| Git commit graph format |
| ======================= |
| |
| The Git commit graph stores a list of commit OIDs and some associated |
| metadata, including: |
| |
| - The generation number of the commit. Commits with no parents have |
| generation number 1; commits with parents have generation number |
| one more than the maximum generation number of its parents. We |
| reserve zero as special, and can be used to mark a generation |
| number invalid or as "not computed". |
| |
| - The root tree OID. |
| |
| - The commit date. |
| |
| - The parents of the commit, stored using positional references within |
| the graph file. |
| |
| These positional references are stored as unsigned 32-bit integers |
| corresponding to the array position withing the list of commit OIDs. We |
| use the most-significant bit for special purposes, so we can store at most |
| (1 << 31) - 1 (around 2 billion) commits. |
| |
| == Commit graph files have the following format: |
| |
| In order to allow extensions that add extra data to the graph, we organize |
| the body into "chunks" and provide a binary lookup table at the beginning |
| of the body. The header includes certain values, such as number of chunks |
| and hash type. |
| |
| All 4-byte numbers are in network order. |
| |
| HEADER: |
| |
| 4-byte signature: |
| The signature is: {'C', 'G', 'P', 'H'} |
| |
| 1-byte version number: |
| Currently, the only valid version is 1. |
| |
| 1-byte Hash Version (1 = SHA-1) |
| We infer the hash length (H) from this value. |
| |
| 1-byte number (C) of "chunks" |
| |
| 1-byte (reserved for later use) |
| Current clients should ignore this value. |
| |
| CHUNK LOOKUP: |
| |
| (C + 1) * 12 bytes listing the table of contents for the chunks: |
| First 4 bytes describe the chunk id. Value 0 is a terminating label. |
| Other 8 bytes provide the byte-offset in current file for chunk to |
| start. (Chunks are ordered contiguously in the file, so you can infer |
| the length using the next chunk position if necessary.) Each chunk |
| ID appears at most once. |
| |
| The remaining data in the body is described one chunk at a time, and |
| these chunks may be given in any order. Chunks are required unless |
| otherwise specified. |
| |
| CHUNK DATA: |
| |
| OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes) |
| The ith entry, F[i], stores the number of OIDs with first |
| byte at most i. Thus F[255] stores the total |
| number of commits (N). |
| |
| OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes) |
| The OIDs for all commits in the graph, sorted in ascending order. |
| |
| Commit Data (ID: {'C', 'G', 'E', 'T' }) (N * (H + 16) bytes) |
| * The first H bytes are for the OID of the root tree. |
| * The next 8 bytes are for the positions of the first two parents |
| of the ith commit. Stores value 0xffffffff if no parent in that |
| position. If there are more than two parents, the second value |
| has its most-significant bit on and the other bits store an array |
| position into the Large Edge List chunk. |
| * The next 8 bytes store the generation number of the commit and |
| the commit time in seconds since EPOCH. The generation number |
| uses the higher 30 bits of the first 4 bytes, while the commit |
| time uses the 32 bits of the second 4 bytes, along with the lowest |
| 2 bits of the lowest byte, storing the 33rd and 34th bit of the |
| commit time. |
| |
| Large Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional] |
| This list of 4-byte values store the second through nth parents for |
| all octopus merges. The second parent value in the commit data stores |
| an array position within this list along with the most-significant bit |
| on. Starting at that array position, iterate through this list of commit |
| positions for the parents until reaching a value with the most-significant |
| bit on. The other bits correspond to the position of the last parent. |
| |
| TRAILER: |
| |
| H-byte HASH-checksum of all of the above. |