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104 lines
4.2 KiB
104 lines
4.2 KiB
// Package commitgraph implements encoding and decoding of commit-graph files.
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//
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// Git commit graph format
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// =======================
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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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//
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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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//
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// - The root tree OID.
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//
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// - The commit date.
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//
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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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//
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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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//
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// == Commit graph files have the following format:
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//
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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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//
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// All 4-byte numbers are in network order.
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//
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// HEADER:
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//
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// 4-byte signature:
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// The signature is: {'C', 'G', 'P', 'H'}
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//
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// 1-byte version number:
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// Currently, the only valid version is 1.
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//
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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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//
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// 1-byte number (C) of "chunks"
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//
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// 1-byte (reserved for later use)
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// Current clients should ignore this value.
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//
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// CHUNK LOOKUP:
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//
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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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//
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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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//
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// CHUNK DATA:
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//
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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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//
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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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//
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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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//
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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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//
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// TRAILER:
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//
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// H-byte HASH-checksum of all of the above.
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//
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// Source:
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// https://raw.githubusercontent.com/git/git/master/Documentation/technical/commit-graph-format.txt
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package commitgraph
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