Re: [PATCH 02/17] pack-mtimes: support reading .mtimes files

On 2021-12-02 at 15:06:07, Derrick Stolee wrote:

On 11/29/2021 5:25 PM, Taylor Blau wrote:

quoted

+== pack-*.mtimes files have the format:
+
+  - A 4-byte magic number '0x4d544d45' ('MTME').
+
+  - A 4-byte version identifier (= 1).
+
+  - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).

I vaguely remember complaints about using a 1-byte identifier in
the commit-graph and multi-pack-index formats because the "standard"
way to refer to these hash functions was a magic number that had a
meaning in ASCII that helped human readers a bit. I cannot find an
example of such 4-byte identifiers, but perhaps brian (CC'd) could
remind us.

You are using a 4-byte identifier, but using the same values as
those 1-byte identifiers.

The preferred value is the_hash_algo->format_id.  For SHA-1, that's
"sha1", big-endian (0x73686131) and for SHA-256 it's "s256", big-endian
(0x73323536).

There's also hash_algo_by_id to turn the format ID into an index into
the hash_algos array, but you need to check for GIT_HASH_UNKNOWN (0)
first.

These will be used in index v3, which I haven't sent out patches for
yet.
-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA