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-rw-r--r--plumbing/format/packfile/doc.go205
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diff --git a/plumbing/format/packfile/doc.go b/plumbing/format/packfile/doc.go
index 0b173ca..2882a7f 100644
--- a/plumbing/format/packfile/doc.go
+++ b/plumbing/format/packfile/doc.go
@@ -1,168 +1,39 @@
-// Package packfile implements a encoder/decoder of packfile format
+// Package packfile implements encoding and decoding of packfile format.
+//
+// == 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)
+// 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.
+//
+//
+// Source:
+// https://www.kernel.org/pub/software/scm/git/docs/v1.7.5/technical/pack-protocol.txt
package packfile
-
-/*
-GIT pack format
-===============
-
-== 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)
- 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.
-
-== 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.
-
- 20-byte object name.
-
- - 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:
-
- -- +--------------------------------+
-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
- 20-byte base object name SHA1 (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 20-byte SHA1 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 20-byte SHA1 checksum at the end of
- corresponding packfile.
-
- 20-byte SHA1-checksum of all of the above.
-
-From:
-https://www.kernel.org/pub/software/scm/git/docs/v1.7.5/technical/pack-protocol.txt
-*/