If the following topic is subscribed to, a tree structure like the one shown in the diagram is formed.
/Alpha/Bravo/Echo
/Alpha/Charlie/Foxtrot
/Alpha/Charlie/Golf
/Alpha/+/Quebec
/Alpha/#
graph TB
A[Alpha]-->B[Bravo]
A-->C[Charlie]
A-->+
B-->E[Echo]
C-->F[Foxtrot]
C-->G[Golf]
+-->Q[Quebec]
A-->#
Each node may contain a value, assuming in the case of an MQTT broker, it might have a transmission channel. The same node can hold multiple values. This requirement corresponds to the scenario where multiple clients can subscribe to the same topic.
Note
Currently, the implementation is using RefCell and Rc, but for multi-threaded implementation, it will be necessary to switch to using Mutex and Arc.
Each node holds a Weak reference to its parent. This is because when MQTT implements Unsubscribe, it must delete the node. Additionally, apart from the tree, I believe it should also hold a Weak indexed by ID. This is because nodes held during client disconnection must be deleted.