Rewrite & optimize LimitedQueue (#3798)

* Use circular buffer for LimitedQueue

* Reduce copying of snapshot

* Small optimizations

* Remove unneeded lock statements

* Add LimitedQueue tests

* Fix includes for limited queue benchmark

* Update CHANGELOG.md

* Use correct boost version iterators

* Use a shared_mutex to clarify reads and writes

* Update `find`/`rfind` to return the result as a boost::optional

* Use `[[nodiscard]]` where applicable

* Update comments

* Add a couple more doc comments

* Replace size with get

get is a safe (locked & checked) version of at

* Use std::vector in LimitedQueueSnapshot

* Update LimitedQueue benchmarks

* Add mutex guard to buffer accessors

We do not know whether T is an atomic type or not
so we can't safely say that we can copy the value
at a certain address of the buffer.

See https://stackoverflow.com/a/2252478

* Update doc comments, add first/last getters

* Make limit_ const

* Omit `else` if the if-case always returns

* Title case category comments

* Remove `at`

* Fix `get` comment

* Privatize/comment/lock property accessors

 - `limit` is now private
 - `space` is now private
 - `full` has been removed
 - `empty` now locks

* Remove `front` function

* Remove `back` method

* Add comment to `first`

* Add comment to `last`

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/widgets/helper/ChannelView.cpp

@@ -381,7 +381,7 @@ void ChannelView::performLayout(bool causedByScrollbar)
     // BenchmarkGuard benchmark("layout");
 
     /// Get messages and check if there are at least 1
-    auto messages = this->getMessagesSnapshot();
+    auto &messages = this->getMessagesSnapshot();
 
     this->showingLatestMessages_ =
         this->scrollBar_->isAtBottom() || !this->scrollBar_->isVisible();
@@ -502,7 +502,7 @@ QString ChannelView::getSelectedText()
 {
     QString result = "";
 
-    LimitedQueueSnapshot<MessageLayoutPtr> messagesSnapshot =
+    LimitedQueueSnapshot<MessageLayoutPtr> &messagesSnapshot =
         this->getMessagesSnapshot();
 
     Selection _selection = this->selection_;
@@ -561,7 +561,7 @@ const boost::optional<MessageElementFlags> &ChannelView::getOverrideFlags()
     return this->overrideFlags_;
 }
 
-LimitedQueueSnapshot<MessageLayoutPtr> ChannelView::getMessagesSnapshot()
+LimitedQueueSnapshot<MessageLayoutPtr> &ChannelView::getMessagesSnapshot()
 {
     if (!this->paused() /*|| this->scrollBar_->isVisible()*/)
     {
@@ -682,11 +682,9 @@ void ChannelView::setChannel(ChannelPtr underlyingChannel)
 
     auto snapshot = underlyingChannel->getMessageSnapshot();
 
-    for (size_t i = 0; i < snapshot.size(); i++)
+    for (const auto &msg : snapshot)
     {
-        MessageLayoutPtr deleted;
-
-        auto messageLayout = new MessageLayout(snapshot[i]);
+        auto messageLayout = new MessageLayout(msg);
 
         if (this->lastMessageHasAlternateBackground_)
         {
@@ -700,11 +698,10 @@ void ChannelView::setChannel(ChannelPtr underlyingChannel)
             messageLayout->flags.set(MessageLayoutFlag::IgnoreHighlights);
         }
 
-        this->messages_.pushBack(MessageLayoutPtr(messageLayout), deleted);
+        this->messages_.pushBack(MessageLayoutPtr(messageLayout));
         if (this->showScrollbarHighlights())
         {
-            this->scrollBar_->addHighlight(
-                snapshot[i]->getScrollBarHighlight());
+            this->scrollBar_->addHighlight(msg->getScrollBarHighlight());
         }
     }
 
@@ -776,8 +773,6 @@ bool ChannelView::hasSourceChannel() const
 void ChannelView::messageAppended(MessagePtr &message,
                                   boost::optional<MessageFlags> overridingFlags)
 {
-    MessageLayoutPtr deleted;
-
     auto *messageFlags = &message->flags;
     if (overridingFlags)
     {
@@ -809,7 +804,7 @@ void ChannelView::messageAppended(MessagePtr &message,
         loop.exec();
     }
 
-    if (this->messages_.pushBack(MessageLayoutPtr(messageRef), deleted))
+    if (this->messages_.pushBack(MessageLayoutPtr(messageRef)))
     {
         if (this->paused())
         {
@@ -915,22 +910,16 @@ void ChannelView::messageRemoveFromStart(MessagePtr &message)
 
 void ChannelView::messageReplaced(size_t index, MessagePtr &replacement)
 {
-    if (index >= this->messages_.getSnapshot().size())
+    auto oMessage = this->messages_.get(index);
+    if (!oMessage)
     {
         return;
     }
 
+    auto message = *oMessage;
+
     MessageLayoutPtr newItem(new MessageLayout(replacement));
-    auto snapshot = this->messages_.getSnapshot();
-    if (index >= snapshot.size())
-    {
-        qCDebug(chatterinoWidget)
-            << "Tried to replace out of bounds message. Index:" << index
-            << ". Length:" << snapshot.size();
-        return;
-    }
 
-    const auto &message = snapshot[index];
     if (message->flags.has(MessageLayoutFlag::AlternateBackground))
     {
         newItem->flags.set(MessageLayoutFlag::AlternateBackground);
@@ -945,11 +934,9 @@ void ChannelView::messageReplaced(size_t index, MessagePtr &replacement)
 
 void ChannelView::updateLastReadMessage()
 {
-    auto _snapshot = this->getMessagesSnapshot();
-
-    if (_snapshot.size() > 0)
+    if (auto lastMessage = this->messages_.last())
     {
-        this->lastReadMessage_ = _snapshot[_snapshot.size() - 1];
+        this->lastReadMessage_ = *lastMessage;
     }
 
     this->update();
@@ -1055,7 +1042,7 @@ void ChannelView::paintEvent(QPaintEvent * /*event*/)
 // such as the grey overlay when a message is disabled
 void ChannelView::drawMessages(QPainter &painter)
 {
-    auto messagesSnapshot = this->getMessagesSnapshot();
+    auto &messagesSnapshot = this->getMessagesSnapshot();
 
     size_t start = size_t(this->scrollBar_->getCurrentValue());
 
@@ -1122,7 +1109,7 @@ void ChannelView::drawMessages(QPainter &painter)
     // add all messages on screen to the map
     for (size_t i = start; i < messagesSnapshot.size(); ++i)
     {
-        std::shared_ptr<MessageLayout> layout = messagesSnapshot[i];
+        const std::shared_ptr<MessageLayout> &layout = messagesSnapshot[i];
 
         this->messagesOnScreen_.insert(layout);
 
@@ -1153,7 +1140,7 @@ void ChannelView::wheelEvent(QWheelEvent *event)
         qreal desired = this->scrollBar_->getDesiredValue();
         qreal delta = event->angleDelta().y() * qreal(1.5) * mouseMultiplier;
 
-        auto snapshot = this->getMessagesSnapshot();
+        auto &snapshot = this->getMessagesSnapshot();
         int snapshotLength = int(snapshot.size());
         int i = std::min<int>(int(desired), snapshotLength);
 
@@ -1553,7 +1540,7 @@ void ChannelView::mousePressEvent(QMouseEvent *event)
     if (!tryGetMessageAt(event->pos(), layout, relativePos, messageIndex))
     {
         setCursor(Qt::ArrowCursor);
-        auto messagesSnapshot = this->getMessagesSnapshot();
+        auto &messagesSnapshot = this->getMessagesSnapshot();
         if (messagesSnapshot.size() == 0)
         {
             return;
@@ -2387,7 +2374,7 @@ bool ChannelView::tryGetMessageAt(QPoint p,
                                   std::shared_ptr<MessageLayout> &_message,
                                   QPoint &relativePos, int &index)
 {
-    auto messagesSnapshot = this->getMessagesSnapshot();
+    auto &messagesSnapshot = this->getMessagesSnapshot();
 
     size_t start = this->scrollBar_->getCurrentValue();