stojkomilos opened a new pull request, #513:
URL: https://github.com/apache/datasketches-cpp/pull/513
## What
Adds a public `get_live_bytes()` accessor that reports, in O(1), the exact
heap a sketch's
mutable internal state is holding **right now**. The sketches already expose
`get_serialized_size_bytes()`, but that is an upper bound on the
*serialized* form; there is
currently no way to ask how much memory a live, mutable sketch is actually
retaining. This is
useful for memory accounting when many mutable sketches are held in memory
at once (e.g. one
sketch per group/window in a streaming aggregation).
## Where
- **theta / tuple** — the accessor lives on `theta_update_sketch_base`, the
hash table shared by
theta and tuple update sketches, unions and intersections. It returns
`(1 << lg_cur_size_) * sizeof(Entry)`, or `0` when the table is
unallocated (`lg_cur_size_ == 0`
leaves `entries_ == nullptr`). `update_theta_sketch`, `theta_union`,
`theta_intersection`, the
tuple update sketch, `tuple_union` and `tuple_intersection` forward to it.
Tuple's union and
intersection reuse the theta base as their `State`, so no separate
implementation is needed
there.
- **kll** — not a hash table, so `kll_sketch::get_live_bytes()` sums the
`items_` buffer
(`items_size_` elements) and the `levels_` vector, mirroring the sketch's
`allocate()` sites.
Unlike `get_serialized_size_bytes()`, this touches no data and does no
iteration; it just reports
the current allocation size.
## Tests
Adds Catch2 cases in `theta_sketch_test`, `theta_union_test`,
`theta_intersection_test`,
`tuple_sketch_test` and `kll_sketch_test` that:
- check the footprint is a power-of-two count of entries where applicable,
- confirm an unallocated intersection reports `0`,
- confirm the footprint only grows (never shrinks) as entries are inserted
and ends up larger than
the initial allocation.
All theta / tuple / kll suites pass locally.
## Notes
Scope is theta, tuple and kll. `hll` and `cpc` could get an equivalent
accessor in a follow-up if
maintainers want the whole family consistent.
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