refactor: update occupancy in an optimal way

1 files changed, 24 insertions, 11 deletions

diff --git a/src/board.rs b/src/board.rs
index ef8dde7..54efab9 100644
--- a/src/board.rs
+++ b/src/board.rs
@@ -201,6 +201,9 @@ impl Board {
     /// *Blindlessly* apply a move without any validation
     /// Move should be validated beforehand
     pub fn make_move(&mut self, mov: Move) -> Option<PieceType> {
+        let move_source_bb = mov.source.to_bitboard();
+        let move_target_bb = mov.target.to_bitboard();
+
         // Remove existing piece (if any) from target square
         let captured_piece = match self.pieces
             .iter()
@@ -208,7 +211,7 @@ impl Board {
             .find(|(piece_type, bitboard)| *bitboard & mov.target.to_bitboard() > 0)
             {
                 Some((target_piece, _)) => {
-                    self.pieces[target_piece] ^= mov.target.to_bitboard();
+                    self.pieces[target_piece] ^= move_target_bb;
                     Some(PieceType::from(target_piece))
                 },
                 None => None,
@@ -221,13 +224,15 @@ impl Board {
             .find(|(piece_type, bitboard)| *bitboard & mov.source.to_bitboard() > 0)
             {
                 Some((source_piece, _)) => {
-                    self.pieces[source_piece] ^= mov.source.to_bitboard();
-                    self.pieces[source_piece] |= mov.target.to_bitboard();
+                    self.pieces[source_piece] ^= move_source_bb;
+                    self.occupancy            ^= move_source_bb;
+
+                    self.pieces[source_piece] |= move_target_bb;
+                    self.occupancy            |= move_target_bb;
                 },
                 None => panic!("Move is malformed: source piece not found"),
             };
 
-        self.update_occupancy();
         self.ply += 1;
 
         captured_piece
@@ -235,15 +240,21 @@ impl Board {
 
     /// Completely reverse make_move as if it never happened
     pub fn unmake_move(&mut self, mov: Move, captured_piece: Option<PieceType>) {
-        // Remove source piece from target square
+        let move_source_bb = mov.source.to_bitboard();
+        let move_target_bb = mov.target.to_bitboard();
+
+        // Move a piece from target square back to source square
         match self.pieces
             .iter()
             .enumerate()
             .find(|(piece_type, bitboard)| *bitboard & mov.target.to_bitboard() > 0)
             {
                 Some((source_piece, _)) => {
-                    self.pieces[source_piece] ^= mov.target.to_bitboard();
-                    self.pieces[source_piece] |= mov.source.to_bitboard();
+                    self.pieces[source_piece] ^= move_target_bb;
+                    self.occupancy            ^= move_target_bb;
+
+                    self.pieces[source_piece] |= move_source_bb;
+                    self.occupancy            |= move_source_bb;
                 },
                 None => panic!("Trying to unmake move which was not made: no piece was found on target square"),
             };
@@ -251,12 +262,12 @@ impl Board {
         // Return captured piece to target square
         match captured_piece {
             Some(target_piece) => {
-                self.pieces[target_piece as usize] |= mov.target.to_bitboard();
+                self.pieces[target_piece as usize] |= move_target_bb;
+                self.occupancy                     |= move_target_bb;
             },
             None => {}
         }
 
-        self.update_occupancy();
         self.ply -= 1;
     }
 }
@@ -332,13 +343,14 @@ mod tests {
     fn test_make_move() {
         let fen = String::from("q1b2k2/5p1p/4p1pb/pPPp4/3N4/3nPB2/P2QKnR1/1R6 w - - 0 25");
         let mut board = Board::from_FEN(fen);
+        let initial_board = board.clone();
         board.print();
 
         let black_move = Move { source: Square::F7, target: Square::F5 };
         println!("\n{:?}", black_move);
 
         match board.make_move(black_move) {
-            Some(..) => panic!("No piece should have been captured"),
+            Some(..) => panic!("No piece should be captured"),
             None => {},
         };
 
@@ -353,7 +365,7 @@ mod tests {
 
         match board.make_move(white_move) {
             Some(captured) => assert!(captured == PieceType::PawnBlack),
-            None => panic!("A piece should have been captured"),
+            None => panic!("A piece should be captured"),
         };
 
         board.print();
@@ -361,6 +373,7 @@ mod tests {
         assert!(board.pieces[PieceType::PawnBlack as usize] & Square::A5.to_bitboard() == 0, "Target piece should be captured");
         assert!(board.pieces[PieceType::Queen as usize] & Square::D2.to_bitboard() == 0);
         assert!(board.pieces[PieceType::Queen as usize] & Square::A5.to_bitboard() > 0);
+        assert_ne!(board.occupancy, initial_board.occupancy, "Occupancy should change after make_move");
         assert!(board.ply == 2);
     }