Re: fast-import deltas

From: Junio C Hamano <gitster@pobox.com>
Date: Apr 1, 2014, 22:32 UTC
Message-ID: <xmqqlhvoy92m.fsf@gitster.dls.corp.google.com>
In-Reply-To: <20140401221003.GA5923@glandium.org>

Mike Hommey <mh@glandium.org> writes:

Show 12 quoted lines

> On Tue, Apr 01, 2014 at 10:14:02AM -0700, Junio C Hamano wrote:
> ...
>> Unless you already have your change in the xdelta on hand, or the
>> format your foreign change is in gives sufficient information to
>> produce a corresponding xdelta without looking at the content that
>> your foreign change applies to, it is silly to try to convert your
>> foreign change into xdelta and feed it to fast-import.
>
> The patch format I'm getting from mercurial boils down to:
>   - replace N bytes from offset in the source material with the given
>     M bytes.
> Which can easily be converted to xdelta without looking at the original.

If that is the case, and if you can identify the original in a way fast-import can understand, it might be interesting [*1*] to add support for accepting <base object, xdelta> pair in place of blob data, as Jonathan already hinted earlier.

It would not be sufficient for the receiving fast-import to store the delta data to its output---it needs to make sure that the base object is stored in the same output file as well, but that should not be too complicated.

[Footnote]

*1* I am still not sure how useful the feature would be outside slurping from Hg and Git---for obvious reasons, though. As long as the change is to a cleanly isolated codepath, it would be OK, I guess.