104 lines
4.1 KiB
Text
104 lines
4.1 KiB
Text
Git commit graph format
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=======================
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The Git commit graph stores a list of commit OIDs and some associated
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metadata, including:
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- The generation number of the commit. Commits with no parents have
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generation number 1; commits with parents have generation number
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one more than the maximum generation number of its parents. We
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reserve zero as special, and can be used to mark a generation
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number invalid or as "not computed".
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- The root tree OID.
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- The commit date.
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- The parents of the commit, stored using positional references within
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the graph file.
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These positional references are stored as unsigned 32-bit integers
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corresponding to the array position within the list of commit OIDs. Due
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to some special constants we use to track parents, we can store at most
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(1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 billion) commits.
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== Commit graph files have the following format:
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In order to allow extensions that add extra data to the graph, we organize
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the body into "chunks" and provide a binary lookup table at the beginning
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of the body. The header includes certain values, such as number of chunks
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and hash type.
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All 4-byte numbers are in network order.
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HEADER:
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4-byte signature:
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The signature is: {'C', 'G', 'P', 'H'}
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1-byte version number:
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Currently, the only valid version is 1.
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1-byte Hash Version (1 = SHA-1)
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We infer the hash length (H) from this value.
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1-byte number (C) of "chunks"
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1-byte number (B) of base commit-graphs
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We infer the length (H*B) of the Base Graphs chunk
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from this value.
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CHUNK LOOKUP:
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(C + 1) * 12 bytes listing the table of contents for the chunks:
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First 4 bytes describe the chunk id. Value 0 is a terminating label.
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Other 8 bytes provide the byte-offset in current file for chunk to
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start. (Chunks are ordered contiguously in the file, so you can infer
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the length using the next chunk position if necessary.) Each chunk
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ID appears at most once.
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The remaining data in the body is described one chunk at a time, and
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these chunks may be given in any order. Chunks are required unless
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otherwise specified.
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CHUNK DATA:
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OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)
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The ith entry, F[i], stores the number of OIDs with first
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byte at most i. Thus F[255] stores the total
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number of commits (N).
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OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)
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The OIDs for all commits in the graph, sorted in ascending order.
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Commit Data (ID: {'C', 'D', 'A', 'T' }) (N * (H + 16) bytes)
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* The first H bytes are for the OID of the root tree.
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* The next 8 bytes are for the positions of the first two parents
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of the ith commit. Stores value 0x7000000 if no parent in that
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position. If there are more than two parents, the second value
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has its most-significant bit on and the other bits store an array
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position into the Extra Edge List chunk.
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* The next 8 bytes store the generation number of the commit and
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the commit time in seconds since EPOCH. The generation number
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uses the higher 30 bits of the first 4 bytes, while the commit
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time uses the 32 bits of the second 4 bytes, along with the lowest
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2 bits of the lowest byte, storing the 33rd and 34th bit of the
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commit time.
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Extra Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]
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This list of 4-byte values store the second through nth parents for
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all octopus merges. The second parent value in the commit data stores
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an array position within this list along with the most-significant bit
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on. Starting at that array position, iterate through this list of commit
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positions for the parents until reaching a value with the most-significant
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bit on. The other bits correspond to the position of the last parent.
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Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional]
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This list of H-byte hashes describe a set of B commit-graph files that
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form a commit-graph chain. The graph position for the ith commit in this
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file's OID Lookup chunk is equal to i plus the number of commits in all
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base graphs. If B is non-zero, this chunk must exist.
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TRAILER:
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H-byte HASH-checksum of all of the above.
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