technical/commit-graph-format.txt

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.

- 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 CHUNK LOOKUP matches the table of contents from
 link:technical/chunk-format.html[the chunk-based file format].

 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 topological level (generation number v1)
 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.

 Generation Data (ID: {'G', 'D', 'A', '2' }) (N * 4 bytes) [Optional]
 * This list of 4-byte values store corrected commit date offsets for the
 commits, arranged in the same order as commit data chunk.
 * If the corrected commit date offset cannot be stored within 31 bits,
 the value has its most-significant bit on and the other bits store
 the position of corrected commit date into the Generation Data Overflow
 chunk.
 * Generation Data chunk is present only when commit-graph file is written
 by compatible versions of Git and in case of split commit-graph chains,
 the topmost layer also has Generation Data chunk.

 Generation Data Overflow (ID: {'G', 'D', 'O', '2' }) [Optional]
 * This list of 8-byte values stores the corrected commit date offsets
 for commits with corrected commit date offsets that cannot be
 stored within 31 bits.
 * Generation Data Overflow chunk is present only when Generation Data
 chunk is present and atleast one corrected commit date offset cannot
 be stored within 31 bits.

 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.

== Historical Notes:

The Generation Data (GDA2) and Generation Data Overflow (GDO2) chunks have
the number '2' in their chunk IDs because a previous version of Git wrote
possibly erroneous data in these chunks with the IDs "GDAT" and "GDOV". By
changing the IDs, newer versions of Git will silently ignore those older
chunks and write the new information without trusting the incorrect data.