Git commit graph format

=======================

The Git commit graph stores a list of commit OIDs and some associated

metadata, including:

- The generation number of the commit. Commits with no parents have

generation number 1; commits with parents have generation number

one more than the maximum generation number of its parents. We

reserve zero as special, and can be used to mark a generation

number invalid or as "not computed".

- The root tree OID.

- The commit date.

- The parents of the commit, stored using positional references within

the graph file.

These positional references are stored as unsigned 32-bit integers

corresponding to the array position withing the list of commit OIDs. We

use the most-significant bit for special purposes, so we can store at most

(1 << 31) - 1 (around 2 billion) commits.

== Commit graph files have the following format:

In order to allow extensions that add extra data to the graph, we organize

the body into "chunks" and provide a binary lookup table at the beginning

of the body. The header includes certain values, such as number of chunks

and hash type.

All 4-byte numbers are in network order.

HEADER:

4-byte signature:

The signature is: {'C', 'G', 'P', 'H'}

1-byte version number:

Currently, the only valid version is 1.

1-byte Hash Version (1 = SHA-1)

We infer the hash length (H) from this value.

1-byte number (C) of "chunks"

1-byte (reserved for later use)

Current clients should ignore this value.

CHUNK LOOKUP:

(C + 1) * 12 bytes listing the table of contents for the chunks:

First 4 bytes describe the chunk id. Value 0 is a terminating label.

Other 8 bytes provide the byte-offset in current file for chunk to

start. (Chunks are ordered contiguously in the file, so you can infer

the length using the next chunk position if necessary.) Each chunk

ID appears at most once.

The remaining data in the body is described one chunk at a time, and

these chunks may be given in any order. Chunks are required unless

otherwise specified.

CHUNK DATA:

OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)

The ith entry, F[i], stores the number of OIDs with first

byte at most i. Thus F[255] stores the total

number of commits (N).

OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)

The OIDs for all commits in the graph, sorted in ascending order.

Commit Data (ID: {'C', 'G', 'E', 'T' }) (N * (H + 16) bytes)

* The first H bytes are for the OID of the root tree.

* The next 8 bytes are for the positions of the first two parents

of the ith commit. Stores value 0xffffffff if no parent in that

position. If there are more than two parents, the second value

has its most-significant bit on and the other bits store an array

position into the Large Edge List chunk.

* The next 8 bytes store the generation number of the commit and

the commit time in seconds since EPOCH. The generation number

uses the higher 30 bits of the first 4 bytes, while the commit

time uses the 32 bits of the second 4 bytes, along with the lowest

2 bits of the lowest byte, storing the 33rd and 34th bit of the

commit time.

Large Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]

This list of 4-byte values store the second through nth parents for

all octopus merges. The second parent value in the commit data stores

an array position within this list along with the most-significant bit

on. Starting at that array position, iterate through this list of commit

positions for the parents until reaching a value with the most-significant

bit on. The other bits correspond to the position of the last parent.

TRAILER:

H-byte HASH-checksum of all of the above.