FluorineDog opened a new pull request, #527:
URL: https://github.com/apache/datasketches-cpp/pull/527
### Problem (AI generated)
`frequent_items_sketch::is_empty()` is defined as `map.get_num_active() ==
0`, conflating "no retained items" with "no observations".
A purge subtracts the median of all counters and keeps only positive ones.
When every counter equals the median — typical for streams with many
low-frequency items — the purge wipes **all** counters, leaving `num_active ==
0` while `offset` and `total_weight` remain non-zero. Such a state is then
misreported as empty, which corrupts two operations:
- `serialize()` emits the empty short format, dropping `total_weight` and
`offset` on the wire. After a round-trip the sketch reads `total_weight == 0`
and `get_maximum_error() == 0` — falsely looking like a fresh, exact sketch.
- `merge()` early-returns on `other.is_empty()`, silently discarding the
other sketch's `total_weight` and `offset`.
### Minimal reproducer
See unittest in this PR. After serde, data is lost.
### Fix
Use the following to implement is_empty instead.
```cpp
bool is_empty() const { return total_weight == 0; }
```
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