/**
     * Get the number of vehicles in a given group.
     * @param group_id The group to get the number of engines in.
     * @param vehicle_type The type of vehicle of the group.
     * @pre IsValidGroup(group_id) || group_id == GROUP_ALL || group_id == GROUP_DEFAULT.
     * @pre IsValidGroup(group_id) || vehicle_type == ScriptVehicle::VT_ROAD || vehicle_type == ScriptVehicle::VT_RAIL ||
     *   vehicle_type == ScriptVehicle::VT_WATER || vehicle_type == ScriptVehicle::VT_AIR
     * @return The number of vehicles in the group with id group_id.
     * @note If the group is valid (neither GROUP_ALL nor GROUP_DEFAULT), the value of
     *  vehicle_type is retrieved from the group itself and not from the input value.
     *  But if the group is invalid, either GROUP_ALL or GROUP_DEFAULT, then a
     *  vehicle type must be specified.
     */
    static int32 GetNumVehicles(GroupID group_id, ScriptVehicle::VehicleType vehicle_type);

/* static */ int32 ScriptGroup::GetNumVehicles(GroupID group_id, ScriptVehicle::VehicleType vehicle_type)
{
    bool valid_group = IsValidGroup(group_id);
    if (!valid_group && group_id != GROUP_DEFAULT && group_id != GROUP_ALL) return -1;
    if (!valid_group && (vehicle_type < ScriptVehicle::VT_RAIL || vehicle_type > ScriptVehicle::VT_AIR)) return -1;

    return GetGroupNumVehicle(ScriptObject::GetCompany(), group_id, valid_group ? ::Group::Get(group_id)->vehicle_type : (::VehicleType)vehicle_type);
}