Subject: Re: [PATCH 04/14] packed-graph: add format document

From: Derrick Stolee <[email-masked]>
Date: Fri, 26 Jan 2018 08:25:11 -0500

On 1/25/2018 5:07 PM, Stefan Beller wrote:
> On Thu, Jan 25, 2018 at 6:02 AM, Derrick Stolee <[email-masked]> 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 <[email-masked]>
>> ---
>>   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?)

20 for binary.

>
>> +       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 ...

bytes.

>
>> +           (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
>>

Thanks for the detailed comments here!
-Stolee