Re: [PATCH 06/11] submodule.c: sort changed_submodule_names before searching it

Stefan Beller <sbeller@google.com> writes:

It is unclear what problem you are solving, especially with subjunctive "could" there. We are creating an unsorted list and then sorting it and you see it as a problem because it is just as easy and efficient to do the insertion sort while building up the list? (don't react and answer without reading all the way to the end; I think I know what is going on).

That may be a statement of a fact, but it is unclear how that fact relates to what the code is doing before this patch, or what the code updated by this patch is doing.

It is unclear what (performance?) trade-off this senttence is trying to make. It sounds as if it is claiming this:

	We can string_list_insert() to maintain sorted-ness of the
	list as we find new items, or we can string_list_append() to
	build an unsorted list and sort it at the end just once.

	To pick which one is more appropriate, we notice the fact
	that we discover new items more or less in the already
	sorted order.  That makes "append then sort" more
	appropriate.

But is that reasoning sensible?  

I'd imagine that append-then-sort would always be more efficient than insert-at-the-right-place-as-we-go, and the reason why we sometimes would need to do the latter is when we need to look up elements in the middle of building the list (e.g. we may want to de-dup, which requires us to look up a name from the ones we collected so far).

And in this application, calculate_changed_submodule_paths() discover paths by calling collect_changed_submodules() which finds a mapping <submodule name, oid of commits> into a list sorted by submodule name, and then goes through that list and builds a list of submodules paths (which could be different from submodule names) by appending. Only after this list is fully built, get_next_submodule() gets called, so making the latter use string_list_lookup() that assumes a sorted list is safe if we built the list by append-then-sort (iow, sortedness while building the list does not matter).

Having analysed all that, I find it somewhat iffy that _append() is used there in the calculate_changed_submodule_paths() function. It would cause the resulting changed_submodule_names list to hold the same name twice (or more), but I do not know if that would pose a problem to the consumer of the list. Using "accumulate then sort before calling look-up" would not change it as string_list_sort() would not dedup, so I do not think this patch would introduce a new problem, though.