Subject: [PATCH v4 0/3] bitmap-format.txt: fix some formatting issues and include checksum info

From: "Abhradeep Chakraborty via GitGitGadget" <[email-masked]>
Date: Thu, 16 Jun 2022 05:03:51 +0000

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 <[email-masked]>
      
       ## 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: ::

-- 
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