Re: [PATCH] Fix -B "very-different" logic.

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"LT" == Linus Torvalds [off-list ref] writes:

LT> Careful. 

LT> I think the amount of new code _should_ matter. Otherwise, an old empty
LT> file would always be considered the source of a new file, since the diff 
LT> doesn't remove anything. Similarly, just because we have a boilerplate 
LT> file shouldn't make that always be considered a "wonderful source", when 
LT> people add the real meat to it.

Yes, I agree that rename/copy logic should use different
heuristics from the one I proposed for breaking.

It is my assumption that people in practice tend to make only
small edits after a rename/copy just to adjust things like:

 - filenames mentioned in the comment of the file itself,

 - include paths that refer other files if the file was
   moved/copied from a different directory,

 - names of functions and variables.

and making sure there would not be too much new stuff is quite
useful to detect rename/copy source correctly as the current
similarity estimator in diffcore-rename does.  I do not intend
to touch that.

The boilderplate example you mention is a very good reason not
to dismiss the amount of new material when doing rename/copy
detection.

LT> In particular, let's say that I used to have two files:

LT> 	a.c - small helper functions
LT> 	b.c - the "meat" of the thing

LT> and I end up deciding that I might as well collapse it all into one file, 
LT> a.c. What happens? There's almost no deletes from a.c, but there's a lot 
LT> of new code in it. 

LT> See what I'm saying?

Yes.  I think I do.

When git-diff-tree -B -C runs your example, it feeds diffcore
with these:

  :100644 100644 sha1-a-helper-only sha1-a-and-meat M   a.c
  :100644 000000 sha1-b-stale-meat  0{40}           D   b.c

The ideal diffcore-break breaks a.c because it looks at
insertions as well:

  :100644 000000 sha1-a-helper-only 0{40}           D   a.c
  :000000 100644 0{40}              sha1-a-and-meat N   a.c
  :100644 000000 sha1-b-stale-meat  0{40}           D   b.c

Then diffcore-rename notices that sha1-b-stale-meat is better
match than sha1-a-helper-only to produce sha1-a-and-meat, and
resolves the above to:

  :100644 100644 sha1-b-stale-meat  sha1-a-and-meat R   b.c	a.c

Up to this point is just a demonstration that I see your point.

But I still want to keep the example I gave in the original
commit message.  Suppose you did not have b.c file under version
control, and did the same operation.  I.e. a.c acquired a lot of
good stuff.  git-diff-tree -B -C feeds:

  :100644 100644 sha1-a-helper-only sha1-a-and-meat M   a.c

which is broken into:

  :100644 000000 sha1-a-helper-only 0{40}           D   a.c
  :000000 100644 0{40}              sha1-a-and-meat N   a.c

Unfortunately, in this case nobody absorbs these pairs.  I want
to allow you to add 1000 lines of new stuff to a file (which was
originally 100 lines long) as long as you do not remove too many
lines from the original 100 lines without triggering "this is a
rewrite" logic in this case.  So after rename/copy runs, we need
to match these up and merge them back into the original.

  :100644 100644 sha1-a-helper-only sha1-a-and-meat M   a.c

We should carry a bit more information about broken entries than
we currently do.  We would break a pair based on both deletion
and insertion, just like the current code (i.e. without the
patch you are responding to) does.  But when we do break a pair,
we need to mark them if the "new" side have enough original
source material remaining.  If we have such mark to tell us that
"these were broken but there are a good chunk of source material
remaining", the clean-up phase, to run after diffcore-rename
finishes, should be able to notice surviving broken pairs and
merge them back accordingly.