Question: the proper way to pass some specific tests.

Is there a recommended way to pass iterator tests passing option as below:

• ..9999 (midway) for start, end, lt, gt, lte, gte
• ..a for start, end, lt, gt, lte, gte

[vweevers]

In a nutshell, they test that the implementation sorts keys lexicographically. Numbers are a good way to do that, because they shouldn’t sort “naturally”.

[vweevers]

Why would ‘40000’ ~ ‘50000’ contains ‘45’ ?

Keys are compared character by character (or byte by byte), stopping at inquality. Comparing '45' and '40000', '4' is equal to '4' and '5' is greater than '0' so therefore '45' comes after '40000'.

If a consumer of an abstract-leveldown implementation wants to sort numbers naturally, they can either zero-pad their keys so that '45' becomes e.g. '00045', or to save space, use an encoding like lexicographic-integer.

If then, looks able to be done with radix tree.

Can’t say if that’s the right choice, without knowing anything about your implementation and its underlying storage.