Re: [PATCH v3 12/16] Documentation/technical: describe multi-pack reverse indexes

[Jeff King <peff@peff.net>]

On Thu, Mar 11, 2021 at 12:05:25PM -0500, Taylor Blau wrote:

> As a prerequisite to implementing multi-pack bitmaps, motivate and
> describe the format and ordering of the multi-pack reverse index.

Nicely written overall. I found a few typos / formatting issues.

> +One solution is to let bits occupy the same position in the oid-sorted
> +index stored by the MIDX. But because oids are effectively random, there

s/there/their/

> +Given the list of packs and their counts of objects, you can
> +naïvely reconstruct that pseudo-pack ordering (e.g., the object at

An HTML entity seems to have snuck in. The source is utf8, so we can
just say ï.

> +position 27 must be (c,1) because packs "a" and "b" consumed 25 of the
> +slots). But there's a catch. Objects may be duplicated between packs, in
> +which case the MIDX only stores one pointer to the object (and thus we'd
> +want only one slot in the bitmap).
> +
> +Callers could handle duplicates themselves by reading objects in order
> +of their bit-position, but that's linear in the number of objects, and
> +much too expensive for ordinary bitmap lookups. Building a reverse index
> +solves this, since it is the logical inverse of the index, and that
> +index has already removed duplicates. But, building a reverse index on
> +the fly can be expensive. Since we already have an on-disk format for
> +pack-based reverse indexes, let's reuse it for the MIDX's pseudo-pack,
> +too.

Yep, I think this nicely builds up the logic explaining the need for the
midx .rev file.

> +Objects from the MIDX are ordered as follows to string together the
> +pseudo-pack. Let _pack(o)_ return the pack from which _o_ was selected
> +by the MIDX, and define an ordering of packs based on their numeric ID
> +(as stored by the MIDX). Let _offset(o)_ return the object offset of _o_
> +within _pack(o)_. Then, compare _o~1~_ and _o~2~_ as follows:

I guess the asciidoc-formatted version of this makes these nicely
italicized and subscripted. Personally I think pack(o) and o1 would be
more readable in the source (which is what I would tend to read). Or
maybe backticks if you want to be fancy.

> +  - If _pack(o~1~) ≠ pack(o~2~)_, then sort the two objects in
> +    descending order based on the pack ID.
> +
> +  - Otherwise, _pack(o~1~) = pack(o~2~)_, and the objects are
> +    sorted in pack-order (i.e., _o~1~_ sorts ahead of _o~2~_ exactly
> +    when _offset(o~1~) < offset(o~2~)_).

A few more HTML bits in the comparison operators.

-Peff