import pygame



def run_game():
    
    ## Initialize the pygame submodules and set up the display window.

    pygame.init()

    width = 640
    height = 480
    my_win = pygame.display.set_mode((width,height))


    # starting position for the circle
    x = 320
    y = 240
    radius = 50

    xspeed = -1
    yspeed = -1

    ## The game loop starts here.
    
    keepGoing = True    
    while (keepGoing):

        ## Handle events.

        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                keepGoing = False

        # update circle position
        x = x + xspeed
        y = y + yspeed

        if (x < 0):
            xspeed = -1 * xspeed
        if (x > width):
            xspeed = -1 * xspeed
        if (y < 0):
            yspeed = -1 * yspeed
        if (y > height):
            yspeed = -1 * yspeed
        

        ## Draw picture and update display

        my_win.fill(pygame.color.Color("black"))

        pygame.draw.circle(my_win, pygame.color.Color('navy'), (x,y),radius)

        pygame.display.update()

    ## The game loop ends here.

    pygame.quit()


## Call the function run_game.

run_game()