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    1 Git commit graph format
    2 =======================
    3 
    4 The Git commit graph stores a list of commit OIDs and some associated
    5 metadata, including:
    6 
    7 - The generation number of the commit. Commits with no parents have
    8   generation number 1; commits with parents have generation number
    9   one more than the maximum generation number of its parents. We
   10   reserve zero as special, and can be used to mark a generation
   11   number invalid or as "not computed".
   12 
   13 - The root tree OID.
   14 
   15 - The commit date.
   16 
   17 - The parents of the commit, stored using positional references within
   18   the graph file.
   19 
   20 These positional references are stored as unsigned 32-bit integers
   21 corresponding to the array position within the list of commit OIDs. Due
   22 to some special constants we use to track parents, we can store at most
   23 (1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 billion) commits.
   24 
   25 == Commit graph files have the following format:
   26 
   27 In order to allow extensions that add extra data to the graph, we organize
   28 the body into "chunks" and provide a binary lookup table at the beginning
   29 of the body. The header includes certain values, such as number of chunks
   30 and hash type.
   31 
   32 All 4-byte numbers are in network order.
   33 
   34 HEADER:
   35 
   36   4-byte signature:
   37       The signature is: {'C', 'G', 'P', 'H'}
   38 
   39   1-byte version number:
   40       Currently, the only valid version is 1.
   41 
   42   1-byte Hash Version (1 = SHA-1)
   43       We infer the hash length (H) from this value.
   44 
   45   1-byte number (C) of "chunks"
   46 
   47   1-byte (reserved for later use)
   48      Current clients should ignore this value.
   49 
   50 CHUNK LOOKUP:
   51 
   52   (C + 1) * 12 bytes listing the table of contents for the chunks:
   53       First 4 bytes describe the chunk id. Value 0 is a terminating label.
   54       Other 8 bytes provide the byte-offset in current file for chunk to
   55       start. (Chunks are ordered contiguously in the file, so you can infer
   56       the length using the next chunk position if necessary.) Each chunk
   57       ID appears at most once.
   58 
   59   The remaining data in the body is described one chunk at a time, and
   60   these chunks may be given in any order. Chunks are required unless
   61   otherwise specified.
   62 
   63 CHUNK DATA:
   64 
   65   OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)
   66       The ith entry, F[i], stores the number of OIDs with first
   67       byte at most i. Thus F[255] stores the total
   68       number of commits (N).
   69 
   70   OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)
   71       The OIDs for all commits in the graph, sorted in ascending order.
   72 
   73   Commit Data (ID: {'C', 'D', 'A', 'T' }) (N * (H + 16) bytes)
   74     * The first H bytes are for the OID of the root tree.
   75     * The next 8 bytes are for the positions of the first two parents
   76       of the ith commit. Stores value 0x7000000 if no parent in that
   77       position. If there are more than two parents, the second value
   78       has its most-significant bit on and the other bits store an array
   79       position into the Large Edge List chunk.
   80     * The next 8 bytes store the generation number of the commit and
   81       the commit time in seconds since EPOCH. The generation number
   82       uses the higher 30 bits of the first 4 bytes, while the commit
   83       time uses the 32 bits of the second 4 bytes, along with the lowest
   84       2 bits of the lowest byte, storing the 33rd and 34th bit of the
   85       commit time.
   86 
   87   Large Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]
   88       This list of 4-byte values store the second through nth parents for
   89       all octopus merges. The second parent value in the commit data stores
   90       an array position within this list along with the most-significant bit
   91       on. Starting at that array position, iterate through this list of commit
   92       positions for the parents until reaching a value with the most-significant
   93       bit on. The other bits correspond to the position of the last parent.
   94 
   95 TRAILER:
   96 
   97 	H-byte HASH-checksum of all of the above.