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Re: [PATCH 4/9] submodule.c: sort changed_submodule_names before searching it

On Wed, Sep 12, 2018 at 11:18 AM Junio C Hamano <gitster@xxxxxxxxx> wrote:
> Stefan Beller <sbeller@xxxxxxxxxx> writes:
> > 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.
> Sorry, but I still do not get the math you are implying in the
> second paragraph.  Are you saying that append-then-sort is efficient
> when items being appended is already sorted?  That depends on the
> sorting algorithm used, so the logic is incomplete unless you say
> "given that we use X for sorting,...", I think.
> Do we really discover new items in sorted order, by the way?  In a
> single diff invocation made inside collect_changed_submodules() for
> one commit in the superproject's history, we will grab changed paths
> in the pathname order (i.e. sorted); if the superproject's tip commit
> touches the submodules at paths A and Z, we will discover these two
> paths in sorted order.
> But because we are walking the superproject's history to collect all
> paths that have been affected in that function, and repeatedly
> calling diff as we discover commit in the superproject's history, I
> am not sure how well the resulting set of paths would be sorted.
> The tip commit in superproject's history may have modified the
> submodule at path X, the parent of that commit may have touched the
> submodule at path M, and its parent may have touched the submodule
> at path A.  Don't we end up grabbing these paths in that discoverd
> order, i.e. X, M and A?

That is true.

> I still think changing it from "insert as we find an item, keeping
> the list sorted" to "append all and then sort before we start
> looking things up from the result" makes sense, but I do not think
> the "we find things in sorted order" is either true, or it would
> affect the choice between the two.  A justification to choose the
> latter I can think of that makes sense is that we don't have to pay
> cost to keep the list sorted while building it because we do not do
> any look-up while building the list.