BSI: add Retain for in-place filtering#517
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lemire merged 2 commits intoRoaringBitmap:masterfrom Mar 9, 2026
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Expand the doc comment to explain: in-place semantics vs NewBSIRetainSet, why skipping bA when dropped==0 is correct (BSI consistency invariant), and when to prefer Retain over the allocating alternative. The in-place form is used in caterwaul's CleanBitmaps, which iterates every term bitmap and filters it down to currently-valid doc IDs before writing it back to storage. No new BSI allocation is needed since the result is immediately serialized.
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Summary
Retainis the in-place equivalent ofNewBSIRetainSet: it removes allvalues whose column IDs are not in the provided bitmap, modifying the BSI
in place and returning the number of column IDs dropped.
Prefer
RetainoverNewBSIRetainSetwhen no copy is needed — forexample when the BSI will be immediately re-serialized — as it avoids
allocating a new BSI and cloning all bit planes.
The implementation uses an optimization: if the existence bitmap is
unchanged after the intersection (i.e.
retaincovers all existing columnIDs), the bit planes are skipped entirely. This is correct because BSI
consistency guarantees that
bAcontains no set bits for column IDsabsent from
eBM.A test is included covering both the drop path and the no-op early-return.