def test_nonempty_group():
g = Group()
s1 = Sphere()
s2 = Sphere()
s2.set_transform(Translation(0, 0, -3))
s3 = Sphere()
s3.set_transform(Translation(5, 0, 0))
g.add_child(s1)
g.add_child(s2)
g.add_child(s3)
r = Ray(Point(0, 0, -5), Vector(0, 0, 1))
xs = g.local_intersect(r)
assert len(xs) == 4
assert xs[0].object == s2
assert xs[1].object == s2
assert xs[2].object == s1
assert xs[3].object == s1
def test_empty_group():
g = Group()
r = Ray(Point(0, 0, 0), Vector(0, 0, 1))
xs = g.local_intersect(r)
assert len(xs) == 0