mirror of
https://github.com/git/git.git
synced 2024-11-16 14:04:52 +01:00
a6080a0a44
This uses "git-apply --whitespace=strip" to fix whitespace errors that have crept in to our source files over time. There are a few files that need to have trailing whitespaces (most notably, test vectors). The results still passes the test, and build result in Documentation/ area is unchanged. Signed-off-by: Junio C Hamano <gitster@pobox.com>
118 lines
3.9 KiB
Text
118 lines
3.9 KiB
Text
GIT pack format
|
|
===============
|
|
|
|
= pack-*.pack file has the following format:
|
|
|
|
- The 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)
|
|
20-byte base object name
|
|
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 20-byte SHA1 checksum of all of the above.
|
|
|
|
= pack-*.idx file has 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 are smaller than N. This is called the
|
|
'first-level fan-out' table.
|
|
|
|
Observation: we would need to extend this to an array of
|
|
8-byte integers to go beyond 4G objects per pack, but it is
|
|
not strictly necessary.
|
|
|
|
- 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.
|
|
|
|
20-byte object name.
|
|
|
|
Observation: we would definitely need to extend this to
|
|
8-byte integer plus 20-byte object name to handle a packfile
|
|
that is larger than 4GB.
|
|
|
|
- The file is concluded with a trailer:
|
|
|
|
A copy of the 20-byte SHA1 checksum at the end of
|
|
corresponding packfile.
|
|
|
|
20-byte SHA1-checksum of all of the above.
|
|
|
|
Pack Idx file:
|
|
|
|
idx
|
|
+--------------------------------+
|
|
| fanout[0] = 2 |-.
|
|
+--------------------------------+ |
|
|
| fanout[1] | |
|
|
+--------------------------------+ |
|
|
| fanout[2] | |
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
| fanout[255] | |
|
|
+--------------------------------+ |
|
|
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 DELTA, then
|
|
20-byte base object name SHA1 (the size above is the
|
|
size of the delta data that follows).
|
|
delta data, deflated.
|