In a real life system what is the usual use case for this? Let's say a user reorders documents with drag and drop. What's the low-level equivalent? Could you h

In a real life system what is the usual use case for this? Let's say a user reorders documents with drag and drop. What's the low-level equivalent? Could you help me come up with an algorithm that depends on this? If write speed is important, I'd just use an in-memory LevelDB-like thing (so a log structured merge tree). If the data is not much, then I'd optimize for code simplicity.