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GITFORMAT-COMMIT-G(5)              Git Manual              GITFORMAT-COMMIT-G(5)


       gitformat-commit-graph - Git commit-graph format




       The Git commit-graph stores a list of commit OIDs and some associated
       metadata, including:

       o   The generation number of the commit.

       o   The root tree OID.

       o   The commit date.

       o   The parents of the commit, stored using positional references within
           the graph file.

       o   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.


       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.

           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

           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.

           (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
           the chunk-based file format, see linkgit:gitformat-chunk[5]

           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.

       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)

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

           o   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.

           o   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]

           o   This list of 4-byte values store corrected commit date offsets
               for the commits, arranged in the same order as commit data chunk.

           o   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.

           o   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

       Generation Data Overflow (ID: {G, D, O, 2 }) [Optional]

           o   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.

           o   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]

           o   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.

           o   The BIDX chunk is ignored if the BDAT chunk is not present.

       Bloom Filter Data (ID: {B, D, A, T}) [Optional]

           o   It starts with header consisting of three unsigned 32-bit

               o   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
          and the
                   double hashing technique using seed values 0x293ae76f and
                   0x7e646e2 as described in
          "Bloom Filters
                   in Probabilistic Verification"

               o   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.

               o   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.

           o   The rest of the chunk is the concatenation of all the computed
               Bloom filters for the commits in lexicographic order.

           o   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

           o   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.

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


       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.


       Part of the git(1) suite

Git 2.39.0                         12/12/2022              GITFORMAT-COMMIT-G(5)

git 2.39.0 - Generated Wed Dec 14 08:07:03 CST 2022
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