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Wed, 29 Oct 2025 13:03:54 -0400 (EDT) From: Junio C Hamano To: "brian m. carlson" Cc: , Patrick Steinhardt , Ezekiel Newren Subject: Re: [PATCH 12/14] rust: add a new binary loose object map format In-Reply-To: <20251027004404.2152927-13-sandals@crustytoothpaste.net> (brian m. carlson's message of "Mon, 27 Oct 2025 00:44:02 +0000") References: <20251027004404.2152927-1-sandals@crustytoothpaste.net> <20251027004404.2152927-13-sandals@crustytoothpaste.net> Date: Wed, 29 Oct 2025 10:03:52 -0700 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain "brian m. carlson" writes: > Our current loose object format has a few problems. First, it is not > efficient: the list of object IDs is not sorted and even if it were, > there would not be an efficient way to look up objects in both > algorithms. I was confused by reading the above, mostly because "our current loose object format" meant to me the " SP NUL " deflated with zlib, which has no list of object IDs. As Patrick commented you are talking about something else? Mapping mechanism for object names between primary and compat hash algorithms? > +== Loose object mapping > + > +When the `compatObjectFormat` option is used, Git needs to store a mapping > +between the repository's main algorithm and the compatibility algorithm. There > +are two formats for this: the legacy mapping and the modern mapping. > + > +=== Legacy mapping > + > +The compatibility mapping is stored in a file called > +`$GIT_DIR/objects/loose-object-idx`. The format of this file looks like this: > + > + # loose-object-idx > + (main-name SP compat-name LF)* > + > +`main-name` refers to hexadecimal object ID of the object in the main > +repository format and `compat-name` refers to the same thing, but for the > +compatibility format. > + > +This format is read if it exists but is not written. > + > +Note that carriage returns are not permitted in this file, regardless of the > +host system or configuration. Unless it is zero cost to keep supporting the reading side, perhaps we want to drop this mapping file format?