From: "Abhradeep Chakraborty via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: Taylor Blau <me@ttaylorr.com>,
Kaartic Sivaraam <kaartic.sivaraam@gmail.com>,
Derrick Stolee <derrickstolee@github.com>,
Junio C Hamano <gitster@pobox.com>,
Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Subject: [PATCH v4 0/3] bitmap-format.txt: fix some formatting issues and include checksum info
Date: Thu, 16 Jun 2022 05:03:51 +0000 [thread overview]
Message-ID: <pull.1246.v4.git.1655355834.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.1246.v3.git.1654858481.gitgitgadget@gmail.com>
There are some issues in the bitmap-format html page. For example, some
nested lists are shown as top-level lists (e.g. [1]- Here
BITMAP_OPT_FULL_DAG (0x1) and BITMAP_OPT_HASH_CACHE (0x4) are shown as
top-level list). There is also a need of adding info about trailing checksum
in the docs.
Changes since v3:
* spaces are used instead of tabs
* fixed remaining <pre> blocks
Changes since v2: The last two commits are updated to address the
suggestions. These changes are -
* previously omitted blank lines are re-added. In the updated commit, use
of <pre> blocks are decreased. Description lists and + are used instead
to add more than one paragraphs under lists. Readability of the source
text might decrease due to the use of +. But other documentation files
(e.g. git-add.txt) also use it to connect two paragraphs. So, I hope this
is acceptable.
* Information about trailing checksum is updated (as suggested by Taylor)
Changes since v1:
* a new commit addressing bitmap-format.txt html page generation is added
* Remove extra indentation from the previous change
* elaborate more about the trailing checksum (as suggested by Kaartic)
initial version:
* first commit fixes some formatting issues
* information about trailing checksum in the bitmap file is added in the
bitmap-format doc.
[1] https://git-scm.com/docs/bitmap-format#_on_disk_format
Abhradeep Chakraborty (3):
bitmap-format.txt: feed the file to asciidoc to generate html
bitmap-format.txt: fix some formatting issues
bitmap-format.txt: add information for trailing checksum
Documentation/Makefile | 1 +
Documentation/technical/bitmap-format.txt | 203 ++++++++++++----------
2 files changed, 108 insertions(+), 96 deletions(-)
base-commit: 5699ec1b0aec51b9e9ba5a2785f65970c5a95d84
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1246%2FAbhra303%2Ffix-doc-formatting-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1246/Abhra303/fix-doc-formatting-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/1246
Range-diff vs v3:
1: a1b9bd9af90 ! 1: 494c1c1bd52 bitmap-format.txt: feed the file to asciidoc to generate html
@@ Documentation/Makefile: TECH_DOCS += MyFirstContribution
TECH_DOCS += ToolsForGit
+TECH_DOCS += technical/bitmap-format
TECH_DOCS += technical/bundle-format
+ TECH_DOCS += technical/cruft-packs
TECH_DOCS += technical/hash-function-transition
- TECH_DOCS += technical/http-protocol
2: c74b9a52c2a ! 2: 25512aa9c5b bitmap-format.txt: fix some formatting issues
@@ Commit message
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
## Documentation/technical/bitmap-format.txt ##
+@@ Documentation/technical/bitmap-format.txt: An object is uniquely described by its bit position within a bitmap:
+ is defined as follows:
+
+ o1 <= o2 <==> pack(o1) <= pack(o2) /\ offset(o1) <= offset(o2)
+-
+- The ordering between packs is done according to the MIDX's .rev file.
+- Notably, the preferred pack sorts ahead of all other packs.
+++
++The ordering between packs is done according to the MIDX's .rev file.
++Notably, the preferred pack sorts ahead of all other packs.
+
+ The on-disk representation (described below) of a bitmap is the same regardless
+ of whether or not that bitmap belongs to a packfile or a MIDX. The only
@@ Documentation/technical/bitmap-format.txt: MIDXs, both the bit-cache and rev-cache extensions are required.
== On-disk format
- - A header appears at the beginning:
-+ * A header appears at the beginning:
-
+-
- 4-byte signature: {'B', 'I', 'T', 'M'}
-+ 4-byte signature: :: {'B', 'I', 'T', 'M'}
-+
-+ 2-byte version number (network byte order): ::
-
+-
- 2-byte version number (network byte order)
- The current implementation only supports version 1
- of the bitmap index (the same one as JGit).
-
+- The current implementation only supports version 1
+- of the bitmap index (the same one as JGit).
+-
- 2-byte flags (network byte order)
-+ 2-byte flags (network byte order): ::
-
- The following flags are supported:
-
+-
+- The following flags are supported:
+-
- - BITMAP_OPT_FULL_DAG (0x1) REQUIRED
-+ ** {empty}
-+ BITMAP_OPT_FULL_DAG (0x1) REQUIRED: :::
-+
- This flag must always be present. It implies that the
- bitmap index has been generated for a packfile or
- multi-pack index (MIDX) with full closure (i.e. where
-@@ Documentation/technical/bitmap-format.txt: MIDXs, both the bit-cache and rev-cache extensions are required.
- JGit, that greatly reduces the complexity of the
- implementation.
-
+- This flag must always be present. It implies that the
+- bitmap index has been generated for a packfile or
+- multi-pack index (MIDX) with full closure (i.e. where
+- every single object in the packfile/MIDX can find its
+- parent links inside the same packfile/MIDX). This is a
+- requirement for the bitmap index format, also present in
+- JGit, that greatly reduces the complexity of the
+- implementation.
+-
- - BITMAP_OPT_HASH_CACHE (0x4)
-+ ** {empty}
-+ BITMAP_OPT_HASH_CACHE (0x4): :::
-+
- If present, the end of the bitmap file contains
- `N` 32-bit name-hash values, one per object in the
- pack/MIDX. The format and meaning of the name-hash is
- described below.
-
+- If present, the end of the bitmap file contains
+- `N` 32-bit name-hash values, one per object in the
+- pack/MIDX. The format and meaning of the name-hash is
+- described below.
+-
- 4-byte entry count (network byte order)
-
-+ 4-byte entry count (network byte order): ::
- The total count of entries (bitmapped commits) in this bitmap index.
-
+- The total count of entries (bitmapped commits) in this bitmap index.
+-
- 20-byte checksum
-
-+ 20-byte checksum: ::
- The SHA1 checksum of the pack/MIDX this bitmap index
- belongs to.
-
+- The SHA1 checksum of the pack/MIDX this bitmap index
+- belongs to.
+-
- - 4 EWAH bitmaps that act as type indexes
-
- Type indexes are serialized after the hash cache in the shape
@@ Documentation/technical/bitmap-format.txt: MIDXs, both the bit-cache and rev-cac
- Each entry contains the following:
-
- - 4-byte object position (network byte order)
-+ * 4 EWAH bitmaps that act as type indexes
+- The position **in the index for the packfile or
+- multi-pack index** where the bitmap for this commit is
+- found.
+-
+- - 1-byte XOR-offset
+- The xor offset used to compress this bitmap. For an entry
+- in position `x`, a XOR offset of `y` means that the actual
+- bitmap representing this commit is composed by XORing the
+- bitmap for this entry with the bitmap in entry `x-y` (i.e.
+- the bitmap `y` entries before this one).
+-
+- Note that this compression can be recursive. In order to
+- XOR this entry with a previous one, the previous entry needs
+- to be decompressed first, and so on.
+-
+- The hard-limit for this offset is 160 (an entry can only be
+- xor'ed against one of the 160 entries preceding it). This
+- number is always positive, and hence entries are always xor'ed
+- with **previous** bitmaps, not bitmaps that will come afterwards
+- in the index.
+-
+- - 1-byte flags for this bitmap
+- At the moment the only available flag is `0x1`, which hints
+- that this bitmap can be re-used when rebuilding bitmap indexes
+- for the repository.
+-
+- - The compressed bitmap itself, see Appendix A.
++ * A header appears at the beginning:
++
++ 4-byte signature: :: {'B', 'I', 'T', 'M'}
++
++ 2-byte version number (network byte order): ::
++
++ The current implementation only supports version 1
++ of the bitmap index (the same one as JGit).
++
++ 2-byte flags (network byte order): ::
++
++ The following flags are supported:
++
++ ** {empty}
++ BITMAP_OPT_FULL_DAG (0x1) REQUIRED: :::
++
++ This flag must always be present. It implies that the
++ bitmap index has been generated for a packfile or
++ multi-pack index (MIDX) with full closure (i.e. where
++ every single object in the packfile/MIDX can find its
++ parent links inside the same packfile/MIDX). This is a
++ requirement for the bitmap index format, also present in
++ JGit, that greatly reduces the complexity of the
++ implementation.
++
++ ** {empty}
++ BITMAP_OPT_HASH_CACHE (0x4): :::
++
++ If present, the end of the bitmap file contains
++ `N` 32-bit name-hash values, one per object in the
++ pack/MIDX. The format and meaning of the name-hash is
++ described below.
++
++ 4-byte entry count (network byte order): ::
++ The total count of entries (bitmapped commits) in this bitmap index.
++
++ 20-byte checksum: ::
++ The SHA1 checksum of the pack/MIDX this bitmap index
++ belongs to.
++
++ * 4 EWAH bitmaps that act as type indexes
++
+Type indexes are serialized after the hash cache in the shape
+of four EWAH bitmaps stored consecutively (see Appendix A for
@@ Documentation/technical/bitmap-format.txt: MIDXs, both the bit-cache and rev-cac
+There is a bitmap for each Git object type, stored in the following
+order:
++
-+ - Commits
-+ - Trees
-+ - Blobs
-+ - Tags
++ - Commits
++ - Trees
++ - Blobs
++ - Tags
+
++
+In each bitmap, the `n`th bit is set to true if the `n`th object
+in the packfile or multi-pack index is of that type.
+++
++The obvious consequence is that the OR of all 4 bitmaps will result
++in a full set (all bits set), and the AND of all 4 bitmaps will
++result in an empty bitmap (no bits set).
+
-+ The obvious consequence is that the OR of all 4 bitmaps will result
-+ in a full set (all bits set), and the AND of all 4 bitmaps will
-+ result in an empty bitmap (no bits set).
-+
-+ * N entries with compressed bitmaps, one for each indexed commit
++ * N entries with compressed bitmaps, one for each indexed commit
++
+Where `N` is the total amount of entries in this bitmap index.
+Each entry contains the following:
+
-+ ** {empty}
-+ 4-byte object position (network byte order): ::
- The position **in the index for the packfile or
- multi-pack index** where the bitmap for this commit is
- found.
-
-- - 1-byte XOR-offset
-+ ** {empty}
-+ 1-byte XOR-offset: ::
- The xor offset used to compress this bitmap. For an entry
- in position `x`, a XOR offset of `y` means that the actual
- bitmap representing this commit is composed by XORing the
- bitmap for this entry with the bitmap in entry `x-y` (i.e.
- the bitmap `y` entries before this one).
--
-- Note that this compression can be recursive. In order to
-- XOR this entry with a previous one, the previous entry needs
-- to be decompressed first, and so on.
--
-- The hard-limit for this offset is 160 (an entry can only be
-- xor'ed against one of the 160 entries preceding it). This
-- number is always positive, and hence entries are always xor'ed
-- with **previous** bitmaps, not bitmaps that will come afterwards
-- in the index.
--
-- - 1-byte flags for this bitmap
++ ** {empty}
++ 4-byte object position (network byte order): ::
++ The position **in the index for the packfile or
++ multi-pack index** where the bitmap for this commit is
++ found.
++
++ ** {empty}
++ 1-byte XOR-offset: ::
++ The xor offset used to compress this bitmap. For an entry
++ in position `x`, a XOR offset of `y` means that the actual
++ bitmap representing this commit is composed by XORing the
++ bitmap for this entry with the bitmap in entry `x-y` (i.e.
++ the bitmap `y` entries before this one).
++
+NOTE: This compression can be recursive. In order to
+XOR this entry with a previous one, the previous entry needs
@@ Documentation/technical/bitmap-format.txt: MIDXs, both the bit-cache and rev-cac
+with **previous** bitmaps, not bitmaps that will come afterwards
+in the index.
+
-+ ** {empty}
-+ 1-byte flags for this bitmap: ::
- At the moment the only available flag is `0x1`, which hints
- that this bitmap can be re-used when rebuilding bitmap indexes
- for the repository.
-
-- - The compressed bitmap itself, see Appendix A.
-+ ** The compressed bitmap itself, see Appendix A.
++ ** {empty}
++ 1-byte flags for this bitmap: ::
++ At the moment the only available flag is `0x1`, which hints
++ that this bitmap can be re-used when rebuilding bitmap indexes
++ for the repository.
++
++ ** The compressed bitmap itself, see Appendix A.
== Appendix A: Serialization format for an EWAH bitmap
+@@ Documentation/technical/bitmap-format.txt: implementation:
+ - 4-byte number of words of the COMPRESSED bitmap, when stored
+
+ - N x 8-byte words, as specified by the previous field
+-
+- This is the actual content of the compressed bitmap.
+++
++This is the actual content of the compressed bitmap.
+
+ - 4-byte position of the current RLW for the compressed
+ bitmap
3: b971558e1cb ! 3: dbb86dca205 bitmap-format.txt: add information for trailing checksum
@@ Commit message
## Documentation/technical/bitmap-format.txt ##
@@ Documentation/technical/bitmap-format.txt: in the index.
- ** The compressed bitmap itself, see Appendix A.
+ ** The compressed bitmap itself, see Appendix A.
+ * {empty}
+ TRAILER: ::
--
gitgitgadget
next prev parent reply other threads:[~2022-06-16 5:04 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-02 13:52 [PATCH 0/2] bitmap-format.txt: fix some formatting issues and include checksum info Abhradeep Chakraborty via GitGitGadget
2022-06-02 13:52 ` [PATCH 1/2] bitmap-format.txt: fix some formatting issues Abhradeep Chakraborty via GitGitGadget
2022-06-06 15:55 ` Junio C Hamano
2022-06-07 10:25 ` Abhradeep Chakraborty
2022-06-02 13:52 ` [PATCH 2/2] bitmap-format.txt: add information for trailing checksum Abhradeep Chakraborty via GitGitGadget
2022-06-07 17:43 ` [PATCH v2 0/3] bitmap-format.txt: fix some formatting issues and include checksum info Abhradeep Chakraborty via GitGitGadget
2022-06-07 17:43 ` [PATCH v2 1/3] bitmap-format.txt: feed the file to asciidoc to generate html Abhradeep Chakraborty via GitGitGadget
2022-06-07 18:39 ` Junio C Hamano
2022-06-08 15:02 ` Abhradeep Chakraborty
2022-06-07 20:21 ` Taylor Blau
2022-06-07 17:43 ` [PATCH v2 2/3] bitmap-format.txt: fix some formatting issues Abhradeep Chakraborty via GitGitGadget
2022-06-07 20:51 ` Taylor Blau
2022-06-07 22:02 ` Junio C Hamano
2022-06-08 16:06 ` Abhradeep Chakraborty
2022-06-08 15:40 ` Abhradeep Chakraborty
2022-06-07 17:43 ` [PATCH v2 3/3] bitmap-format.txt: add information for trailing checksum Abhradeep Chakraborty via GitGitGadget
2022-06-07 20:56 ` Taylor Blau
2022-06-08 16:15 ` Abhradeep Chakraborty
2022-06-07 18:28 ` [PATCH v2 0/3] bitmap-format.txt: fix some formatting issues and include checksum info Junio C Hamano
2022-06-07 20:58 ` Taylor Blau
2022-06-07 21:00 ` Junio C Hamano
2022-06-08 17:12 ` Abhradeep Chakraborty
2022-06-10 10:54 ` [PATCH v3 " Abhradeep Chakraborty via GitGitGadget
2022-06-10 10:54 ` [PATCH v3 1/3] bitmap-format.txt: feed the file to asciidoc to generate html Abhradeep Chakraborty via GitGitGadget
2022-06-10 10:54 ` [PATCH v3 2/3] bitmap-format.txt: fix some formatting issues Abhradeep Chakraborty via GitGitGadget
2022-06-15 2:27 ` Taylor Blau
2022-06-15 14:28 ` Abhradeep Chakraborty
2022-06-10 10:54 ` [PATCH v3 3/3] bitmap-format.txt: add information for trailing checksum Abhradeep Chakraborty via GitGitGadget
2022-06-10 17:01 ` [PATCH v3 0/3] bitmap-format.txt: fix some formatting issues and include checksum info Junio C Hamano
2022-06-15 2:28 ` Taylor Blau
2022-06-15 22:41 ` Junio C Hamano
2022-06-16 5:03 ` Abhradeep Chakraborty via GitGitGadget [this message]
2022-06-16 5:03 ` [PATCH v4 1/3] bitmap-format.txt: feed the file to asciidoc to generate html Abhradeep Chakraborty via GitGitGadget
2022-06-16 5:03 ` [PATCH v4 2/3] bitmap-format.txt: fix some formatting issues Abhradeep Chakraborty via GitGitGadget
2022-06-16 5:03 ` [PATCH v4 3/3] bitmap-format.txt: add information for trailing checksum Abhradeep Chakraborty via GitGitGadget
2022-06-16 18:53 ` [PATCH v4 0/3] bitmap-format.txt: fix some formatting issues and include checksum info Junio C Hamano
2022-06-16 21:18 ` Taylor Blau
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