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-Git pack format
-===============
-
-== Checksums and object IDs
-
-In a repository using the traditional SHA-1, pack checksums, index checksums,
-and object IDs (object names) mentioned below are all computed using SHA-1.
-Similarly, in SHA-256 repositories, these values are computed using SHA-256.
-
-== pack-*.pack files have the following format:
-
-   - A header appears at the beginning and consists of the following:
-
-     4-byte signature:
-         The signature is: {'P', 'A', 'C', 'K'}
-
-     4-byte version number (network byte order):
-	 Git currently accepts version number 2 or 3 but
-         generates version 2 only.
-
-     4-byte number of objects contained in the pack (network byte order)
-
-     Observation: we cannot have more than 4G versions ;-) and
-     more than 4G objects in a pack.
-
-   - The header is followed by number of object entries, each of
-     which looks like this:
-
-     (undeltified representation)
-     n-byte type and length (3-bit type, (n-1)*7+4-bit length)
-     compressed data
-
-     (deltified representation)
-     n-byte type and length (3-bit type, (n-1)*7+4-bit length)
-     base object name if OBJ_REF_DELTA or a negative relative
-	 offset from the delta object's position in the pack if this
-	 is an OBJ_OFS_DELTA object
-     compressed delta data
-
-     Observation: length of each object is encoded in a variable
-     length format and is not constrained to 32-bit or anything.
-
-  - The trailer records a pack checksum of all of the above.
-
-=== Object types
-
-Valid object types are:
-
-- OBJ_COMMIT (1)
-- OBJ_TREE (2)
-- OBJ_BLOB (3)
-- OBJ_TAG (4)
-- OBJ_OFS_DELTA (6)
-- OBJ_REF_DELTA (7)
-
-Type 5 is reserved for future expansion. Type 0 is invalid.
-
-=== Deltified representation
-
-Conceptually there are only four object types: commit, tree, tag and
-blob. However to save space, an object could be stored as a "delta" of
-another "base" object. These representations are assigned new types
-ofs-delta and ref-delta, which is only valid in a pack file.
-
-Both ofs-delta and ref-delta store the "delta" to be applied to
-another object (called 'base object') to reconstruct the object. The
-difference between them is, ref-delta directly encodes base object
-name. If the base object is in the same pack, ofs-delta encodes
-the offset of the base object in the pack instead.
-
-The base object could also be deltified if it's in the same pack.
-Ref-delta can also refer to an object outside the pack (i.e. the
-so-called "thin pack"). When stored on disk however, the pack should
-be self contained to avoid cyclic dependency.
-
-The delta data is a sequence of instructions to reconstruct an object
-from the base object. If the base object is deltified, it must be
-converted to canonical form first. Each instruction appends more and
-more data to the target object until it's complete. There are two
-supported instructions so far: one for copy a byte range from the
-source object and one for inserting new data embedded in the
-instruction itself.
-
-Each instruction has variable length. Instruction type is determined
-by the seventh bit of the first octet. The following diagrams follow
-the convention in RFC 1951 (Deflate compressed data format).
-
-==== Instruction to copy from base object
-
-  +----------+---------+---------+---------+---------+-------+-------+-------+
-  | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
-  +----------+---------+---------+---------+---------+-------+-------+-------+
-
-This is the instruction format to copy a byte range from the source
-object. It encodes the offset to copy from and the number of bytes to
-copy. Offset and size are in little-endian order.
-
-All offset and size bytes are optional. This is to reduce the
-instruction size when encoding small offsets or sizes. The first seven
-bits in the first octet determines which of the next seven octets is
-present. If bit zero is set, offset1 is present. If bit one is set
-offset2 is present and so on.
-
-Note that a more compact instruction does not change offset and size
-encoding. For example, if only offset2 is omitted like below, offset3
-still contains bits 16-23. It does not become offset2 and contains
-bits 8-15 even if it's right next to offset1.
-
-  +----------+---------+---------+
-  | 10000101 | offset1 | offset3 |
-  +----------+---------+---------+
-
-In its most compact form, this instruction only takes up one byte
-(0x80) with both offset and size omitted, which will have default
-values zero. There is another exception: size zero is automatically
-converted to 0x10000.
-
-==== Instruction to add new data
-
-  +----------+============+
-  | 0xxxxxxx |    data    |
-  +----------+============+
-
-This is the instruction to construct target object without the base
-object. The following data is appended to the target object. The first
-seven bits of the first octet determines the size of data in
-bytes. The size must be non-zero.
-
-==== Reserved instruction
-
-  +----------+============
-  | 00000000 |
-  +----------+============
-
-This is the instruction reserved for future expansion.
-
-== Original (version 1) pack-*.idx files have the following format:
-
-  - The header consists of 256 4-byte network byte order
-    integers.  N-th entry of this table records the number of
-    objects in the corresponding pack, the first byte of whose
-    object name is less than or equal to N.  This is called the
-    'first-level fan-out' table.
-
-  - The header is followed by sorted 24-byte entries, one entry
-    per object in the pack.  Each entry is:
-
-    4-byte network byte order integer, recording where the
-    object is stored in the packfile as the offset from the
-    beginning.
-
-    one object name of the appropriate size.
-
-  - The file is concluded with a trailer:
-
-    A copy of the pack checksum at the end of the corresponding
-    packfile.
-
-    Index checksum of all of the above.
-
-Pack Idx file:
-
-	--  +--------------------------------+
-fanout	    | fanout[0] = 2 (for example)    |-.
-table	    +--------------------------------+ |
-	    | fanout[1]                      | |
-	    +--------------------------------+ |
-	    | fanout[2]                      | |
-	    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
-	    | fanout[255] = total objects    |---.
-	--  +--------------------------------+ | |
-main	    | offset                         | | |
-index	    | object name 00XXXXXXXXXXXXXXXX | | |
-table	    +--------------------------------+ | |
-	    | offset                         | | |
-	    | object name 00XXXXXXXXXXXXXXXX | | |
-	    +--------------------------------+<+ |
-	  .-| offset                         |   |
-	  | | object name 01XXXXXXXXXXXXXXXX |   |
-	  | +--------------------------------+   |
-	  | | offset                         |   |
-	  | | object name 01XXXXXXXXXXXXXXXX |   |
-	  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
-	  | | offset                         |   |
-	  | | object name FFXXXXXXXXXXXXXXXX |   |
-	--| +--------------------------------+<--+
-trailer	  | | packfile checksum              |
-	  | +--------------------------------+
-	  | | idxfile checksum               |
-	  | +--------------------------------+
-          .-------.
-                  |
-Pack file entry: <+
-
-     packed object header:
-	1-byte size extension bit (MSB)
-	       type (next 3 bit)
-	       size0 (lower 4-bit)
-        n-byte sizeN (as long as MSB is set, each 7-bit)
-		size0..sizeN form 4+7+7+..+7 bit integer, size0
-		is the least significant part, and sizeN is the
-		most significant part.
-     packed object data:
-        If it is not DELTA, then deflated bytes (the size above
-		is the size before compression).
-	If it is REF_DELTA, then
-	  base object name (the size above is the
-		size of the delta data that follows).
-          delta data, deflated.
-	If it is OFS_DELTA, then
-	  n-byte offset (see below) interpreted as a negative
-		offset from the type-byte of the header of the
-		ofs-delta entry (the size above is the size of
-		the delta data that follows).
-	  delta data, deflated.
-
-     offset encoding:
-	  n bytes with MSB set in all but the last one.
-	  The offset is then the number constructed by
-	  concatenating the lower 7 bit of each byte, and
-	  for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
-	  to the result.
-
-
-
-== Version 2 pack-*.idx files support packs larger than 4 GiB, and
-   have some other reorganizations.  They have the format:
-
-  - A 4-byte magic number '\377tOc' which is an unreasonable
-    fanout[0] value.
-
-  - A 4-byte version number (= 2)
-
-  - A 256-entry fan-out table just like v1.
-
-  - A table of sorted object names.  These are packed together
-    without offset values to reduce the cache footprint of the
-    binary search for a specific object name.
-
-  - A table of 4-byte CRC32 values of the packed object data.
-    This is new in v2 so compressed data can be copied directly
-    from pack to pack during repacking without undetected
-    data corruption.
-
-  - A table of 4-byte offset values (in network byte order).
-    These are usually 31-bit pack file offsets, but large
-    offsets are encoded as an index into the next table with
-    the msbit set.
-
-  - A table of 8-byte offset entries (empty for pack files less
-    than 2 GiB).  Pack files are organized with heavily used
-    objects toward the front, so most object references should
-    not need to refer to this table.
-
-  - The same trailer as a v1 pack file:
-
-    A copy of the pack checksum at the end of
-    corresponding packfile.
-
-    Index checksum of all of the above.
-
-== multi-pack-index (MIDX) files have the following format:
-
-The multi-pack-index files refer to multiple pack-files and loose objects.
-
-In order to allow extensions that add extra data to the MIDX, we organize
-the body into "chunks" and provide a lookup table at the beginning of the
-body. The header includes certain length values, such as the number of packs,
-the number of base MIDX files, hash lengths and types.
-
-All 4-byte numbers are in network order.
-
-HEADER:
-
-	4-byte signature:
-	    The signature is: {'M', 'I', 'D', 'X'}
-
-	1-byte version number:
-	    Git only writes or recognizes version 1.
-
-	1-byte Object Id Version
-	    We infer the length of object IDs (OIDs) from this value:
-		1 => SHA-1
-		2 => SHA-256
-	    If the hash type does not match the repository's hash algorithm,
-	    the multi-pack-index file should be ignored with a warning
-	    presented to the user.
-
-	1-byte number of "chunks"
-
-	1-byte number of base multi-pack-index files:
-	    This value is currently always zero.
-
-	4-byte number of pack files
-
-CHUNK LOOKUP:
-
-	(C + 1) * 12 bytes providing the chunk offsets:
-	    First 4 bytes describe chunk id. Value 0 is a terminating label.
-	    Other 8 bytes provide offset in current file for chunk to start.
-	    (Chunks are provided in file-order, so you can infer the length
-	    using the next chunk position if necessary.)
-
-	The remaining data in the body is described one chunk at a time, and
-	these chunks may be given in any order. Chunks are required unless
-	otherwise specified.
-
-CHUNK DATA:
-
-	Packfile Names (ID: {'P', 'N', 'A', 'M'})
-	    Stores the packfile names as concatenated, null-terminated strings.
-	    Packfiles must be listed in lexicographic order for fast lookups by
-	    name. This is the only chunk not guaranteed to be a multiple of four
-	    bytes in length, so should be the last chunk for alignment reasons.
-
-	OID Fanout (ID: {'O', 'I', 'D', 'F'})
-	    The ith entry, F[i], stores the number of OIDs with first
-	    byte at most i. Thus F[255] stores the total
-	    number of objects.
-
-	OID Lookup (ID: {'O', 'I', 'D', 'L'})
-	    The OIDs for all objects in the MIDX are stored in lexicographic
-	    order in this chunk.
-
-	Object Offsets (ID: {'O', 'O', 'F', 'F'})
-	    Stores two 4-byte values for every object.
-	    1: The pack-int-id for the pack storing this object.
-	    2: The offset within the pack.
-		If all offsets are less than 2^32, then the large offset chunk
-		will not exist and offsets are stored as in IDX v1.
-		If there is at least one offset value larger than 2^32-1, then
-		the large offset chunk must exist, and offsets larger than
-		2^31-1 must be stored in it instead. If the large offset chunk
-		exists and the 31st bit is on, then removing that bit reveals
-		the row in the large offsets containing the 8-byte offset of
-		this object.
-
-	[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
-	    8-byte offsets into large packfiles.
-
-TRAILER:
-
-	Index checksum of the above contents.