public void Test_S2CellId_ParentChildRelationships()
{
S2CellId id = S2CellId.FromFacePosLevel(3, 0x12345678, S2.kMaxCellLevel - 4);
Assert.True(id.IsValid());
Assert.Equal(3UL, id.Face());
Assert.Equal(0x12345700UL, id.Pos());
Assert.Equal(S2.kMaxCellLevel - 4, id.Level());
Assert.False(id.IsLeaf());
Assert.Equal(0x12345610UL, id.ChildBegin(id.Level() + 2).Pos());
Assert.Equal(0x12345640UL, id.ChildBegin().Pos());
Assert.Equal(0x12345400UL, id.Parent().Pos());
Assert.Equal(0x12345000UL, id.Parent(id.Level() - 2).Pos());
// Check ordering of children relative to parents.
Assert.True(id.ChildBegin() < id);
Assert.True(id.ChildEnd() > id);
Assert.Equal(id.ChildEnd(), id.ChildBegin().Next().Next().Next().Next());
Assert.Equal(id.RangeMin(), id.ChildBegin(S2.kMaxCellLevel));
Assert.Equal(id.RangeMax().Next(), id.ChildEnd(S2.kMaxCellLevel));
// Check that cells are represented by the position of their center
// along the Hilbert curve.
Assert.Equal(2 * id.Id, id.RangeMin().Id + id.RangeMax().Id);
}