Codechange: Create API for special hotkeys used to select items from list.

src/hotkeys.h

@@ -62,7 +62,64 @@ bool IsQuitKey(uint16_t keycode);
 void LoadHotkeysFromConfig();
 void SaveHotkeysToConfig();
 
-
 void HandleGlobalHotkeys(char32_t key, uint16_t keycode);
 
+static constexpr int SPECIAL_HOTKEY_BIT = 30; ///< Bit which denotes that hotkey isn't bound to UI button.
+
+/**
+ * Checks if hotkey index is special or not.
+ * @param hotkey Hotkey index to check.
+ * @return True iff the index is for special hotkey.
+ */
+inline bool IsSpecialHotkey(const int &hotkey)
+{
+	return HasBit(hotkey, SPECIAL_HOTKEY_BIT);
+}
+
+/**
+ * Indexes for special hotkeys to navigate in lists.
+ * Values have to have SPECIAL_HOTKEY_BIT set.
+ */
+enum class SpecialListHotkeys : int {
+	PreviousItem = 1 << SPECIAL_HOTKEY_BIT, ///< Hotkey to select previous item in the list.
+	NextItem, ///< Hotkey to select next item in the list.
+	FirstItem, ///< Hotkey to select first item in the list.
+	LastItem, ///< Hotkey to select last item in the list.
+};
+
+/**
+ * Gets the first index in the list for given hotkey and the step to look for another if first is invalid.
+ * @tparam ItemType The type of items stored in the list.
+ * @tparam ListType The type of the list. For example: std::vector<ItemType>, std::list<ItemType>.
+ * @param hotkey The special hotkey to get the index and step for.
+ * @param list The list containing items.
+ * @param current_item The item that is currently chosen, used for next and previous hotkeys.
+ * @return Tuple containing index as first element and step as second. The step for backward direction has positive value, use `% list.size()` to remain in bounds.
+ */
+template<class ItemType, class ListType>
+std::tuple<size_t, size_t> GetListIndexStep(SpecialListHotkeys hotkey, const ListType &list, const ItemType &current_item)
+{
+	/* Don't use -1, because how % is implemented for negative numbers. */
+	size_t step_back = list.size() - 1;
+	auto get_relative_index_step = [list, current_item](size_t step) -> std::tuple<size_t, size_t> {
+		size_t index = std::distance(list.begin(), std::ranges::find(list, current_item));
+		return {(index + step) % list.size(), step};
+	};
+
+	switch (hotkey) {
+		default: NOT_REACHED();
+		case SpecialListHotkeys::FirstItem:
+			return {0, 1};
+
+		case SpecialListHotkeys::LastItem:
+			return {list.size() - 1, step_back};
+
+		case SpecialListHotkeys::PreviousItem:
+			return get_relative_index_step(step_back);
+
+		case SpecialListHotkeys::NextItem:
+			return get_relative_index_step(1);
+	}
+}
+
 #endif /* HOTKEYS_H */