feat: use a ring buffer for the RoomListService cache

This will limit the memory used by the cache entries (while it was unbounded before). It's now possible to do this,
since we have the `latest_room_event` handy for all the rooms; using the unbounded cache before was papering over
the lack of that feature.

We can bikeshed on the number of entries in this cache. It has to be small enough to not blow up memory (and keep
the linear search over room id fast, but it's secondary), and high enough that we don't hit the full timeline
re-build path that often.

crates/matrix-sdk-ui/src/room_list_service/mod.rs

@@ -66,7 +66,7 @@ mod room;
 mod room_list;
 mod state;
 
-use std::{collections::BTreeMap, future::ready, sync::Arc};
+use std::{future::ready, sync::Arc};
 
 use async_stream::stream;
 use eyeball::{SharedObservable, Subscriber};
@@ -76,6 +76,7 @@ use matrix_sdk::{
     sliding_sync::Ranges, Client, Error as SlidingSyncError, SlidingSync, SlidingSyncList,
     SlidingSyncListBuilder, SlidingSyncMode,
 };
+use matrix_sdk_base::ring_buffer::RingBuffer;
 pub use room::*;
 pub use room_list::*;
 use ruma::{
@@ -102,7 +103,7 @@ pub struct RoomListService {
     state: SharedObservable<State>,
 
     /// Room cache, to avoid recreating `Room`s everytime users fetch them.
-    rooms: Arc<RwLock<BTreeMap<OwnedRoomId, Room>>>,
+    rooms: Arc<RwLock<RingBuffer<Room>>>,
 
     /// The current viewport ranges.
     ///
@@ -112,6 +113,13 @@ pub struct RoomListService {
 }
 
 impl RoomListService {
+    /// Size of the room's ring buffer.
+    ///
+    /// This number should be high enough so that navigating to a room
+    /// previously visited is almost instant, but also not too high so as to
+    /// avoid exhausting memory.
+    const ROOM_OBJECT_CACHE_SIZE: usize = 128;
+
     /// Create a new `RoomList`.
     ///
     /// A [`matrix_sdk::SlidingSync`] client will be created, with a cached list
@@ -168,7 +176,7 @@ impl RoomListService {
         Ok(Self {
             sliding_sync,
             state: SharedObservable::new(State::Init),
-            rooms: Default::default(),
+            rooms: Arc::new(RwLock::new(RingBuffer::new(Self::ROOM_OBJECT_CACHE_SIZE))),
             viewport_ranges: Mutex::new(vec![VISIBLE_ROOMS_DEFAULT_RANGE]),
         })
     }
@@ -305,7 +313,7 @@ impl RoomListService {
     pub async fn room(&self, room_id: &RoomId) -> Result<Room, Error> {
         {
             let rooms = self.rooms.read().await;
-            if let Some(room) = rooms.get(room_id) {
+            if let Some(room) = rooms.iter().find(|room| room.id() == room_id) {
                 return Ok(room.clone());
             }
         }
@@ -315,7 +323,7 @@ impl RoomListService {
             None => return Err(Error::RoomNotFound(room_id.to_owned())),
         };
 
-        self.rooms.write().await.insert(room_id.to_owned(), room.clone());
+        self.rooms.write().await.push(room.clone());
 
         Ok(room)
     }