class GameStats():
    """ Track statistics for Alien Invasion. """
    
    def __init__(self, ai_settings):
        """ Initialize statistic. """
        self.ai_settings = ai_settings
        self.reset_stats()
        
        # Start Alien Invasion in an inactive state.
        self.game_active = False
        
        # Highest score of history
        self.high_score = 0
    
    def reset_stats(self):
        """ Initialize statistics that can change during the game. """
        self.ships_left = self.ai_settings.ship_limit
        self.score = 0
        self.level = 1