Shockp opened a new pull request, #49848:
URL: https://github.com/apache/arrow/pull/49848
### Rationale for this change
In Arrow, null bitmaps are optional and represented by `nullptr` when a
column contains no nulls. Previously, conjoining two optional bitmaps required
downstream code to manually handle the `nullptr` checks and memory allocations.
This change centralizes that logic into a single, highly optimized utility
function.
### What changes are included in this PR?
* Added `OptionalBitmapAnd` to `cpp/src/arrow/util/bitmap_ops.h` and
`bitmap_ops.cc`.
* The implementation safely handles the four possible memory permutations
with minimal allocations:
1. Both buffers `nullptr`: Returns `nullptr`.
2. Left is `nullptr`: Returns a copy of the right buffer.
3. Right is `nullptr`: Returns a copy of the left buffer.
4. Both valid: Defers to the low-level `BitmapAnd` function.
### Are these changes tested?
Yes. I added a new test case (`OptionalBitmapAnd`) to
`cpp/src/arrow/util/bitmap_test.cc` that explicitly verifies the correct buffer
allocation and bitwise output for all four memory states.
### Are there any user-facing changes?
No. This simply exposes a new C++ utility for internal development.
Closes #45819
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