1Git commit graph format 2======================= 3 4The Git commit graph stores a list of commit OIDs and some associated 5metadata, 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 20These positional references are stored as unsigned 32-bit integers 21corresponding to the array position within the list of commit OIDs. Due 22to 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 27In order to allow extensions that add extra data to the graph, we organize 28the body into "chunks" and provide a binary lookup table at the beginning 29of the body. The header includes certain values, such as number of chunks 30and hash type. 31 32All 4-byte numbers are in network order. 33 34HEADER: 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 number (B) of base commit-graphs 48 We infer the length (H*B) of the Base Graphs chunk 49 from this value. 50 51CHUNK LOOKUP: 52 53 (C + 1) * 12 bytes listing the table of contents for the chunks: 54 First 4 bytes describe the chunk id. Value 0 is a terminating label. 55 Other 8 bytes provide the byte-offset in current file for chunk to 56 start. (Chunks are ordered contiguously in the file, so you can infer 57 the length using the next chunk position if necessary.) Each chunk 58 ID appears at most once. 59 60 The remaining data in the body is described one chunk at a time, and 61 these chunks may be given in any order. Chunks are required unless 62 otherwise specified. 63 64CHUNK DATA: 65 66 OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes) 67 The ith entry, F[i], stores the number of OIDs with first 68 byte at most i. Thus F[255] stores the total 69 number of commits (N). 70 71 OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes) 72 The OIDs for all commits in the graph, sorted in ascending order. 73 74 Commit Data (ID: {'C', 'D', 'A', 'T' }) (N * (H + 16) bytes) 75 * The first H bytes are for the OID of the root tree. 76 * The next 8 bytes are for the positions of the first two parents 77 of the ith commit. Stores value 0x7000000 if no parent in that 78 position. If there are more than two parents, the second value 79 has its most-significant bit on and the other bits store an array 80 position into the Extra Edge List chunk. 81 * The next 8 bytes store the generation number of the commit and 82 the commit time in seconds since EPOCH. The generation number 83 uses the higher 30 bits of the first 4 bytes, while the commit 84 time uses the 32 bits of the second 4 bytes, along with the lowest 85 2 bits of the lowest byte, storing the 33rd and 34th bit of the 86 commit time. 87 88 Extra Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional] 89 This list of 4-byte values store the second through nth parents for 90 all octopus merges. The second parent value in the commit data stores 91 an array position within this list along with the most-significant bit 92 on. Starting at that array position, iterate through this list of commit 93 positions for the parents until reaching a value with the most-significant 94 bit on. The other bits correspond to the position of the last parent. 95 96 Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional] 97 This list of H-byte hashes describe a set of B commit-graph files that 98 form a commit-graph chain. The graph position for the ith commit in this 99 file's OID Lookup chunk is equal to i plus the number of commits in all 100 base graphs. If B is non-zero, this chunk must exist. 101 102TRAILER: 103 104 H-byte HASH-checksum of all of the above.