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From: Duy Nguyen <pclouds@gmail.com>
To: Stefan Beller <sbeller@google.com>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	git <git@vger.kernel.org>, "Junio C Hamano" <gitster@pobox.com>,
	"Jeff King" <peff@peff.net>,
	"brian m. carlson" <sandals@crustytoothpaste.net>,
	"Derrick Stolee" <stolee@gmail.com>,
	"Eric Sunshine" <sunshine@sunshineco.com>
Subject: Re: [PATCH v2] pack-format.txt: more details on pack file format
Date: Fri, 11 May 2018 08:41:08 +0200	[thread overview]
Message-ID: <CACsJy8Btz_yYzdMds5SsJ+roSHj4RgiN4eCuPOh0+fod3e2mpw@mail.gmail.com> (raw)
In-Reply-To: <CAGZ79kYYFBE+39aeHX22w3ARmujFcgE8YDUQpp6Hps264r0wfQ@mail.gmail.com>

On Thu, May 10, 2018 at 7:06 PM, Stefan Beller <sbeller@google.com> wrote:
>> +=== Deltified representation
>> +
>> +Conceptually there are only four object types: commit, tree, tag and
>> +blob. However to save space, an object could be stored as a "delta" of
>> +another "base" object. These representations are assigned new types
>> +ofs-delta and ref-delta, which is only valid in a pack file.
>
> ...only valid...
>
> as opposed to loose objects or as opposed to referencing cross-packs?
> I would think the former, not the latter.

Yeah. This is pretty much an implementation detail of a pack. The
"real" type is always blob/commit/tree/tag. But you only see this when
you dig deep down in pack-related code.

>> +Both ofs-delta and ref-delta store the "delta" against another
>> +object. The difference between them is, ref-delta directly encodes
>> +20-byte base object name. If the base object is in the same pack,
>> +ofs-delta encodes the offset of the base object in the pack instead.
>
> Reading this paragraph clears up the question from before.
> The ref delta is a delta to another "reference by hash id (sha1)".
> What abbreviation is OFS? OFfSet ?

I guess so. I never bothered to track down the source for that.

>> +The delta data is a sequence of instructions to reconstruct an object
>> +from the base object.
>
> As said before the base object is of type 1..4, we do not "delta-on-delta"
> yet, but to construct the object we have to create the base object first,
> which itself can be represented as a deltified object leading to a delta
> chain.

Yeah that's the delta chain concept. I'll just make a note here about
base object potentially being a delta object as well.
-- 
Duy

  reply	other threads:[~2018-05-11  6:41 UTC|newest]

Thread overview: 99+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-30 22:07 [PATCH 0/9] get_short_oid UI improvements Ævar Arnfjörð Bjarmason
2018-04-30 22:07 ` [PATCH 1/9] sha1-name.c: remove stray newline Ævar Arnfjörð Bjarmason
2018-04-30 22:07 ` [PATCH 2/9] sha1-array.h: align function arguments Ævar Arnfjörð Bjarmason
2018-04-30 22:07 ` [PATCH 3/9] sha1-name.c: move around the collect_ambiguous() function Ævar Arnfjörð Bjarmason
2018-04-30 22:07 ` [PATCH 4/9] get_short_oid: sort ambiguous objects by type, then SHA-1 Ævar Arnfjörð Bjarmason
2018-05-01 11:11   ` Derrick Stolee
2018-05-01 11:27     ` Ævar Arnfjörð Bjarmason
2018-05-01 12:26       ` Derrick Stolee
2018-05-01 12:36         ` Ævar Arnfjörð Bjarmason
2018-05-01 13:05           ` Derrick Stolee
2018-04-30 22:07 ` [PATCH 5/9] get_short_oid: learn to disambiguate by ^{tag} Ævar Arnfjörð Bjarmason
2018-04-30 22:07 ` [PATCH 6/9] get_short_oid: learn to disambiguate by ^{blob} Ævar Arnfjörð Bjarmason
2018-04-30 22:07 ` [PATCH 7/9] get_short_oid / peel_onion: ^{tree} should mean tree, not treeish Ævar Arnfjörð Bjarmason
2018-05-01  1:13   ` brian m. carlson
2018-04-30 22:07 ` [PATCH 8/9] get_short_oid / peel_onion: ^{tree} should mean commit, not commitish Ævar Arnfjörð Bjarmason
2018-04-30 23:22   ` Eric Sunshine
2018-04-30 22:07 ` [PATCH 9/9] config doc: document core.disambiguate Ævar Arnfjörð Bjarmason
2018-04-30 22:34 ` [PATCH 0/9] get_short_oid UI improvements Stefan Beller
2018-05-01  1:27 ` brian m. carlson
2018-05-01 11:16 ` Derrick Stolee
2018-05-01 12:06 ` [PATCH v2 00/12] " Ævar Arnfjörð Bjarmason
2018-05-01 13:03   ` [PATCH v2 06/11] get_short_oid: sort ambiguous objects by type, then SHA-1 Derrick Stolee
2018-05-01 13:39     ` Ævar Arnfjörð Bjarmason
2018-05-01 13:44       ` Derrick Stolee
2018-05-01 14:10         ` Ævar Arnfjörð Bjarmason
2018-05-01 14:15           ` Derrick Stolee
2018-05-01 18:40   ` [PATCH v3 00/12] get_short_oid UI improvements Ævar Arnfjörð Bjarmason
2018-05-02 12:42     ` Derrick Stolee
2018-05-02 13:45       ` Derrick Stolee
2018-05-03  6:43         ` Jacob Keller
2018-05-01 18:40   ` [PATCH v3 01/12] sha1-name.c: remove stray newline Ævar Arnfjörð Bjarmason
2018-05-01 18:40   ` [PATCH v3 02/12] sha1-array.h: align function arguments Ævar Arnfjörð Bjarmason
2018-05-01 18:40   ` [PATCH v3 03/12] git-p4: change "commitish" typo to "committish" Ævar Arnfjörð Bjarmason
2018-05-01 18:40   ` [PATCH v3 04/12] cache.h: add comment explaining the order in object_type Ævar Arnfjörð Bjarmason
2018-05-03  5:05     ` Junio C Hamano
2018-05-08 15:35     ` Duy Nguyen
2018-05-08 15:56       ` [PATCH] pack-format.txt: more details on pack file format Nguyễn Thái Ngọc Duy
2018-05-08 17:23         ` Stefan Beller
2018-05-08 18:22           ` Duy Nguyen
2018-05-08 18:58             ` Stefan Beller
2018-05-08 18:21         ` Ævar Arnfjörð Bjarmason
2018-05-08 18:24           ` Duy Nguyen
2018-05-10 15:09         ` [PATCH v2] " Nguyễn Thái Ngọc Duy
2018-05-10 17:06           ` Stefan Beller
2018-05-11  6:41             ` Duy Nguyen [this message]
2018-05-11  3:54           ` Junio C Hamano
2018-05-11  6:55           ` [PATCH v3] " Nguyễn Thái Ngọc Duy
2018-05-01 18:40   ` [PATCH v3 05/12] sha1-name.c: move around the collect_ambiguous() function Ævar Arnfjörð Bjarmason
2018-05-01 18:40   ` [PATCH v3 06/12] get_short_oid: sort ambiguous objects by type, then SHA-1 Ævar Arnfjörð Bjarmason
2018-05-03  5:13     ` Junio C Hamano
2018-05-08 14:44     ` Jeff King
2018-05-01 18:40   ` [PATCH v3 07/12] get_short_oid: learn to disambiguate by ^{tag} Ævar Arnfjörð Bjarmason
2018-05-01 18:40   ` [PATCH v3 08/12] get_short_oid: learn to disambiguate by ^{blob} Ævar Arnfjörð Bjarmason
2018-05-01 18:40   ` [PATCH v3 09/12] get_short_oid / peel_onion: ^{tree} should be tree, not treeish Ævar Arnfjörð Bjarmason
2018-05-03  5:28     ` Junio C Hamano
2018-05-03  7:28       ` Ævar Arnfjörð Bjarmason
2018-05-04  2:19         ` Junio C Hamano
2018-05-04  8:42           ` Ævar Arnfjörð Bjarmason
2018-05-07  4:08             ` Junio C Hamano
2018-05-08 14:34               ` Jeff King
2018-05-08 18:53                 ` Ævar Arnfjörð Bjarmason
2018-05-09  7:56                   ` Jeff King
2018-05-09 10:48                     ` Ævar Arnfjörð Bjarmason
2018-05-10  4:21                       ` Junio C Hamano
2018-05-10  6:50                         ` Jeff King
2018-05-10 12:42     ` [PATCH v4 0/6] get_short_oid UI improvements Ævar Arnfjörð Bjarmason
2018-05-10 16:04       ` Jeff King
2018-05-10 12:42     ` [PATCH v4 1/6] sha1-name.c: remove stray newline Ævar Arnfjörð Bjarmason
2018-05-10 12:42     ` [PATCH v4 2/6] sha1-array.h: align function arguments Ævar Arnfjörð Bjarmason
2018-05-10 15:06       ` Jeff King
2018-05-11  3:07         ` Junio C Hamano
2018-05-11  3:09           ` Junio C Hamano
2018-05-10 12:43     ` [PATCH v4 3/6] git-p4: change "commitish" typo to "committish" Ævar Arnfjörð Bjarmason
2018-05-10 15:00       ` Luke Diamand
2018-05-10 12:43     ` [PATCH v4 4/6] sha1-name.c: move around the collect_ambiguous() function Ævar Arnfjörð Bjarmason
2018-05-10 12:43     ` [PATCH v4 5/6] get_short_oid: sort ambiguous objects by type, then SHA-1 Ævar Arnfjörð Bjarmason
2018-05-10 15:22       ` Jeff King
2018-05-11  5:36       ` Junio C Hamano
2018-05-10 12:43     ` [PATCH v4 6/6] get_short_oid: document & warn if we ignore the type selector Ævar Arnfjörð Bjarmason
2018-05-10 13:15       ` Martin Ågren
2018-05-10 16:03       ` Jeff King
2018-05-10 16:10         ` Jeff King
2018-05-10 16:15         ` Jeff King
2018-05-01 18:40   ` [PATCH v3 10/12] get_short_oid / peel_onion: ^{commit} should be commit, not committish Ævar Arnfjörð Bjarmason
2018-05-01 18:40   ` [PATCH v3 11/12] config doc: document core.disambiguate Ævar Arnfjörð Bjarmason
2018-05-08 14:41     ` Jeff King
2018-05-01 18:40   ` [PATCH v3 12/12] get_short_oid: document & warn if we ignore the type selector Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 01/12] sha1-name.c: remove stray newline Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 02/12] sha1-array.h: align function arguments Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 03/12] git-p4: change "commitish" typo to "committish" Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 04/12] cache.h: add comment explaining the order in object_type Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 05/12] sha1-name.c: move around the collect_ambiguous() function Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 06/12] get_short_oid: sort ambiguous objects by type, then SHA-1 Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 07/12] get_short_oid: learn to disambiguate by ^{tag} Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 08/12] get_short_oid: learn to disambiguate by ^{blob} Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 09/12] get_short_oid / peel_onion: ^{tree} should be tree, not treeish Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 10/12] get_short_oid / peel_onion: ^{commit} should be commit, not committish Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 11/12] config doc: document core.disambiguate Ævar Arnfjörð Bjarmason
2018-05-01 12:06 ` [PATCH v2 12/12] get_short_oid: document & warn if we ignore the type selector Ævar Arnfjörð Bjarmason

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