import pygame


def initialize():
    """
    This function sets some global constants (i.e., the values of these global
    variables never change).
    """
    pygame.init()

    # set up the display window
    global width, height, screen
    width = 640
    height = 480
    screen = pygame.display.set_mode((width,height))

    # load the ballon picture
    global balloon
    balloon = pygame.image.load("red_balloon.gif")
    balloon = balloon.convert()



def move_and_bounce_object(dt, x, y, speed_x, speed_y, obj_width, obj_height):
    """
    This function calculates an object's new position based on its
    old position, the time that has passed since the last update,
    and its speed. It also makes sure that the object bounces
    off the edge of the screen.
    """

    x = x + dt * speed_x
    if (x < 0):
        x = 0
        speed_x = - speed_x
    elif (x > width - obj_width):
        x = width - obj_width
        speed_x = - speed_x
        
    y = y + dt * speed_y
    if (y < 0):
        y = 0
        speed_y = - speed_y
    elif (y > height - obj_height):
        y = height - obj_height
        speed_y = - speed_y

    # Here you see that in Python we can return more than one value.
    # This is a specialy of Python.
    return x, y, speed_x, speed_y



def run_game():

    # initialize the balloon's position and speed
    b_x = 100
    b_y = 100
    b_speed_x = 0.2
    b_speed_y = 0.2

    # start clock
    clock = pygame.time.Clock()
    
    keepGoing = True

    while (keepGoing):

        dt = clock.tick()

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

        # calculate the new position and speed of the balloon
        # Since the function move_and_bounce_object returns several values
        # we need an assignment statement that gives a name to each of these
        # return values.
        b_x, b_y, b_speed_x, b_speed_y = move_and_bounce_object(dt, b_x, b_y, b_speed_x, b_speed_y, balloon.get_width(),balloon.get_height())

        # set background color
        screen.fill(pygame.color.Color("darkblue"))

        # draw balloon image
        screen.blit(balloon,(b_x,b_y))

        # update display
        pygame.display.update()


def done():
    
    pygame.quit()
    


def main():

    initialize()

    run_game()

    done()



main()