On 2/21/2018 2:23 PM, Stefan Beller wrote:
On Mon, Feb 19, 2018 at 10:53 AM, Derrick Stolee <stolee@xxxxxxxxx> wrote:
+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 number (C) of "chunks"
+
+ 1-byte (reserved for later use)
What should clients of today do with it?
* ignore it completely [as they have no idea what it is] or
* throw hands up in the air if it is anything other than 0 ?
[because clearly we will increment the version
or have new information in a new chunk instead of just sneaking
in information here?]
They should ignore it completely, which will allow using the value for
something meaningful later without causing a version change (which we DO
die() for). A user could downgrade from a version that uses this byte
for something meaningful and not require a new commit-graph file.
The "commit-graph read" subcommand does output this byte, so we can
verify that the "write" subcommand places a 0 in this position.
+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? I could imagine having a larger granularity here,
because chunks don't sound small.]
It is good to specify "offset 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 in small repos, where there are fewer than 256 objects,
F[i] == F[i+1], for all i'th where there is no object starting with i byte]
Correct. I'm not sure this additional information is valuable for the
document, though.
+ 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', '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 array
+ position into the Large 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.
+
+ Large 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 int-ids
+ for the parents 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.
+
--
2.7.4
Makes sense so far, I'll read on.
I agree with Junio, that I could read this documentation without
the urge to point out nits. :)
Thanks,
Stefan
