package plumbing documentation improvements (#248)

1 files changed, 38 insertions, 167 deletions

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
-*/