Merge branch 'ac/bitmap-format-doc' into maint

Adjust technical/bitmap-format to be formatted by AsciiDoc, and
add some missing information to the documentation.
source: <pull.1246.v4.git.1655355834.gitgitgadget@gmail.com>

* ac/bitmap-format-doc:
  bitmap-format.txt: add information for trailing checksum
  bitmap-format.txt: fix some formatting issues
  bitmap-format.txt: feed the file to asciidoc to generate html
pull/1309/head
Junio C Hamano 2 months ago
commit 8f6b482d24
  1. 1
      Documentation/Makefile
  2. 203
      Documentation/technical/bitmap-format.txt

@ -94,6 +94,7 @@ TECH_DOCS += MyFirstContribution
TECH_DOCS += MyFirstObjectWalk
TECH_DOCS += SubmittingPatches
TECH_DOCS += ToolsForGit
TECH_DOCS += technical/bitmap-format
TECH_DOCS += technical/bundle-format
TECH_DOCS += technical/cruft-packs
TECH_DOCS += technical/hash-function-transition

@ -25,9 +25,9 @@ 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
@ -39,97 +39,108 @@ MIDXs, both the bit-cache and rev-cache extensions are required.
== On-disk format
- 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:
- 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.
- 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
the serialization format of an EWAH bitmap).
There is a bitmap for each Git object type, stored in the following
order:
- 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).
- 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:
- 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
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
the serialization format of an EWAH bitmap).
+
There is a bitmap for each Git object type, stored in the following
order:
+
- 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).
* 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.
** {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
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.
** {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.
* {empty}
TRAILER: ::
Trailing checksum of the preceding contents.
== Appendix A: Serialization format for an EWAH bitmap
@ -142,8 +153,8 @@ 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

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