1 Git commit graph format
2 =======================
4 The Git commit graph stores a list of commit OIDs and some associated
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".
17 - The parents of the commit, stored using positional references within
20 - The Bloom filter of the commit carrying the paths that were changed between
21 the commit and its first parent, if requested.
23 These positional references are stored as unsigned 32-bit integers
24 corresponding to the array position within the list of commit OIDs. Due
25 to some special constants we use to track parents, we can store at most
26 (1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 billion) commits.
28 == Commit graph files have the following format:
30 In order to allow extensions that add extra data to the graph, we organize
31 the body into "chunks" and provide a binary lookup table at the beginning
32 of the body. The header includes certain values, such as number of chunks
35 All multi-byte numbers are in network byte order.
40 The signature is: {'C', 'G', 'P', 'H'}
42 1-byte version number:
43 Currently, the only valid version is 1.
45 1-byte Hash Version (1 = SHA-1)
46 We infer the hash length (H) from this value.
48 1-byte number (C) of "chunks"
50 1-byte number (B) of base commit-graphs
51 We infer the length (H*B) of the Base Graphs chunk
56 (C + 1) * 12 bytes listing the table of contents for the chunks:
57 First 4 bytes describe the chunk id. Value 0 is a terminating label.
58 Other 8 bytes provide the byte-offset in current file for chunk to
59 start. (Chunks are ordered contiguously in the file, so you can infer
60 the length using the next chunk position if necessary.) Each chunk
61 ID appears at most once.
63 The remaining data in the body is described one chunk at a time, and
64 these chunks may be given in any order. Chunks are required unless
69 OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)
70 The ith entry, F[i], stores the number of OIDs with first
71 byte at most i. Thus F[255] stores the total
72 number of commits (N).
74 OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)
75 The OIDs for all commits in the graph, sorted in ascending order.
77 Commit Data (ID: {'C', 'D', 'A', 'T' }) (N * (H + 16) bytes)
78 * The first H bytes are for the OID of the root tree.
79 * The next 8 bytes are for the positions of the first two parents
80 of the ith commit. Stores value 0x7000000 if no parent in that
81 position. If there are more than two parents, the second value
82 has its most-significant bit on and the other bits store an array
83 position into the Extra Edge List chunk.
84 * The next 8 bytes store the generation number of the commit and
85 the commit time in seconds since EPOCH. The generation number
86 uses the higher 30 bits of the first 4 bytes, while the commit
87 time uses the 32 bits of the second 4 bytes, along with the lowest
88 2 bits of the lowest byte, storing the 33rd and 34th bit of the
91 Extra Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]
92 This list of 4-byte values store the second through nth parents for
93 all octopus merges. The second parent value in the commit data stores
94 an array position within this list along with the most-significant bit
95 on. Starting at that array position, iterate through this list of commit
96 positions for the parents until reaching a value with the most-significant
97 bit on. The other bits correspond to the position of the last parent.
99 Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]
100 * The ith entry, BIDX[i], stores the number of bytes in all Bloom filters
101 from commit 0 to commit i (inclusive) in lexicographic order. The Bloom
102 filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header
103 length), where BIDX[-1] is 0.
104 * The BIDX chunk is ignored if the BDAT chunk is not present.
106 Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
107 * It starts with header consisting of three unsigned 32-bit integers:
108 - Version of the hash algorithm being used. We currently only support
109 value 1 which corresponds to the 32-bit version of the murmur3 hash
110 implemented exactly as described in
111 https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double
112 hashing technique using seed values 0x293ae76f and 0x7e646e2 as
113 described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters
114 in Probabilistic Verification"
115 - The number of times a path is hashed and hence the number of bit positions
116 that cumulatively determine whether a file is present in the commit.
117 - The minimum number of bits 'b' per entry in the Bloom filter. If the filter
118 contains 'n' entries, then the filter size is the minimum number of 64-bit
119 words that contain n*b bits.
120 * The rest of the chunk is the concatenation of all the computed Bloom
121 filters for the commits in lexicographic order.
122 * Note: Commits with no changes or more than 512 changes have Bloom filters
124 * The BDAT chunk is present if and only if BIDX is present.
126 Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional]
127 This list of H-byte hashes describe a set of B commit-graph files that
128 form a commit-graph chain. The graph position for the ith commit in this
129 file's OID Lookup chunk is equal to i plus the number of commits in all
130 base graphs. If B is non-zero, this chunk must exist.
134 H-byte HASH-checksum of all of the above.
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