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Re: How DELTA objects values work and are calculated

On Sat, Jan 5, 2019 at 9:49 AM Farhan Khan <khanzf@xxxxxxxxx> wrote:
> Hi all,
> I'm having trouble understanding how OBJ_REF_DELTA and OBJ_REF_DELTA
> (deltas) work in git. Where does git calculate the sha1 hash values
> when doing "git index-pack" in builtin/index-pack.c. I think my lack
> of understanding of the code is compounded the fact that I do not
> understand what the two object types are.
> From tracing the code starting from index-pack, all non-delta object
> type hashes are calculated in index-pack.c:1131 (parse_pack_objects).
> However, when the function ends, the delta objects hash values are set
> to all 0's.

Delta objects depend on other objects (and even delta ones). To
calculate its sha1 values we may need to recursively calculate sha1
values of its base objects. This is why we do it in a separate phase
because the calculation is more complicated than non-delta objects.

> My questions are:
> A) How do Delta objects work?

A delta object consists of a reference to the base object (either an
sha1 value, or the offset to where the object is) and a "delta" to be
applied on (it's basically a binary diff).

> B) Where and how are the sha1 values calculated?

Start at threaded_second_pass() in index-pack.c, we go through all
delta objects here and try to calculate their sha1 values. Eventually
you'll hit resolve_delta(), where the delta is actually applied to the
base object in the patch_delta() call, and the sha1 value calculated
in the following hash_object_file() call.

> I have read Documentation/technical/pack-format.txt, but am still not clear.
> Thank you!
> --
> Farhan Khan
> PGP Fingerprint: B28D 2726 E2BC A97E 3854 5ABE 9A9F 00BC D525 16EE