is `table-groupby` guaranteed to be stable?

[jbclements]

I'm wondering if the rows in the tables generated by table-groupby always appear in the order in which they occurred in the original table? I'm guessing the answer is yes, and I'd love to nail this down by adding a few tests and some documentation for this.

[massung]

It's been a while since I wrote this, but the applicable code is in table-groupby.

I think there are a couple things to note and consider:

The groups are collected and sorted by their index (see: group-indices). This little snippet does two things:

• Pushes each group key into a list of keys, which is the order each new key appeared in the table.
• Maintains an internal hash of each group: key -> list of row indices.

So, the default behavior is to return each group - and the rows inside that group - to be exactly the same as they were in the source table.

However, it's also possible to supply a less-than? sorting parameter to the function, which will sort the final list of keys generated and return each table in sorted-key order. But the order of the rows within that table should still be in original-table order.

Definitely not opposed to more tests and documentation, though. 😄