def snapToGrid(originalPos, offset, step):
    index = int((originalPos - offset) / step) #truncates the remainder away
    return index * gap + offset

def mouseMotion(self, event):
    pixelPos = event.pos[Y]
    odePos = Scroll.pixelPosToOdePos(pixelPos)
    self.tool.positionChanged(odePos)

def pixelPosToOdePos(pixelPos):
    offsetPixelPos = pixelPos - self.viewPortOffset
    return pixelsToOde(offsetPixelPos)

def pixelsToOde(pixels):
    return float(pixels) / float(pixels_in_an_ode_unit)

def positionChanged(self, newPos):
    self.snappedPos = snapToGrid(originalPos, Y_OFFSET, Y_STEP)

#in Tool
def render(self, screen):
    Scroll.render(screen, self.image, self.snappedPos)

#in Scroll
def render(self, screen, image, odePos):
    pixelPos = self.odePosToPixelPos(odePos)
    screen.blit(image, pixelPos) # screen is a surface from pygame for the curious

def odePosToPixelPos(self.odePos):
    offsetPos = odePos + self.viewPortOffset
    return odeToPixels(offsetPos)

def odeToPixels(odeUnits):
    return int(odeUnits * pixels_in_an_ode_unit)

def positionChanged(self, newPos):
    self.snappedPos = snapToGrid(newPos, Y_OFFSET, Y_STEP)

def snapToGrid(originalPos, offset, step):
    EPS = 1e-6
    index = int((originalPos - offset) / step  + EPS) #truncates the remainder away
    return index * gap + offset