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    use lru_cache::LruCache;
    use parking_lot::Mutex;
    use hash::H256;
    use chain::BlockHeader;
    use kv::{KeyValueDatabase, KeyState, Operation, KeyValue, Key, Value, Transaction};

    pub struct CacheDatabase<T> where T: KeyValueDatabase {
    	db: T,
    	header: Mutex<LruCache<H256, KeyState<BlockHeader>>>,
    }

    impl<T> CacheDatabase<T> where T: KeyValueDatabase {
    	pub fn new(db: T) -> Self {
    		CacheDatabase {
    			db: db,
    			// 144 (blocks per day) * 14 (days) + 100 (arbitrary number)
    			header: Mutex::new(LruCache::new(2116)),
    		}
    	}
    }

    impl<T> KeyValueDatabase for CacheDatabase<T> where T: KeyValueDatabase {
    	fn write(&self, tx: Transaction) -> Result<(), String> {
    		for op in &tx.operations {
    			match *op {
    				Operation::Insert(KeyValue::BlockHeader(ref hash, ref header)) => {
    					self.header.lock().insert(hash.clone(), KeyState::Insert(header.clone()));
    				},
    				Operation::Delete(Key::BlockHeader(ref hash)) => {
    					self.header.lock().insert(hash.clone(), KeyState::Delete);
    				},
    				_ => (),
    			}
    		}
    		self.db.write(tx)
    	}

    	fn get(&self, key: &Key) -> Result<KeyState<Value>, String> {
    		if let Key::BlockHeader(ref hash) = *key {
    			let mut header = self.header.lock();
    			if let Some(state) = header.get_mut(hash) {
    				return Ok(state.clone().map(Value::BlockHeader))
    			}
    		}
    		self.db.get(key)
    	}
    }

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