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From: Stefan Beller <sbeller@google.com>
To: "Nguyễn Thái Ngọc Duy" <pclouds@gmail.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] pack-format.txt: more details on pack file format
Date: Tue, 8 May 2018 10:23:56 -0700	[thread overview]
Message-ID: <CAGZ79kZiwX-QFnkTfRHby38GYBDwj-0Dyv3_PWPXtnWr+112CA@mail.gmail.com> (raw)
In-Reply-To: <20180508155627.19245-1-pclouds@gmail.com>

On Tue, May 8, 2018 at 8:56 AM, Nguyễn Thái Ngọc Duy <pclouds@gmail.com> wrote:
> The current document mentions OBJ_* constants without their actual
> values. A git developer would know these are from cache.h but that's
> not very friendly to a person who wants to read this file to implement
> a pack file parser.
>
> Similarly, the deltified representation is not documented at all (the
> "document" is basically patch-delta.c). Translate that C code in
> English.
>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  I noticed that these object type values are not documented in
>  pack-format.txt so here's my attempt to improve it.

Thanks for sending this patch!

>
>  While at there, I also add some text about this obscure delta format.
>  We occasionally have questions about this on the mailing list if I
>  remember correctly.

Let me see if I can understand it, as I am not well versed in the
delta format, so ideally I would understand it from the patch here?

>
>  Documentation/technical/pack-format.txt | 41 +++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
>
> diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
> index 8e5bf60be3..2c7d5c0e74 100644
> --- a/Documentation/technical/pack-format.txt
> +++ b/Documentation/technical/pack-format.txt
> @@ -36,6 +36,47 @@ Git pack format
>
>    - The trailer records 20-byte SHA-1 checksum of all of the above.
>
> +Valid object types are:
> +
> +- OBJ_COMMIT (1)
> +- OBJ_TREE (2)
> +- OBJ_BLOB (3)
> +- OBJ_TAG (4)
> +- OBJ_OFS_DELTA (6)
> +- OBJ_REF_DELTA (7)
> +
> +Type 5 is reserved for future expansion.

and type 0 as well, but that is not spelled out?

> +Deltified representation

Does this refer to OFS delta as well as REF deltas?

> is a sequence of one byte command optionally
> +followed by more data for the command. The following commands are
> +recognized:

So a Deltified representation of an object is a 6 or 7 in the 3 bit type
and then the length. Then a command is shown how to construct
the object based on other objects. Can there be more commands?

> +- If bit 7 is set, the remaining bits in the command byte specifies
> +  how to extract copy offset and size to copy. The following must be
> +  evaluated in this exact order:

So there are 2 modes, and the high bit indicates which mode is used.
You start describing the more complicated mode first,
maybe give names to both of them? "direct copy" (below) and
"compressed copy with offset" ?


> +  - If bit 0 is set, the following byte contains bits 0-7 of the copy
> +    offset (this also resets all other bits in the copy offset to
> +    zero).
> +  - If bit 1 is set, the following byte contains bits 8-15 of the copy
> +    offset.
> +  - If bit 2 is set, the following byte contains bits 16-23 of the
> +    copy offset.
> +  - If bit 3 is set, the following byte contains bits 24-31 of the
> +    copy offset.

I assume these bits are exclusive, i.e. if bit 3 is set, bits 0-2 are not
allowed to be set. What happens if they are set, do we care?

If bit 3 is set, then the following byte contains 24-31 of the copy offset,
where is the rest? Do I wait for another command byte with
bits 2,1,0 to learn about the body offsets, or do they follow the
following byte? Something like:

  "If bit 3 is set, then the next 4 bytes are the copy offset,
  in network byte order"


> +  - If bit 4 is set, the following byte contains bits 0-7 of the copy
> +    size (this also resets all other bits in the copy size to zero_.
> +  - If bit 5 is set, the following byte contains bits 8-15 of the copy
> +    size.
> +  - If bit 6 is set, the following byte contains bits 16-23 of the
> +    copy size.

bits 4-7 seem to be another group of mutually exclusive bits.
The same question as above:
If bit 6 is set, where are bits 0-15 of the copy size?

> +
> +  Copy size zero means 0x10000 bytes.

This is an interesting caveat. So we can only copy 1-0x10000 bytes,
and cannot express to copy 0 bytes?

> The data from source object at
> +  the given copy offset is copied back to the destination buffer.
> +
> +- If bit 7 is not set, it is the copy size in bytes. The following
> +  bytes are copied to destination buffer
> +- Command byte zero is reserved for future expansion.

Thanks,
Stefan

  reply	other threads:[~2018-05-08 17:24 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 [this message]
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
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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