#This is a simple first pygame example program
#I've taken the example from the book, annotated it with comments (which the book
#would explain on the following pages) and then broken it into functions
import pygame

#I need to use screen in more than one function till we see classes
#I'll use this 'global' variable to allow the same variable in more than one method
screen = None
background = None
box = None
box_x=0
box_y =0

def initAndDisplay():
    """initialize pygame and setup the display screen"""
    global screen
    pygame.init()
    # set mode is a method to create a window or full screen display
    # it takes a single parameter a 'tuple' with the desired window width and
    #height in pixals.
    screen = pygame.display.set_mode((800,600))
    pygame.display.set_caption("hello world!") #puts a window title on

def setupEntities():
    global background
    global box
    global box_x
    global box_y
    """just one entity for now - the background"""
    #get the drawable part of the window - create a surface for that whole thing
    
    background = pygame.Surface(screen.get_size())
     
    #change the background from python standard to colors for this system
    #saves time when displaying
    background = background.convert()
    #set the background color
    background.fill((255,0,0))
    box = pygame.Surface((25,25))
    box = box.convert()
    box.fill((0,200,0))
    box_x=0
    box_y=300

def action():
    #create a clock object to set the framerate
    global box_x
    clock = pygame.time.Clock()
    stillWorking = True

    while stillWorking:
        clock.tick(30) #update 30 times/sec (framerate)

        #handle events - only quit for now
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                stillWorking = False
        box_x = box_x + 5
        if box_x > screen.get_width():
            box_x =0
        #now actually draw something
        screen.blit(background, (0,0))
        screen.blit(box, (box_x,box_y))
        pygame.display.flip()
    #one some systems we need to free pygame resources before the program can end
    pygame.quit()


#now actually invoke the functions to run the program
initAndDisplay()
setupEntities()
action()