fps = 30
screenWidth = 640
screenHeight = 480
#global alienImage

# check to see if the pygame module is in the namespace
try:
    pygame
except NameError:
    # pygame doesn't exsist, so import it.
    # this only happens once.
    import pygame
    from pygame.locals import *

    # initialize the pygame display
    pygame.init()
    pygame.display.set_mode((screenWidth,screenHeight),0,32)
    display = pygame.display.get_surface()

    # initialize the pygame clock
    clk = pygame.time.Clock()


def getMotion(framecount, shapeSize, screenSize, velocity):
    """
        Calculates a linear position using a step number, a
        velocity, and a derived range of motion.
        
        Returns the position.
    """
    
    # calculate the range of motion:
    workingSize = (screenSize - shapeSize) / velocity
    
    # use mod to find position in the range of motion.
    # *2 is used to bounce back and forth inside the range
    pos = framecount % (workingSize * 2)   
    
    # if we are greater than the initial size, we are moving
    # in the opposite direction.
    if pos > workingSize:
        pos = 2*workingSize - pos
    
    return pos * velocity


def play(framecount):
    """ Main game logic, called once per frame."""

    # fill the display with black.  Clears the screen.
    pygame.draw.rect(display, (0,0,0,255),(0,0,screenWidth,screenHeight))

    
    shapeColor = (10,80,10,255)
    shapeWidth = 30
    shapeHeight = 30

    xVel = 10.0 # velocity in pixels per frame
    yVel = 10.0
    
    x = getMotion(framecount, shapeWidth, screenWidth, xVel)
    y = getMotion(framecount, shapeHeight, screenHeight, yVel)


    # draw a shape.
    pygame.draw.ellipse(display, shapeColor,(x,y,shapeWidth,shapeHeight))
       
    
    # update the display.
    pygame.display.flip()

    # check for events, and return true if we get an exit signal.
    events = pygame.event.get()
    for event in events:
        if event.type == QUIT or (event.type == KEYDOWN 
                                    and event.key == K_ESCAPE):
            return True
    
    # try to maintain a set frame rate.
    clk.tick(fps)
    
    # No exit signal was recived, so return false.
    return False