# gitformat-commit-graph

## NAME



           gitformat-commit-graph - Git commit-graph format
    


## SYNOPSIS



           $GIT_DIR/objects/info/commit-graph
           $GIT_DIR/objects/info/commit-graphs/*
    


## DESCRIPTION



           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
               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.
    
       **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 at least 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
                       support value 2 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". Version 1
                       Bloom filters have a bug that appears when char is
                       signed and the repository has path names that have
                       characters >= 0x80; Git supports reading and writing
                       them, but this ability will be removed in a future
                       version of Git.
    
                   •   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.
    


## GIT



           Part of the git(1) suite
    


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