Harden Steam Input gamepad slot mapping

docs/input.js

@@ -893,6 +893,8 @@
 /**
  * @func steam_input_get_controller_for_gamepad_index
  * @desc This function returns the input handle for an XInput gamepad slot, or 0 if that slot is not powered by Steam Input. That can be used to match between native GameMaker `gamepad_` slots and Steam Input controllers on Windows. Since on Windows the GameMaker pad slots from 0 to 3 are XInput controllers, and from 4 to 12 are DirectInput controllers. This function only works with emulated XInput controllers.
+ *
+ * [[NOTE: Invalid slot values return `0`.]]
  * 
  * @param {real} index XInput slot from 0 to 3 included.
  * 
@@ -911,7 +913,7 @@
 
 /**
  * @func steam_input_get_gamepad_index_for_controller
- * @desc This function is the reverse of ${function.steam_input_get_controller_for_gamepad_index}, except it allows you to determine whether a Steam Input handle is being emulated as XInput as well or not. See the definition of the reverse function for more information about slots.
+ * @desc This function is the reverse of ${function.steam_input_get_controller_for_gamepad_index}, except it allows you to determine whether a Steam Input handle is being emulated as XInput as well or not. It returns `-1` when no XInput slot maps to the controller handle. See the definition of the reverse function for more information about slots.
  * 
  * @param {real} controller Input handle of the controller.
  * 

source/Steamworks_gml/extensions/steamworks/steamworks_cpp/GMLSteam/steam_input.cpp

@@ -998,6 +998,9 @@ YYEXPORT void steam_input_get_input_type_for_handle(RValue& Result, CInstance* s
 	Result.val = static_cast<double>(API->GetInputTypeForHandle(con));
 }
 
+static constexpr int steam_input_xinput_slot_count = 4;
+static constexpr int steam_input_no_gamepad_index = -1;
+
 /// (index:real)->input_handle
 YYEXPORT void steam_input_get_controller_for_gamepad_index(RValue& Result, CInstance* selfinst, CInstance* otherinst, int argc, RValue* arg)
 {
@@ -1012,6 +1015,12 @@ YYEXPORT void steam_input_get_controller_for_gamepad_index(RValue& Result, CInst
 		return;
 	}
 
+	if (xinput_ind < 0 || xinput_ind >= steam_input_xinput_slot_count)
+	{
+		Result.v64 = 0;
+		return;
+	}
+
 	Result.v64 = static_cast<int64>(API->GetControllerForGamepadIndex(xinput_ind));
 }
 
@@ -1029,7 +1038,23 @@ YYEXPORT void steam_input_get_gamepad_index_for_controller(RValue& Result, CInst
 		return;
 	}
 
-	Result.val = API->GetGamepadIndexForController(con);
+	int gamepadIndex = API->GetGamepadIndexForController(con);
+	if (gamepadIndex >= 0 && gamepadIndex < steam_input_xinput_slot_count)
+	{
+		Result.val = gamepadIndex;
+		return;
+	}
+
+	for (int i = 0; i < steam_input_xinput_slot_count; ++i)
+	{
+		if (API->GetControllerForGamepadIndex(i) == con)
+		{
+			Result.val = i;
+			return;
+		}
+	}
+
+	Result.val = steam_input_no_gamepad_index;
 }
 
 /// (origin:xbox_origin)->string