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- // Package commitgraph implements encoding and decoding of commit-graph files.
- //
- // 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 within the list of commit OIDs. Due
- // to some special constants we use to track parents, we can store at most
- // (1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 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', 'D', 'A', '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 0x7000000 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 Extra 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.
- //
- // Extra 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.
- //
- // Source:
- // https://raw.githubusercontent.com/git/git/master/Documentation/technical/commit-graph-format.txt
- package commitgraph
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