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

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

src/controllers/completion/sources/Source.hpp

@@ -0,0 +1,47 @@
+#pragma once
+
+#include "widgets/listview/GenericListModel.hpp"
+#include "widgets/splits/InputCompletionItem.hpp"
+
+#include <QStringList>
+
+#include <memory>
+#include <utility>
+#include <vector>
+
+namespace chatterino::completion {
+
+/// @brief A Source represents a source for generating completion suggestions.
+///
+/// The source can be queried to update its suggestions and then write the completion
+/// suggestions to a GenericListModel or QStringList depending on the consumer's
+/// requirements.
+///
+/// For example, consider providing emotes for completion. The Source instance
+/// initialized with every available emote in the channel (including  global
+/// emotes). As the user updates their query by typing, the suggestions are
+/// refined and the output model is updated.
+class Source
+{
+public:
+    virtual ~Source() = default;
+
+    /// @brief Updates the internal completion suggestions for the given query
+    /// @param query Query to complete against
+    virtual void update(const QString &query) = 0;
+
+    /// @brief Appends the internal completion suggestions to a GenericListModel
+    /// @param model GenericListModel to add suggestions to
+    /// @param maxCount Maximum number of suggestions. Zero indicates unlimited.
+    virtual void addToListModel(GenericListModel &model,
+                                size_t maxCount = 0) const = 0;
+
+    /// @brief Appends the internal completion suggestions to a QStringList
+    /// @param list QStringList to add suggestions to
+    /// @param maxCount Maximum number of suggestions. Zero indicates unlimited.
+    /// @param isFirstWord Whether the completion is the first word in the input
+    virtual void addToStringList(QStringList &list, size_t maxCount = 0,
+                                 bool isFirstWord = false) const = 0;
+};
+
+};  // namespace chatterino::completion