GIT bitmap v1 format

====================

== Pack and multi-pack bitmaps

Bitmaps store reachability information about the set of objects in a packfile,

or a multi-pack index (MIDX). The former is defined obviously, and the latter is

defined as the union of objects in packs contained in the MIDX.

A bitmap may belong to either one pack, or the repository's multi-pack index (if

it exists). A repository may have at most one bitmap.

An object is uniquely described by its bit position within a bitmap:

- If the bitmap belongs to a packfile, the __n__th bit corresponds to

the __n__th object in pack order. For a function `offset` which maps

objects to their byte offset within a pack, pack order is defined as

follows:

o1 <= o2 <==> offset(o1) <= offset(o2)

- If the bitmap belongs to a MIDX, the __n__th bit corresponds to the

__n__th object in MIDX order. With an additional function `pack` which

maps objects to the pack they were selected from by the MIDX, MIDX order

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 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

difference is the interpretation of the bits, which is described above.

Certain bitmap extensions are supported (see: Appendix B). No extensions are

required for bitmaps corresponding to packfiles. For bitmaps that correspond to

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:

** {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.

** {empty}

BITMAP_OPT_LOOKUP_TABLE (0x10): :::

If present, the end of the bitmap file contains a table

containing a list of `N` <commit_pos, offset, xor_row>

triplets. The format and meaning of the table is described

below.

+

NOTE: Unlike the xor_offset used to compress an individual bitmap,

`xor_row` stores an *absolute* index into the lookup table, not a location

relative to the current entry.

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 number 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`, an 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

Ewah bitmaps are serialized in the same protocol as the JAVAEWAH

library, making them backwards compatible with the JGit

implementation:

- 4-byte number of bits of the resulting UNCOMPRESSED bitmap

- 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.

- 4-byte position of the current RLW for the compressed

bitmap

All words are stored in network byte order for their corresponding

sizes.

The compressed bitmap is stored in a form of run-length encoding, as

follows. It consists of a concatenation of an arbitrary number of

chunks. Each chunk consists of one or more 64-bit words

H L_1 L_2 L_3 .... L_M

H is called RLW (run length word). It consists of (from lower to higher

order bits):

- 1 bit: the repeated bit B

- 32 bits: repetition count K (unsigned)

- 31 bits: literal word count M (unsigned)

The bitstream represented by the above chunk is then:

- K repetitions of B

- The bits stored in `L_1` through `L_M`. Within a word, bits at

lower order come earlier in the stream than those at higher

order.

The next word after `L_M` (if any) must again be a RLW, for the next

chunk. For efficient appending to the bitstream, the EWAH stores a

pointer to the last RLW in the stream.

== Appendix B: Optional Bitmap Sections

These sections may or may not be present in the `.bitmap` file; their

presence is indicated by the header flags section described above.

Name-hash cache

---------------

If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains

a cache of 32-bit values, one per object in the pack/MIDX. The value at

position `i` is the hash of the pathname at which the `i`th object

(counting in index or multi-pack index order) in the pack/MIDX can be found.

This can be fed into the delta heuristics to compare objects with similar

pathnames.

The hash algorithm used is:

hash = 0;

while ((c = *name++))

if (!isspace(c))

hash = (hash >> 2) + (c << 24);

Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.

If implementations want to choose a different hashing scheme, they are

free to do so, but MUST allocate a new header flag (because comparing

hashes made under two different schemes would be pointless).

Commit lookup table

-------------------

If the BITMAP_OPT_LOOKUP_TABLE flag is set, the last `N * (4 + 8 + 4)`

bytes (preceding the name-hash cache and trailing hash) of the `.bitmap`

file contains a lookup table specifying the information needed to get

the desired bitmap from the entries without parsing previous unnecessary

bitmaps.

For a `.bitmap` containing `nr_entries` reachability bitmaps, the table

contains a list of `nr_entries` <commit_pos, offset, xor_row> triplets

(sorted in the ascending order of `commit_pos`). The content of the i'th

triplet is -

* {empty}

commit_pos (4 byte integer, network byte order): ::

It stores the object position of a commit (in the midx or pack

index).

* {empty}

offset (8 byte integer, network byte order): ::

The offset from which that commit's bitmap can be read.

* {empty}

xor_row (4 byte integer, network byte order): ::

The position of the triplet whose bitmap is used to compress

this one, or `0xffffffff` if no such bitmap exists.