Re: Pack v4 again..

Jeff King [off-list ref] writes:

  2. Calculating reachability for packing[1] spends a lot of time in
     lookup_object, as we have to go through each tree saying "have we
     seen object 1234abcd yet?". If we could instead just view the
     differences, we would not have to make those hash lookups for
     entries whose objects we know we have seen.

Hmm, that is an interesting idea.

So in short, we may end up anywhere within a tree object and need to
walk backwards to the start of the entry. But we cannot walk backwards
looking for a NUL, because it may also be part of a sha1. You can only
reliably parse a git tree by starting at the beginning of the stream and
going forwards[3].

That in general is true for non deltified tree object (you cannot
take advantage of the fact that entries are sorted by pathname to
bisect to directly jump to an entry).

If we knew that our deltas were always produced on entry-boundaries (a
"character" in your description above), this would be much simpler.

;-)

[2] One option, of course, is to generate byte-wise deltas, but with a
    promise to always align them on entry boundaries. I'm tempted by
    this, because the result would be readable by existing packv2
    readers. We'd have to set a flag somewhere that indicates the pack
    was written with this property, though.

Yes, that may work.