import pygame
import random

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))


def move_and_bounce_circle(dt, x, y, speed_x, speed_y, radius):
    """
    This function calculates a circle'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 + radius):
        x = 0 + radius
        speed_x = - speed_x
    elif (x > width - radius):
        x = width - radius
        speed_x = - speed_x

    y = y + dt * speed_y
    if (y < 0 + radius):
        y = 0 + radius
        speed_y = - speed_y
    elif (y > height - radius):
        y = height - radius
        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 balls' position and speed with random numbers; the radius is set to 20
    # the variable ball is used to refer to a 4-tuple representing the ball's x-, and y-coordinates
    # and its x- and y-speed
    radius = 20
    x = random.randint(0+radius,width-radius)
    y = random.randint(0+radius,height-radius)
    x_speed = float(random.randint(-30,30))/100
    y_speed = float(random.randint(-30,30))/100
    ball = (x, y, x_speed, y_speed)


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

    while (keepGoing):

        dt = clock.tick()

        # go through all events that happened and check ...
        for event in pygame.event.get():
            
            # ... whether the event was a quit event
            if event.type == pygame.QUIT:
                keepGoing = False


        # update ball's position and speed; notice that ball refers to a 4-tuple
        ball = move_and_bounce_circle(dt, ball[0], ball[1], ball[2], ball[3], radius)


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

        # draw all ball
        pygame.draw.circle(screen,(255,0,0),(ball[0],ball[1]),radius)

        # update display
        pygame.display.update()


def done():

    pygame.quit()
    


def main():

    initialize()

    run_game()

    done()



main()