
+                    6 => Some(mov.source.sout_one()),
+                    rank => panic!("Double-push was used from invalid rank({}) when trying to make {:?}", rank, mov),
+                };
+            },
+            _ => {
+                self.ep_target = None
+            },
+        }
+
         self.ply += 1;
 
         captured_piece
     }
 
     /// Completely reverse make_move as if it never happened
-    pub fn unmake_move(&mut self, mov: Move, captured_piece: Option<PieceType>) {
+    pub fn unmake_move(&mut self, mov: Move, captured_piece: Option<PieceType>, previous_ep_target: Option<Square>) {
         let move_source_bb = mov.source.to_bitboard();
         let move_target_bb = mov.target.to_bitboard();
 
@@ -311,12 +345,24 @@ impl Board {
         // Return captured piece to target square
         match captured_piece {
             Some(target_piece) => {
-                self.pieces[target_piece as usize] |= move_target_bb;
-                self.occupancy                     |= move_target_bb;
+                match mov.kind {
+                    // Return pawn captured by En Passant pawn if needed
+                    MoveKind::EnPassant => {
+                        let original_dead_pawn_bb = Square::from(mov.source.rank() * 8 + mov.target.file()).to_bitboard();
+                        self.pieces[target_piece as usize] |= original_dead_pawn_bb;
+                        self.occupancy                     |= original_dead_pawn_bb;
+                    },
+                    _ => {
+                        self.pieces[target_piece as usize] |= move_target_bb;
+                        self.occupancy                     |= move_target_bb;
+                    },
+                }
             },
             None => {}
         }
 
+
+        self.ep_target = previous_ep_target;
         self.ply -= 1;
     }
 
@@ -338,6 +384,7 @@ impl Board {
         let mut checks = 0;
 
         for mov in moves {
+            let ep_target_before = self.ep_target.clone();
             let captured_piece = self.make_move(mov);
             // King can not be in check after our own move
             if !self.is_king_in_check(color) {
@@ -357,7 +404,7 @@ impl Board {
                 checks += children_checks;
 
             }
-            self.unmake_move(mov, captured_piece);
+            self.unmake_move(mov, captured_piece, ep_target_before);
         }
 
         if print {
@@ -539,7 +586,7 @@ mod tests {
         let captured_piece = board.make_move(mov);
         board.print();
 
-        board.unmake_move(mov, captured_piece);
+        board.unmake_move(mov, captured_piece, None);
         board.print();
 
         assert_eq!(board, initial_board, "Board state after unmake_move should be the same as before make_move");
diff --git a/src/moves.rs b/src/moves.rs
index d6695a1..d0e3eb6 100644
--- a/src/moves.rs
+++ b/src/moves.rs
@@ -6,6 +6,7 @@ pub enum MoveKind {
     Capture,
     Castle,
     EnPassant,
+    DoublePush,
 }
 
 #[derive(Debug, Clone, Copy)]
diff --git a/src/square.rs b/src/square.rs
index ed2877c..1adca83 100644
--- a/src/square.rs
+++ b/src/square.rs
@@ -19,6 +19,22 @@ impl Square {
     pub fn to_bitboard(&self) -> Bitboard {
         1u64 << *self as u8
     }
+    /// 0-based rank
+    pub fn rank(&self) -> u8 {
+        *self as u8 / 8
+    }
+    /// 0-based file
+    pub fn file(&self) -> u8 {
+        *self as u8 % 8
+    }
+
+    pub fn nort_one(&self) -> Self {
+        Self::from(*self as u8 + 8)
+    }
+
+    pub fn sout_one(&self) -> Self {
+        Self::from(*self as u8 - 8)
+    }
 }