Facets and numeric filters

The index splits filters into two shapes by data type. Categorical filters live in FacetIndex; numeric range filters live in NumericIndex.

Categorical facets

Each facet axis interns its values to dense u16 ids at build time and stores a Vec<RoaringBitmap> indexed by id. Lookup is one indexed access plus a HashMap<String, u16> (or HashMap<T, u16> for integer axes) to translate user input. RoaringBitmap intersections drive the AND across multiple selected values within an axis, and across multiple axes within a query.

Axes the index currently exposes:

• dept (string) — first segment of the course code, e.g. 15, 21
• level (string) — 100, 200, …, or grad
• school (string) — SCS, CIT, MCS, etc.
• attribute_tags, gened_tags, skills (string, multi-valued per course)
• has_syllabus_terms (string, multi-valued)
• units_int (integer) — discrete units bucket

Multi-valued axes simply OR together the per-value bitmaps within a single course before pushing into the postings layer.

Smart facet pruning

When a query asks for facet counts (Query.with_facet_counts(true)), the index sorts axis values by current set size and returns at most 100 entries per axis. Long-tail values stay reachable via direct lookup but do not flood the response payload.

Numeric ranges

NumericIndex keeps three flavors that match the catalog data:

• NumericFieldF32 — used for FCE-derived means (workload, instruction quality).
• NumericFieldU32 — used for pagerank (scaled), section IDs, etc.
• NumericFieldU16Optional — used for units (some courses have variable/no units).

Each variant stores values in a flat Vec keyed by doc id, so a range filter is a linear scan with predicate, intersected with whatever bitmap the rest of the query produces.

Interning of low-cardinality fields

Course fields with few distinct values use Arc<str> interning across the catalog: description (rare duplicates from cross-listed courses), level, school, and the multi-valued tag fields. The interning pool lives in the courses region itself; bincode emits each unique string once and references it by index.