Documentation/technical/bitmap-format.txt - jrn/git - Git at Google

 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.

 Pseudo-merge bitmaps
 --------------------

 If the `BITMAP_OPT_PSEUDO_MERGES` flag is set, a variable number of
 bytes (preceding the name-hash cache, commit lookup table, and trailing
 checksum) of the `.bitmap` file is used to store pseudo-merge bitmaps.

 For more information on what pseudo-merges are, why they are useful, and
 how to configure them, see the information in linkgit:gitpacking[7].

 === File format

 If enabled, pseudo-merge bitmaps are stored in an optional section at
 the end of a `.bitmap` file. The format is as follows:

 ....
 +-------------------------------------------+
 |               .bitmap File                |
 +-------------------------------------------+
 |                                           |
 |  Pseudo-merge bitmaps (Variable Length)   |
 |  +---------------------------+            |
 |  | commits_bitmap (EWAH)     |            |
 |  +---------------------------+            |
 |  | merge_bitmap (EWAH)       |            |
 |  +---------------------------+            |
 |                                           |
 +-------------------------------------------+
 |                                           |
 |  Lookup Table                             |
 |  +---------------------------+            |
 |  | commit_pos (4 bytes)      |            |
 |  +---------------------------+            |
 |  | offset (8 bytes)          |            |
 |  +------------+--------------+            |
 |                                           |
 |  Offset Cases:                            |
 |  -------------                            |
 |                                           |
 |  1. MSB Unset: single pseudo-merge bitmap |
 |     + offset to pseudo-merge bitmap       |
 |                                           |
 |  2. MSB Set: multiple pseudo-merges       |
 |     + offset to extended lookup table     |
 |                                           |
 +-------------------------------------------+
 |                                           |
 |  Extended Lookup Table (Optional)         |
 |  +----+----------+----------+----------+  |
 |  | N  | Offset 1 |   ....   | Offset N |  |
 |  +----+----------+----------+----------+  |
 |  |    |  8 bytes |   ....   |  8 bytes |  |
 |  +----+----------+----------+----------+  |
 |                                           |
 +-------------------------------------------+
 |                                           |
 |  Pseudo-merge position table              |
 |  +----+----------+----------+----------+  |
 |  | N  | Offset 1 |   ....   | Offset N |  |
 |  +----+----------+----------+----------+  |
 |  |    |  8 bytes |   ....   |  8 bytes |  |
 |  +----+----------+----------+----------+  |
 |                                           |
 +-------------------------------------------+
 |                                           |
 |  Pseudo-merge Metadata                    |
 |  +-----------------------------------+    |
 |  | # pseudo-merges (4 bytes)         |    |
 |  +-----------------------------------+    |
 |  | # commits (4 bytes)               |    |
 |  +-----------------------------------+    |
 |  | Lookup offset (8 bytes)           |    |
 |  +-----------------------------------+    |
 |  | Extension size (8 bytes)          |    |
 |  +-----------------------------------+    |
 |                                           |
 +-------------------------------------------+
 ....

 * One or more pseudo-merge bitmaps, each containing:

   ** `commits_bitmap`, an EWAH-compressed bitmap describing the set of
      commits included in the this psuedo-merge.

   ** `merge_bitmap`, an EWAH-compressed bitmap describing the union of
      the set of objects reachable from all commits listed in the
      `commits_bitmap`.

 * A lookup table, mapping pseudo-merged commits to the pseudo-merges
   they belong to. Entries appear in increasing order of each commit's
   bit position. Each entry is 12 bytes wide, and is comprised of the
   following:

   ** `commit_pos`, a 4-byte unsigned value (in network byte-order)
      containing the bit position for this commit.

   ** `offset`, an 8-byte unsigned value (also in network byte-order)
   containing either one of two possible offsets, depending on whether or
   not the most-significant bit is set.

     *** If unset (i.e. `offset & ((uint64_t)1<<63) == 0`), the offset
 	(relative to the beginning of the `.bitmap` file) at which the
 	pseudo-merge bitmap for this commit can be read. This indicates
 	only a single pseudo-merge bitmap contains this commit.

     *** If set (i.e. `offset & ((uint64_t)1<<63) != 0`), the offset
 	(again relative to the beginning of the `.bitmap` file) at which
 	the extended offset table can be located describing the set of
 	pseudo-merge bitmaps which contain this commit. This indicates
 	that multiple pseudo-merge bitmaps contain this commit.

 * An (optional) extended lookup table (written if and only if there is
   at least one commit which appears in more than one pseudo-merge).
   There are as many entries as commits which appear in multiple
   pseudo-merges. Each entry contains the following:

   ** `N`, a 4-byte unsigned value equal to the number of pseudo-merges
      which contain a given commit.

   ** An array of `N` 8-byte unsigned values, each of which is
      interpreted as an offset (relative to the beginning of the
      `.bitmap` file) at which a pseudo-merge bitmap for this commit can
      be read. These values occur in no particular order.

 * Positions for all pseudo-merges, each stored as an 8-byte unsigned
   value (in network byte-order) containing the offset (relative to the
   beginning of the `.bitmap` file) of each consecutive pseudo-merge.

 * A 4-byte unsigned value (in network byte-order) equal to the number of
   pseudo-merges.

 * A 4-byte unsigned value (in network byte-order) equal to the number of
   unique commits which appear in any pseudo-merge.

 * An 8-byte unsigned value (in network byte-order) equal to the number
   of bytes between the start of the pseudo-merge section and the
   beginning of the lookup table.

 * An 8-byte unsigned value (in network byte-order) equal to the number
   of bytes in the pseudo-merge section (including this field).
	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.

	Pseudo-merge bitmaps
	--------------------

	If the `BITMAP_OPT_PSEUDO_MERGES` flag is set, a variable number of
	bytes (preceding the name-hash cache, commit lookup table, and trailing
	checksum) of the `.bitmap` file is used to store pseudo-merge bitmaps.

	For more information on what pseudo-merges are, why they are useful, and
	how to configure them, see the information in linkgit:gitpacking[7].

	=== File format

	If enabled, pseudo-merge bitmaps are stored in an optional section at
	the end of a `.bitmap` file. The format is as follows:

	....
	+-------------------------------------------+
	\| .bitmap File \|
	+-------------------------------------------+
	\| \|
	\| Pseudo-merge bitmaps (Variable Length) \|
	\| +---------------------------+ \|
	\| \| commits_bitmap (EWAH) \| \|
	\| +---------------------------+ \|
	\| \| merge_bitmap (EWAH) \| \|
	\| +---------------------------+ \|
	\| \|
	+-------------------------------------------+
	\| \|
	\| Lookup Table \|
	\| +---------------------------+ \|
	\| \| commit_pos (4 bytes) \| \|
	\| +---------------------------+ \|
	\| \| offset (8 bytes) \| \|
	\| +------------+--------------+ \|
	\| \|
	\| Offset Cases: \|
	\| ------------- \|
	\| \|
	\| 1. MSB Unset: single pseudo-merge bitmap \|
	\| + offset to pseudo-merge bitmap \|
	\| \|
	\| 2. MSB Set: multiple pseudo-merges \|
	\| + offset to extended lookup table \|
	\| \|
	+-------------------------------------------+
	\| \|
	\| Extended Lookup Table (Optional) \|
	\| +----+----------+----------+----------+ \|
	\| \| N \| Offset 1 \| .... \| Offset N \| \|
	\| +----+----------+----------+----------+ \|
	\| \| \| 8 bytes \| .... \| 8 bytes \| \|
	\| +----+----------+----------+----------+ \|
	\| \|
	+-------------------------------------------+
	\| \|
	\| Pseudo-merge position table \|
	\| +----+----------+----------+----------+ \|
	\| \| N \| Offset 1 \| .... \| Offset N \| \|
	\| +----+----------+----------+----------+ \|
	\| \| \| 8 bytes \| .... \| 8 bytes \| \|
	\| +----+----------+----------+----------+ \|
	\| \|
	+-------------------------------------------+
	\| \|
	\| Pseudo-merge Metadata \|
	\| +-----------------------------------+ \|
	\| \| # pseudo-merges (4 bytes) \| \|
	\| +-----------------------------------+ \|
	\| \| # commits (4 bytes) \| \|
	\| +-----------------------------------+ \|
	\| \| Lookup offset (8 bytes) \| \|
	\| +-----------------------------------+ \|
	\| \| Extension size (8 bytes) \| \|
	\| +-----------------------------------+ \|
	\| \|
	+-------------------------------------------+
	....

	* One or more pseudo-merge bitmaps, each containing:

	** `commits_bitmap`, an EWAH-compressed bitmap describing the set of
	commits included in the this psuedo-merge.

	** `merge_bitmap`, an EWAH-compressed bitmap describing the union of
	the set of objects reachable from all commits listed in the
	`commits_bitmap`.

	* A lookup table, mapping pseudo-merged commits to the pseudo-merges
	they belong to. Entries appear in increasing order of each commit's
	bit position. Each entry is 12 bytes wide, and is comprised of the
	following:

	** `commit_pos`, a 4-byte unsigned value (in network byte-order)
	containing the bit position for this commit.

	** `offset`, an 8-byte unsigned value (also in network byte-order)
	containing either one of two possible offsets, depending on whether or
	not the most-significant bit is set.

	*** If unset (i.e. `offset & ((uint64_t)1<<63) == 0`), the offset
	(relative to the beginning of the `.bitmap` file) at which the
	pseudo-merge bitmap for this commit can be read. This indicates
	only a single pseudo-merge bitmap contains this commit.

	*** If set (i.e. `offset & ((uint64_t)1<<63) != 0`), the offset
	(again relative to the beginning of the `.bitmap` file) at which
	the extended offset table can be located describing the set of
	pseudo-merge bitmaps which contain this commit. This indicates
	that multiple pseudo-merge bitmaps contain this commit.

	* An (optional) extended lookup table (written if and only if there is
	at least one commit which appears in more than one pseudo-merge).
	There are as many entries as commits which appear in multiple
	pseudo-merges. Each entry contains the following:

	** `N`, a 4-byte unsigned value equal to the number of pseudo-merges
	which contain a given commit.

	** An array of `N` 8-byte unsigned values, each of which is
	interpreted as an offset (relative to the beginning of the
	`.bitmap` file) at which a pseudo-merge bitmap for this commit can
	be read. These values occur in no particular order.

	* Positions for all pseudo-merges, each stored as an 8-byte unsigned
	value (in network byte-order) containing the offset (relative to the
	beginning of the `.bitmap` file) of each consecutive pseudo-merge.

	* A 4-byte unsigned value (in network byte-order) equal to the number of
	pseudo-merges.

	* A 4-byte unsigned value (in network byte-order) equal to the number of
	unique commits which appear in any pseudo-merge.

	* An 8-byte unsigned value (in network byte-order) equal to the number
	of bytes between the start of the pseudo-merge section and the
	beginning of the lookup table.

	* An 8-byte unsigned value (in network byte-order) equal to the number
	of bytes in the pseudo-merge section (including this field).