plumbing: commitgraph, fix types and handle commit-graph-chains

Unfortunately the original variant makes some incorrect typing
assumptions about commit-graphs which make handling graph chains
difficult to do correctly. This creates a new subpackage and deprecates
the old one. It then adds support commit graph chains.

Signed-off-by: Andrew Thornton <art27@cantab.net>

1 files changed, 106 insertions, 0 deletions

diff --git a/plumbing/format/commitgraph/v2/doc.go b/plumbing/format/commitgraph/v2/doc.go
new file mode 100644
index 0000000..157621d
--- /dev/null
+++ b/plumbing/format/commitgraph/v2/doc.go
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+// Package v2 implements encoding and decoding of commit-graph files.
+//
+// This package was created to work around the issues of the incorrect types in
+// the commitgraph package.
+//
+// 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 v2