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 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 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 0x7000000 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 8-byte word blocks 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 zero.
    * 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.