Documentation/gitformat-commit-graph.txt

   1 gitformat-commit-graph(5)
   2 =========================
   3
   4 NAME
   5 ----
   6 gitformat-commit-graph - Git commit graph format
   7
   8 SYNOPSIS
   9 --------
  10 [verse]
  11 $GIT_DIR/objects/info/commit-graph
  12 $GIT_DIR/objects/info/commit-graphs/*
  13
  14 DESCRIPTION
  15 -----------
  16
  17 The Git commit graph stores a list of commit OIDs and some associated
  18 metadata, including:
  19
  20 - The generation number of the commit.
  21
  22 - The root tree OID.
  23
  24 - The commit date.
  25
  26 - The parents of the commit, stored using positional references within
  27   the graph file.
  28
  29 - The Bloom filter of the commit carrying the paths that were changed between
  30   the commit and its first parent, if requested.
  31
  32 These positional references are stored as unsigned 32-bit integers
  33 corresponding to the array position within the list of commit OIDs. Due
  34 to some special constants we use to track parents, we can store at most
  35 (1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 billion) commits.
  36
  37 == Commit graph files have the following format:
  38
  39 In order to allow extensions that add extra data to the graph, we organize
  40 the body into "chunks" and provide a binary lookup table at the beginning
  41 of the body. The header includes certain values, such as number of chunks
  42 and hash type.
  43
  44 All multi-byte numbers are in network byte order.
  45
  46 === HEADER:
  47
  48   4-byte signature:
  49       The signature is: {'C', 'G', 'P', 'H'}
  50
  51   1-byte version number:
  52       Currently, the only valid version is 1.
  53
  54   1-byte Hash Version
  55       We infer the hash length (H) from this value:
  56         1 => SHA-1
  57         2 => SHA-256
  58       If the hash type does not match the repository's hash algorithm, the
  59       commit-graph file should be ignored with a warning presented to the
  60       user.
  61
  62   1-byte number (C) of "chunks"
  63
  64   1-byte number (B) of base commit-graphs
  65       We infer the length (H*B) of the Base Graphs chunk
  66       from this value.
  67
  68 === CHUNK LOOKUP:
  69
  70   (C + 1) * 12 bytes listing the table of contents for the chunks:
  71       First 4 bytes describe the chunk id. Value 0 is a terminating label.
  72       Other 8 bytes provide the byte-offset in current file for chunk to
  73       start. (Chunks are ordered contiguously in the file, so you can infer
  74       the length using the next chunk position if necessary.) Each chunk
  75       ID appears at most once.
  76
  77   The CHUNK LOOKUP matches the table of contents from
  78   the chunk-based file format, see linkgit:gitformat-chunk[5]
  79
  80   The remaining data in the body is described one chunk at a time, and
  81   these chunks may be given in any order. Chunks are required unless
  82   otherwise specified.
  83
  84 === CHUNK DATA:
  85
  86 ==== OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)
  87       The ith entry, F[i], stores the number of OIDs with first
  88       byte at most i. Thus F[255] stores the total
  89       number of commits (N).
  90
  91 ====  OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)
  92       The OIDs for all commits in the graph, sorted in ascending order.
  93
  94 ====  Commit Data (ID: {'C', 'D', 'A', 'T' }) (N * (H + 16) bytes)
  95     * The first H bytes are for the OID of the root tree.
  96     * The next 8 bytes are for the positions of the first two parents
  97       of the ith commit. Stores value 0x70000000 if no parent in that
  98       position. If there are more than two parents, the second value
  99       has its most-significant bit on and the other bits store an array
 100       position into the Extra Edge List chunk.
 101     * The next 8 bytes store the topological level (generation number v1)
 102       of the commit and
 103       the commit time in seconds since EPOCH. The generation number
 104       uses the higher 30 bits of the first 4 bytes, while the commit
 105       time uses the 32 bits of the second 4 bytes, along with the lowest
 106       2 bits of the lowest byte, storing the 33rd and 34th bit of the
 107       commit time.
 108
 109 ==== Generation Data (ID: {'G', 'D', 'A', '2' }) (N * 4 bytes) [Optional]
 110     * This list of 4-byte values store corrected commit date offsets for the
 111       commits, arranged in the same order as commit data chunk.
 112     * If the corrected commit date offset cannot be stored within 31 bits,
 113       the value has its most-significant bit on and the other bits store
 114       the position of corrected commit date into the Generation Data Overflow
 115       chunk.
 116     * Generation Data chunk is present only when commit-graph file is written
 117       by compatible versions of Git and in case of split commit-graph chains,
 118       the topmost layer also has Generation Data chunk.
 119
 120 ==== Generation Data Overflow (ID: {'G', 'D', 'O', '2' }) [Optional]
 121     * This list of 8-byte values stores the corrected commit date offsets
 122       for commits with corrected commit date offsets that cannot be
 123       stored within 31 bits.
 124     * Generation Data Overflow chunk is present only when Generation Data
 125       chunk is present and atleast one corrected commit date offset cannot
 126       be stored within 31 bits.
 127
 128 ==== Extra Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]
 129       This list of 4-byte values store the second through nth parents for
 130       all octopus merges. The second parent value in the commit data stores
 131       an array position within this list along with the most-significant bit
 132       on. Starting at that array position, iterate through this list of commit
 133       positions for the parents until reaching a value with the most-significant
 134       bit on. The other bits correspond to the position of the last parent.
 135
 136 ==== Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]
 137     * The ith entry, BIDX[i], stores the number of bytes in all Bloom filters
 138       from commit 0 to commit i (inclusive) in lexicographic order. The Bloom
 139       filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header
 140       length), where BIDX[-1] is 0.
 141     * The BIDX chunk is ignored if the BDAT chunk is not present.
 142
 143 ==== Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
 144     * It starts with header consisting of three unsigned 32-bit integers:
 145       - Version of the hash algorithm being used. We currently only support
 146         value 1 which corresponds to the 32-bit version of the murmur3 hash
 147         implemented exactly as described in
 148         https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double
 149         hashing technique using seed values 0x293ae76f and 0x7e646e2 as
 150         described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters
 151         in Probabilistic Verification"
 152       - The number of times a path is hashed and hence the number of bit positions
 153               that cumulatively determine whether a file is present in the commit.
 154       - The minimum number of bits 'b' per entry in the Bloom filter. If the filter
 155               contains 'n' entries, then the filter size is the minimum number of 64-bit
 156               words that contain n*b bits.
 157     * The rest of the chunk is the concatenation of all the computed Bloom
 158       filters for the commits in lexicographic order.
 159     * Note: Commits with no changes or more than 512 changes have Bloom filters
 160       of length one, with either all bits set to zero or one respectively.
 161     * The BDAT chunk is present if and only if BIDX is present.
 162
 163 ==== Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional]
 164       This list of H-byte hashes describe a set of B commit-graph files that
 165       form a commit-graph chain. The graph position for the ith commit in this
 166       file's OID Lookup chunk is equal to i plus the number of commits in all
 167       base graphs.  If B is non-zero, this chunk must exist.
 168
 169 === TRAILER:
 170
 171         H-byte HASH-checksum of all of the above.
 172
 173 == Historical Notes:
 174
 175 The Generation Data (GDA2) and Generation Data Overflow (GDO2) chunks have
 176 the number '2' in their chunk IDs because a previous version of Git wrote
 177 possibly erroneous data in these chunks with the IDs "GDAT" and "GDOV". By
 178 changing the IDs, newer versions of Git will silently ignore those older
 179 chunks and write the new information without trusting the incorrect data.
 180
 181 GIT
 182 ---
 183 Part of the linkgit:git[1] suite