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.
- The Bloom filter of the commit carrying the paths that were changed between
the commit and its first parent, if requested.
These positional references are stored as unsigned 32-bit integers
corresponding to the array position within the list of commit OIDs. Due
to some special constants we use to track parents, we can store at most
(1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 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 multi-byte numbers are in network byte 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
We infer the hash length (H) from this value:
1 => SHA-1
2 => SHA-256
If the hash type does not match the repository's hash algorithm, the
commit-graph file should be ignored with a warning presented to the
user.
1-byte number (C) of "chunks"
1-byte number (B) of base commit-graphs
We infer the length (H*B) of the Base Graphs chunk
from 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', 'D', 'A', '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 0x70000000 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 Extra 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.
Extra 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.
Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]
* The ith entry, BIDX[i], stores the number of bytes in all Bloom filters
from commit 0 to commit i (inclusive) in lexicographic order. The Bloom
filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header
length), where BIDX[-1] is 0.
* The BIDX chunk is ignored if the BDAT chunk is not present.
Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
* It starts with header consisting of three unsigned 32-bit integers:
- Version of the hash algorithm being used. We currently only support
value 1 which corresponds to the 32-bit version of the murmur3 hash
implemented exactly as described in
https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double
hashing technique using seed values 0x293ae76f and 0x7e646e2 as
described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters
in Probabilistic Verification"
- The number of times a path is hashed and hence the number of bit positions
that cumulatively determine whether a file is present in the commit.
- The minimum number of bits 'b' per entry in the Bloom filter. If the filter
contains 'n' entries, then the filter size is the minimum number of 64-bit
words that contain n*b bits.
* The rest of the chunk is the concatenation of all the computed Bloom
filters for the commits in lexicographic order.
* Note: Commits with no changes or more than 512 changes have Bloom filters
of length one, with either all bits set to zero or one respectively.
* The BDAT chunk is present if and only if BIDX is present.
Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional]
This list of H-byte hashes describe a set of B commit-graph files that
form a commit-graph chain. The graph position for the ith commit in this
file's OID Lookup chunk is equal to i plus the number of commits in all
base graphs. If B is non-zero, this chunk must exist.
TRAILER:
H-byte HASH-checksum of all of the above.
