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    use parking_lot::Mutex;
    use kv::{Transaction, Value, KeyValueDatabase, MemoryDatabase, KeyState, Key};

    pub struct OverlayDatabase<'a, T> where T: 'a + KeyValueDatabase {
    	db: &'a T,
    	overlay: MemoryDatabase,
    }

    impl<'a, T> OverlayDatabase<'a, T> where T: 'a + KeyValueDatabase {
    	pub fn new(db: &'a T) -> Self {
    		OverlayDatabase {
    			db: db,
    			overlay: MemoryDatabase::default(),
    		}
    	}

    	pub fn flush(&self) -> Result<(), String> {
    		self.db.write(self.overlay.drain_transaction())
    	}
    }

    impl<'a, T> KeyValueDatabase for OverlayDatabase<'a, T> where T: 'a + KeyValueDatabase {
    	fn write(&self, tx: Transaction) -> Result<(), String> {
    		self.overlay.write(tx)
    	}

    	fn get(&self, key: &Key) -> Result<KeyState<Value>, String> {
    		match self.overlay.get(key)? {
    			KeyState::Unknown => self.db.get(key),
    			exists => Ok(exists)
    		}
    	}
    }

    pub struct AutoFlushingOverlayDatabase<T> where T: KeyValueDatabase {
    	db: T,
    	overlay: MemoryDatabase,
    	operations: Mutex<usize>,
    	max_operations: usize,
    }

    impl<T> AutoFlushingOverlayDatabase<T> where T: KeyValueDatabase {
    	pub fn new(db: T, max_operations: usize) -> Self {
    		AutoFlushingOverlayDatabase {
    			db: db,
    			overlay: MemoryDatabase::default(),
    			operations: Mutex::default(),
    			max_operations: max_operations,
    		}
    	}

    	fn flush(&self) -> Result<(), String> {
    		self.db.write(self.overlay.drain_transaction())
    	}
    }

    impl<T> KeyValueDatabase for AutoFlushingOverlayDatabase<T> where T: KeyValueDatabase {
    	fn write(&self, tx: Transaction) -> Result<(), String> {
    		let mut operations = self.operations.lock();
    		*operations += 1;
    		self.overlay.write(tx)?;
    		if *operations == self.max_operations {
    			self.flush()?;
    			*operations = 0;
    		}
    		Ok(())
    	}

    	fn get(&self, key: &Key) -> Result<KeyState<Value>, String> {
    		match self.overlay.get(key)? {
    			KeyState::Unknown => self.db.get(key),
    			exists => Ok(exists)
    		}
    	}
    }

    impl<T> Drop for AutoFlushingOverlayDatabase<T> where T: KeyValueDatabase {
    	fn drop(&mut self) {
    		self.flush().expect("Failed to save database");
    	}
    }

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