Re: [PATCH 2/2] repack: repack promisor objects if -a or -A is set

Jonathan Tan [off-list ref] writes:

quoted

each other [*1*], so listing them in the offset order (with made-up
pathname information to _force_ that objects that live close
together in the original pack are grouped together by getting
similar names) might give us a better than horrible deltification.
I dunno.

Before I write the NEEDSWORK comment, just to clarify - do you mean
better than horrible object locality? I think deltification still occurs
because pack-objects sorts the input when doing the windowed
deltification, for example:

But what to delta against what else is determined by the pathname
info, which is now lost by enumerating objects without tree/history
walking.  By giving phoney pathnames to objects while enumerating
them in offset order and giving similar pathnames to objects closer
to each other, I was hoping that better bases will likely to be in
the same window.  The order in which objects are prepared and fed to
try_delta() is "group by type, and then sort by path-hash (major
key) and size (descending order, used as minor key)", so that
the largest among similarly named blobs is stored as base and other
blobs with similar name try to become delta against that one.

  git rev-parse HEAD:fetch-pack.c HEAD HEAD^ HEAD^^ \
	HEAD^^^ v2.17.0:fetch-pack.c | git pack-objects --window=2 abc

produces a packfile with a delta, as checked by `verify-pack -v`.

Is that interesting?  You can make the same argument that
fast-import produces a packfile with a delta.

It is known to produce horrible delta---its delta base selection
essentially is "you are giving me a new object?  let me see if it
deltas well with the object you gave me immediately before that (and
nothing else)".