Null case: both strings empty

Second string null
[0, 1, "a", undef]
[1, 1, "b", undef]
[2, 1, "c", undef]
[3, 1, "d", undef]
[4, 1, "e", undef]
[5, 1, "f", undef]

First string null
[0, -1, undef, "a"]
[1, -1, undef, "b"]
[2, -1, undef, "d"]
[3, -1, undef, "f"]
[4, -1, undef, "e"]
[5, -1, undef, "c"]

Both strings same length
[0, 0, "a", "a"]
[1, 0, "b", "b"]
[2, -1, "c", "d"]
[3, -1, "d", "f"]
[4, 0, "e", "e"]
[5, 1, "f", "c"]

First string longer
[0, 0, "a", "a"]
[1, 0, "b", "b"]
[2, -1, "c", "d"]
[3, -1, "d", "f"]
[4, 0, "e", "e"]
[5, 1, "f", "c"]
[6, 1, "a", undef]
[7, 1, "b", undef]
[8, 1, "d", undef]
[9, 1, "f", undef]
[10, 1, "e", undef]
[11, 1, "c", undef]