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
