Add Null-move pruning

src/board/game.rs

@@ -249,6 +249,41 @@ impl Game {
         }
     }
 
+    pub fn make_null_move(&mut self) {
+        let move_params = MoveParameters::build_null(self);
+        self.history.push_move_parameters(move_params);
+        self.hash.update_side_to_move_key();
+
+        self.board
+            .state
+            .update_null_game_state(self.current_player());
+    }
+
+    pub fn unmake_null_move(&mut self) {
+        let board = &mut self.board;
+        let move_parameters = &mut self
+            .history
+            .pop_move_parameters()
+            .expect("History stack is empty");
+
+        let color_before_move = board.state.change_side();
+        board.state.revert_full_move(color_before_move);
+
+        board.state.en_passant_square = move_parameters.en_passant_square;
+
+        if let Some(hash) = move_parameters.zobrist_hash {
+            self.hash = hash;
+        }
+
+        if let Some(new_castling_ability) = move_parameters.castling_ability {
+            board.state.castling_ability = new_castling_ability;
+        }
+
+        if let Some(new_halfmove_clock) = move_parameters.halfmove_clock {
+            board.state.halfmove_clock = new_halfmove_clock;
+        }
+    }
+
     pub fn in_repetition(&self) -> bool {
         if self.board.state.halfmove_clock < 4 {
             return false;
@@ -462,4 +497,22 @@ mod tests {
 
         Ok(())
     }
+
+    const FEN_W: &str = "rnbqkbnr/pppppp1p/8/5Pp1/8/8/PPPPP1PP/RNBQKBNR w KQkq g6 0 1";
+    const FEN_B: &str = "rnbqkbnr/pppppp1p/8/5Pp1/8/8/PPPPP1PP/RNBQKBNR b KQkq - 1 1";
+
+    #[test]
+    fn test_make_and_unmake_null_move() -> Result<(), String> {
+        let mut game = from_fen(FEN_W)?;
+
+        game.make_null_move();
+
+        assert_eq!(game, from_fen(FEN_B)?);
+
+        game.unmake_null_move();
+
+        assert!(game == from_fen(FEN_W)?);
+
+        Ok(())
+    }
 }