Restore mergeAndSetValidity as a single ColumnView API

[rishic3]

Description

Supersedes #23249. This reverts the move of mergeAndSetValidity into ColumnVector.

By keeping the API on ColumnView, this ensures that for a caller holding a ColumnView, they may use this API without incurring an additional copy in the event that the mask changes (the call is not a no-op). I.e. consider this table comparing the placement of the API and the copies incurred:

Receiver and result	Single API on ColumnView (this PR)	Strictly ColumnVector
ColumnVector, no-op	0 data copies; refcount increment	0 data copies
ColumnVector, mask changes	1 deep copy	1 deep copy
True ColumnView, no-op	1 necessary materialization	1 materialization before calling
True ColumnView, mask changes	1 deep copy	2 deep copies

An example of the 4th case is NVIDIA/cudf-spark#14846.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[rishic3]

/ok to test aeb0926

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📥 Commits

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📒 Files selected for processing (1)

• java/src/main/java/ai/rapids/cudf/ColumnView.java

🚧 Files skipped from review as they are similar to previous changes (1)

• java/src/main/java/ai/rapids/cudf/ColumnView.java

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📝 Summary

Summary by CodeRabbit

• Changes
  • Removed the column-vector validity-merging operation.
  • Retained validity merging for non-owning column views.
  • Clarified that column views support zero-copy validity merging when possible and materialize results when necessary.
  • Updated the API documentation to distinguish column-vector and column-view behavior.
  • Other column-vector functionality remains unchanged.

Walkthrough

Changes

Validity merge API cleanup

Layer / File(s)	Summary
Java validity merge contract java/src/main/java/ai/rapids/cudf/ColumnView.java, java/src/main/java/ai/rapids/cudf/ColumnVector.java	ColumnView.mergeAndSetValidity now documents owning and non-owning no-op behavior and is no longer deprecated. The ColumnVector implementation was removed.
Native binding cleanup java/src/main/native/src/ColumnVectorJni.cpp	The native validity merge binding and its binary-operation and struct utility dependencies were removed.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to ec35f

This change restores the validity-merge API on ColumnView to avoid unnecessary copies for callers holding views. No current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains that the pull request restores mergeAndSetValidity on ColumnView, removes its ColumnVector placement, and reduces unnecessary copies.
Title check	✅ Passed	The title accurately and concisely summarizes the main change: restoring mergeAndSetValidity as a single ColumnView API.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[ttnghia]

I'm still not clear and don't see how the original column returns no-op incRefCount (to output ColumnVector) if calling from a ColumnView object?

[rishic3]

It does not. As the docstring states if we have a ColumnView we have to do a deep copy to produce an owning result. (This would be row 3 of the table in the PR desc). Only if we have a ColumnVector do we have the no-op incRefCount.

[ttnghia]

But this PR is removing the ColumnVector overload thus the input is going to always be a ColumnView.

[rishic3]

It removes the ColumnVector override, but ColumnVector extends ColumnView. So this will work:

ColumnVector cv = ...;
cv.mergeAndSetValidity(...);

and if the above is a no-op, we invoke copyToColumnVector(), which ColumnVector overrides to be just an incRefCount() and is thus zero copy.

[ttnghia]

copyToColumnVector(), which ColumnVector overrides to be just an incRefCount()

This is the main point. Everything is explained by it. Yay!
Please add more comment on the docs clarifying this -- it is useful to understand what is going on.

[rishic3]

👍 added a comment to clarify.

[rishic3]

/ok to test ec35f42