Consolidate input completion code in preparation for advanced completion strategies (#4639)

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

src/controllers/completion/TabCompletionModel.hpp

@@ -0,0 +1,55 @@
+#pragma once
+
+#include "controllers/completion/sources/Source.hpp"
+
+#include <QObject>
+#include <QString>
+#include <QStringListModel>
+
+#include <optional>
+
+namespace chatterino {
+
+class Channel;
+
+/// @brief TabCompletionModel is a QStringListModel intended to provide tab
+/// completion to a ResizingTextInput. The model automatically selects a completion
+/// source based on the current query before updating the results.
+class TabCompletionModel : public QStringListModel
+{
+public:
+    /// @brief Initializes a new TabCompletionModel bound to a Channel.
+    /// The reference to the Channel must live as long as the TabCompletionModel.
+    /// @param channel Channel reference
+    /// @param parent Model parent
+    explicit TabCompletionModel(Channel &channel, QObject *parent);
+
+    /// @brief Updates the model based on the completion query
+    /// @param query Completion query
+    /// @param isFirstWord Whether the completion is the first word in the input
+    void updateResults(const QString &query, bool isFirstWord = false);
+
+private:
+    enum class SourceKind { Emote, User, Command, EmoteAndUser };
+
+    /// @brief Updates the internal completion source based on the current query.
+    /// The completion source will only change if the deduced completion kind
+    /// changes (see deduceSourceKind).
+    /// @param query Completion query
+    void updateSourceFromQuery(const QString &query);
+
+    /// @brief Attempts to deduce the source kind from the current query. If the
+    /// bound Channel is not a TwitchChannel or if the query is too short, no
+    /// query type will be deduced to prevent completions.
+    /// @param query Completion query
+    /// @return An optional SourceKind deduced from the query
+    std::optional<SourceKind> deduceSourceKind(const QString &query) const;
+
+    std::unique_ptr<completion::Source> buildSource(SourceKind kind) const;
+
+    Channel &channel_;
+    std::unique_ptr<completion::Source> source_{};
+    std::optional<SourceKind> sourceKind_{};
+};
+
+}  // namespace chatterino