Document how to get the next ULID

[tibbe]

If you receive an ULID from some external source (e.g. a database) you might want to compute the next following ULID. This is useful for range-style queries where you are trying to retrieve every item after the aforementioned ULID. The library already does so internally to provide monotonic values but it's not entirely clear how to get the monotonically "next" ULID, given another one.

Example:

prev = ulid.parse(some_str)  # From external source
next = ...  # ???

I was playing with ulid.create but I couldn't quite figure it out. It seems to be that bumping the randomness by one and if that overflow bumping the timestamp by one is what we want.

A ULID.next method would be really nice.

[tibbe]

I would have thought something like ulid.monotonic.create(prev, prev)) would work but it doesn't (it produces the same ULID as prev).

[tibbe]

Here's a possible implementation:

def next_ulid(prev: ulid.ULID):
    randomness = prev.randomness()
    if randomness == ulid.MAX_RANDOMNESS:
        timestamp = prev.timestamp()
        return ulid.create(timestamp.int + 1, randomness)
    return ulid.create(prev, randomness.int + 1)