This code is a bit flatter and just preps the data
for a single user. There is never any interaction
between the data for user A and user B, so we can
mostly avoid complicated nested data structures
and do most of the data-crunching on a per-user basis.
We also do an explicit sort of the data before
running it through groupby. The explicit sort
simplifies how we calculate `most_recent_info`
and also avoids needing to add `dt` to an intermediate
data structure.
Finally, when it comes to the individual client data,
the code has relied on the assumption that there is
only one row per client, which I believe to be true,
but now the code is more explicit about that.
Steve Howell