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 (reserved for later use) 48 Current clients should ignore this value. 49 50CHUNK 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 63CHUNK 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 Extra 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 Extra 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 95TRAILER: 96 97 H-byte HASH-checksum of all of the above.