On Thu, Jan 25, 2018 at 6:02 AM, Derrick Stolee <sto...@gmail.com> wrote: > Add document specifying the binary format for packed graphs. This > format allows for: > > * New versions. > * New hash functions and hash lengths. > * Optional extensions. > > Basic header information is followed by a binary table of contents > into "chunks" that include: > > * An ordered list of commit object IDs. > * A 256-entry fanout into that list of OIDs. > * A list of metadata for the commits. > * A list of "large edges" to enable octopus merges. > > Signed-off-by: Derrick Stolee <dsto...@microsoft.com> > --- > Documentation/technical/graph-format.txt | 88 > ++++++++++++++++++++++++++++++++
So this is different from Documentation/technical/packed-graph.txt, which gives high level design and this gives the details on how to set bits. > 1 file changed, 88 insertions(+) > create mode 100644 Documentation/technical/graph-format.txt > > diff --git a/Documentation/technical/graph-format.txt > b/Documentation/technical/graph-format.txt > new file mode 100644 > index 0000000000..a15e1036d7 > --- /dev/null > +++ b/Documentation/technical/graph-format.txt > @@ -0,0 +1,88 @@ > +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. > + > +== graph-*.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, > +hash lengths and types. > + > +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 Object Id Version (1 = SHA-1) > + > + 1-byte Object Id Length (H) This is 20 or 40 for sha1 ? (binary or text representation?) > + 1-byte number (C) of "chunks" > + > +CHUNK LOOKUP: > + > + (C + 1) * 12 bytes listing the table of contents for the chunks: > + First 4 bytes describe chunk id. Value 0 is a terminating label. > + Other 8 bytes provide offset in current file for chunk to start. ... offset [in bytes/words/4k blocks?] in ... > + (Chunks are ordered contiguously in the file, so you can infer > + the length using the next chunk position if necessary.) > + > + 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). So F[0] > 0 for git.git for example. Or another way: To lookup a 01xxx, I need to look at entry(F[00] + 1 )...entry(F[01]). Makes sense. > + > + OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes) > + The OIDs for all commits in the graph. ... sorted ascending. > + 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 int-ids 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 offset into the > Large > + Edge List chunk. s/an offset into/position in/ ? (otherwise offset in bytes?) > + * 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. This allows for a maximum generation number of 1.073.741.823 (2^30 -1) = 1 billion, and a max time stamp of later than 2100. Do you allow negative time stamps? > + > + [Optional] Large Edge List (ID: {'E', 'D', 'G', 'E'}) > + This list of 4-byte values store the second through nth parents > for > + all octoput merges. The second parent value in the commit data is > a octopus > + negative number pointing into this list. Then iterate through this > + list starting at that position until reaching a value with the > most- > + significant bit on. The other bits correspond to the int-id of the > + last parent. > + > +TRAILER: > + > + H-byte HASH-checksum of all of the above. > -- > 2.16.0 >