def ccw(a, b, c):
    """
    1: c is left of the line a->b
    -1: c is on the right of line a->b
    0: c is on the line a->b

    >>> ccw(Vector(0, 0), Vector(3, 4), Vector(6, 8))
    0
    >>> ccw(Vector(0, 0), Vector(3, 4), Vector(6, 9))
    1
    >>> ccw(Vector(0, 0), Vector(3, 4), Vector(6, 7))
    -1
    """
    cp = (b - a).crossp(c - a)
    if cp > 0.0:
        return 1
    elif cp < 0.0:
        return -1
    return 0