Method put

datafusion/execution/src/cache/lru_queue.rs:104–134

Inserts an entry in the queue, becoming the most recently used. If the entry already exists, returns the previous value.

(&mut self, key: K, value: V)

    /// Inserts an entry in the queue, becoming the most recently used.
    /// If the entry already exists, returns the previous value.
    pub fn put(&mut self, key: K, value: V) -> Option<V> {
        let old_value = self.remove(&key);

        let node = Arc::new(Mutex::new(LruNode {
            key: key.clone(),
            prev: None,
            next: None,
        }));

        match self.queue.head {
            // queue is not empty
            Some(ref old_head) => {
                old_head
                    .upgrade()
                    .expect("value has been unexpectedly dropped")
                    .lock()
                    .prev = Some(Arc::downgrade(&node));
                node.lock().next = Some(Weak::clone(old_head));
                self.queue.head = Some(Arc::downgrade(&node));
            }
            // queue is empty
            _ => {
                self.queue.head = Some(Arc::downgrade(&node));
                self.queue.tail = Some(Arc::downgrade(&node));
            }
        }

        self.data.insert(key, (node, value));

        old_value
    }

    /// Removes and returns the least recently used value.
    /// Returns `None` if the queue is empty.