On 1/25/2018 5:07 PM, Stefan Beller wrote:
On Thu, Jan 25, 2018 at 6:02 AM, Derrick Stolee <stolee@xxxxxxxxx> 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 <dstolee@xxxxxxxxxxxxx>
---
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
