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