text: import pygame, sys
  from pygame.locals import *
  from bullet import Bullet
  from alien import Alien
  from time import sleep
  
  def check_keydown_events(event, ai_settings, aliens, stats, screen, ship, bullets):
  	if event.key == K_RIGHT:
  		ship.moving_right = True
  	elif event.key == K_LEFT:
  		ship.moving_left = True
  	elif event.key == K_SPACE:
  		fire_bullet(ai_settings, screen, ship, bullets)
  	elif event.key == K_q:
  		pygame.quit()
  		sys.exit()
  	elif event.key == K_p and not stats.game_active:
  		start_game(stats, aliens, bullets, ship)
  
  def fire_bullet(ai_settings, screen, ship, bullets):
  	if len(bullets) < ai_settings.bullets_allowed:
  			new_bullet = Bullet(ai_settings, screen, ship)
  			bullets.add(new_bullet)
  
  def check_keyup_events(event, ship):
  	if event.key == K_RIGHT:
  		ship.moving_right = False
  	elif event.key == K_LEFT:
  		ship.moving_left = False
  
  def start_game(stats, aliens, bullets, ship):
  	pygame.mouse.set_visible(False)
  	stats.reset_stats()
  	stats.game_active = True
  	aliens.empty()
  	bullets.empty()
  	ship.center_ship()
  
  def check_play_button(ai_settings, screen, stats, play_button, ship, aliens, bullets, mouse_x, mouse_y):
  	if play_button.rect.collidepoint(mouse_x, mouse_y) and not stats.game_active:
  		ai_settings.initialize_dynamic_settings()
  		start_game(stats, aliens, bullets, ship)
  
  def check_events(ai_settings, screen, stats, play_button, ship, aliens, bullets):
  	for event in pygame.event.get():
  		if event.type == QUIT:
  			pygame.quit()
  			sys.exit()
  
  		elif event.type == KEYDOWN:
  			check_keydown_events(event, ai_settings, aliens, stats, screen, ship, bullets)
  
  		elif event.type == KEYUP:
  			check_keyup_events(event, ship)
  
  		elif event.type == MOUSEBUTTONDOWN:
  			mouse_x, mouse_y = pygame.mouse.get_pos()
  			check_play_button(ai_settings, screen, stats, play_button, ship, aliens, bullets, mouse_x, mouse_y)
  		
  def update_screen(ai_settings, screen, stats, sb, ship, aliens, bullets, play_button):
  	screen.fill(ai_settings.bg_color)
  	for bullet in bullets.sprites():
  		bullet.draw_bullet()
  	ship.blitme()
  	aliens.draw(screen)
  	sb.show_score()
  	if not stats.game_active:
  		play_button.draw_button()
  	pygame.display.flip()
  
  def update_bullets(ai_settings, screen, stats, sb, ship, aliens, bullets):
  	bullets.update()
  	for bullet in bullets.copy():
  		if bullet.rect.bottom <= 0:
  			bullets.remove(bullet)
  	check_bullet_alien_collision(bullets, aliens, screen, stats, sb, ship, ai_settings)
  	
  def check_bullet_alien_collision(bullets, aliens, screen, stats, sb, ship, ai_settings):
  	collisions = pygame.sprite.groupcollide(bullets, aliens, True, True)
  	if collisions:
  			for aliens in collisions.values():
  				stats.score += ai_settings.alien_points * len(aliens)
  				sb.prep_score()
  			check_high_score(stats, sb)
  	if len(aliens) == 0:
  		bullets.empty()
  		create_fleet(ai_settings, screen, ship, aliens)
  		ai_settings.increase_speed()
  
  def get_number_aliens_x(ai_settings, alien_width):
  	available_space_x = ai_settings.screen_width - (2 * alien_width)
  	number_aliens_x = int(available_space_x / (2 * alien_width))
  	return number_aliens_x
  
  def create_alien(ai_settings, screen, aliens, alien_number, row_number):
  	alien = Alien(ai_settings, screen)
  	alien_width = alien.rect.width
  	alien.x = alien_width + 2 * alien_width * alien_number
  	alien.rect.x = alien.x
  	alien.rect.y = alien.rect.height + 2 * alien.rect.height * row_number
  	aliens.add(alien)
  
  def get_number_rows(ai_settings, ship_height, alien_height):
  	available_space_y = (ai_settings.screen_height - (3 * alien_height) - ship_height)
  	number_rows = int(available_space_y / (2 * alien_height))
  	return number_rows
  
  def create_fleet(ai_settings, screen, ship, aliens):
  	alien = Alien(ai_settings, screen)
  	number_aliens_x = get_number_aliens_x(ai_settings, alien.rect.x)
  	number_rows = get_number_rows(ai_settings, ship.rect.height, alien.rect.height)
  
  	for row in range(number_rows):
  		for alien_number in range(number_aliens_x):
  			create_alien(ai_settings, screen, aliens, alien_number, row)
  
  def check_fleet_edges(ai_settings, aliens):
  	for alien in aliens.sprites():
  		if alien.check_edges():
  			change_fleet_direction(ai_settings, aliens)
  			break
  
  def change_fleet_direction(ai_settings, aliens):
  	for alien in aliens.sprites():
  		alien.rect.y += ai_settings.fleet_drop_speed
  	ai_settings.fleet_direction *= -1
  
  def ship_hit(ai_settings, stats, screen, ship, aliens, bullets):
  	if stats.ships_left > 1:
  		stats.ships_left -= 1
  		aliens.empty()
  		bullets.empty()
  
  		create_fleet(ai_settings, screen, ship, aliens)
  		ship.center_ship()
  
  		sleep(0.5)
  	else:
  		stats.game_active = False
  		pygame.mouse.set_visible(True)
  
  def check_aliens_bottom(ai_settings, stats, screen, ship, aliens, bullets):
  	screen_rect = screen.get_rect()
  	for alien in aliens.sprites():
  		if alien.rect.bottom >= screen_rect.bottom:
  			ship_hit(ai_settings, stats, screen, ship, aliens, bullets)
  			break
  
  def update_aliens(ai_settings, aliens, stats, screen, ship, bullets):
  	check_fleet_edges(ai_settings, aliens)
  	aliens.update()
  	if pygame.sprite.spritecollideany(ship, aliens):
  		ship_hit(ai_settings, stats, screen, ship, aliens, bullets)
  	check_aliens_bottom(ai_settings, stats, screen, ship, aliens, bullets)
  
  def check_high_score(stats, sb):
  	if stats.score > stats.high_score:
  		stats.high_score = stats.score
  		sb.prep_high_score()