optimize chatter list (#2814)

* optimize chatter list

* changelog

* Fix tests

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

src/common/ChatterSet.hpp

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <QString>
+#include <functional>
+#include <set>
+#include <unordered_map>
+#include <unordered_set>
+#include "lrucache/lrucache.hpp"
+#include "util/QStringHash.hpp"
+
+namespace chatterino {
+
+/// ChatterSet is a limited container that contains a list of recent chatters
+/// that can be referenced by name.
+class ChatterSet
+{
+public:
+    /// The limit of how many chatters can be saved for a channel.
+    static constexpr size_t chatterLimit = 2000;
+
+    ChatterSet();
+
+    /// Inserts a user name if it isn't contained. Doesn't replace the original
+    /// if the casing hasn't changed.
+    void addRecentChatter(const QString &userName);
+
+    /// Removes chatters that aren't online anymore. Adds chatters that aren't
+    /// in the list yet.
+    void updateOnlineChatters(
+        const std::unordered_set<QString> &lowerCaseUsernames);
+
+    /// Checks if a username is in the list.
+    bool contains(const QString &userName) const;
+
+    /// Get filtered usernames by a prefix for autocompletion. Contained items
+    /// are in mixed case if available.
+    std::vector<QString> filterByPrefix(const QString &prefix) const;
+
+private:
+    // user name in lower case -> user name in normal case
+    cache::lru_cache<QString, QString> items;
+};
+
+using ChatterSet = ChatterSet;
+
+}  // namespace chatterino