Speedup startup by caching last message timestamp

The cache is only approximate, i.e. it doesn't skip edits and similar,
but this might be good enough? Also no migration right now.

Speeds up startup by about 5x on my system.

Half the startup time is now loading the powerlevels for each room. We
can probably lazily load those too in the future.

3 files changed, 18 insertions, 6 deletions

diff --git a/src/timeline/RoomlistModel.cpp b/src/timeline/RoomlistModel.cpp

index 1e0b4c23..972f061d 100644
--- a/src/timeline/RoomlistModel.cpp
+++ b/src/timeline/RoomlistModel.cpp
@@ -121,7 +121,7 @@ RoomlistModel::data(const QModelIndex &index, int role) const
             case Roles::Time:
                 return room->lastMessage().descriptiveTime;
             case Roles::Timestamp:
-                return QVariant{static_cast<quint64>(room->lastMessage().timestamp)};
+                return QVariant{static_cast<quint64>(room->lastMessageTimestamp())};
             case Roles::HasUnreadMessages:
                 return this->roomReadStatus.count(roomid) && this->roomReadStatus.at(roomid);
             case Roles::HasLoudNotification:
@@ -333,7 +333,7 @@ RoomlistModel::addRoom(const QString &room_id, bool suppressInsertNotification)
             emit totalUnreadMessageCountUpdated(total_unread_msgs);
         });
 
-        newRoom->updateLastMessage();
+        // newRoom->updateLastMessage();
 
         std::vector<QString> previewsToAdd;
         if (newRoom->isSpace()) {
diff --git a/src/timeline/TimelineModel.cpp b/src/timeline/TimelineModel.cpp

index 142ca793..578d63b7 100644
--- a/src/timeline/TimelineModel.cpp
+++ b/src/timeline/TimelineModel.cpp
@@ -353,14 +353,13 @@ TimelineModel::TimelineModel(TimelineViewManager *manager, QString room_id, QObj
   , manager_(manager)
   , permissions_{room_id_}
 {
-    lastMessage_.timestamp = 0;
-
     this->isEncrypted_ = cache::isRoomEncrypted(room_id_.toStdString());
 
     auto roomInfo            = cache::singleRoomInfo(room_id_.toStdString());
     this->isSpace_           = roomInfo.is_space;
     this->notification_count = roomInfo.notification_count;
     this->highlight_count    = roomInfo.highlight_count;
+    lastMessage_.timestamp   = roomInfo.approximate_last_modification_ts;
 
     // this connection will simplify adding the plainRoomNameChanged() signal everywhere that it
     // needs to be
@@ -1025,10 +1024,21 @@ isYourJoin(const mtx::events::Event<T> &)
     return false;
 }
 
+DescInfo
+TimelineModel::lastMessage() const
+{
+    if (lastMessage_.event_id.isEmpty())
+        QTimer::singleShot(0, this, &TimelineModel::updateLastMessage);
+
+    return lastMessage_;
+}
+
 void
 TimelineModel::updateLastMessage()
 {
-    for (auto it = events.size() - 1; it >= 0; --it) {
+    // only try to generate a preview for the last 1000 messages
+    auto end = std::max(events.size() - 1001, 0);
+    for (auto it = events.size() - 1; it >= end; --it) {
         auto event = events.get(it, decryptDescription);
         if (!event)
             continue;
diff --git a/src/timeline/TimelineModel.h b/src/timeline/TimelineModel.h

index dae64094..ec9a34f1 100644
--- a/src/timeline/TimelineModel.h
+++ b/src/timeline/TimelineModel.h
@@ -311,7 +311,9 @@ public:
     void sendMessageEvent(const T &content, mtx::events::EventType eventType);
     RelatedInfo relatedInfo(const QString &id);
 
-    DescInfo lastMessage() const { return lastMessage_; }
+    DescInfo lastMessage() const;
+    uint64_t lastMessageTimestamp() const { return lastMessage_.timestamp; }
+
     bool isSpace() const { return isSpace_; }
     bool isEncrypted() const { return isEncrypted_; }
     crypto::Trust trustlevel() const;