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package packfile
// Code from:
// https://github.com/gitchain/gitchain/tree/master/git @ 4c2fabdf9
//
// 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
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