pack format

pack-format

Pack-*.pack 文件具有以下格式：

标题出现在开头，包含以下内容：4字节签名：签名是：{'P'，'A'，'C'，'K'}

  4-byte version number (network byte order): Git currently accepts version number 2 or 3 but        generates version 2 only.

包中包含4个字节的对象数（网络字节顺序）

观察：我们不能超过 4G 版本;-)和一个包中超过 4G 的对象。

头部后面跟着一些对象条目，每条对象条目如下所示：（未分割表示）n 字节类型和长度（3位类型，（n-1）* 7 + 4位长度）压缩数据

  (deltified representation)    n-byte type and length (3-bit type, (n-1)\*7+4-bit length)    20-byte base object name if OBJ\_REF\_DELTA or a negative relative offset from the delta object's position in the pack if this is an OBJ\_OFS\_DELTA object    compressed delta data

观察：每个对象的长度都是以可变长度格式编码的，并不限于32位或任何其他内容。

预告片记录了上述所有内容的20字节 SHA-1 校验之和。

原始（版本1）pack-*.idx 文件具有以下格式：

标题由256个4字节的网络字节顺序整数组成。此表的第 N 个条目记录对应包中对象的数量，其对象名称的第一个字节小于或等于 N 。这称为first-level fan-out表格。

20字节的对象名称。

该文件以一个结尾结束：在相应的包文件末尾的20字节SHA-1校验和的副本。

上述所有的20字节 SHA-1 校验和。

包 Idx 文件：

        --  +--------------------------------+
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 SHA-1 (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.

版本2 pack-*.idx 文件支持包大于4 gib ，并且

have some other reorganizations.  They have the format:

一个4字节的幻数\377tOc，这是一个不合理的扇出值。

上述所有的20字节 SHA-1 校验和。