pub fn touch(&mut self, key: &K, expires: Option<Timestamp>, now: Timestamp) -> bool {
        // update the timestamp and last-used field of a row without copying the
        // whole contents
        match self._get_full_entry(key, now) {
            Some(full_entry) => {
                self._touch(key,
                            (*full_entry).expires, expires,
                            (*full_entry).used, now);
                (*full_entry).expires = expires;
                (*full_entry).used = now;
                true
            },
            _ => false
        }
    }

    fn _touch(&mut self, key: &K,
              old_expires: Option<Timestamp>, new_expires: Option<Timestamp>,
              old_used: Timestamp, now: Timestamp) {

        if old_expires != new_expires {
            if let Some(old_expires_ts) = old_expires {
                // if it expired before, we have to remove it
                let old_expires_key = (old_expires_ts, key.clone());
                self.expires.remove(&old_expires_key);
            }

            if let Some(expires_ts) = new_expires {
                // if it expires now, we have to add it
                let expires_key = (expires_ts, key.clone());
                self.expires.insert(expires_key);
            }
        }

        if old_used != now {
            let old_lru_key = (old_used, (*key).clone());
            self.lru.remove(&old_lru_key);
            let new_lru_key = (now, (*key).clone());
            self.lru.insert(new_lru_key);
        }
    }