Replace hand-rolled output iterators with cuda::tabulate_output_iterator

[PointKernel]

Description

There are a few places in cuDF that hand-roll output iterators where cuda::make_tabulate_output_iterator would do. This PR replaces them.

Each one reimplements the iterator internals just to write a single struct field, when all that is actually needed is the functor.

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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[PointKernel]

/ok to test

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📥 Commits

Reviewing files that changed from the base of the PR and between f588e0c and 4925c87.

📒 Files selected for processing (3)

• cpp/src/copying/contiguous_split.cu
• cpp/src/io/parquet/reader_impl_preprocess.cu
• cpp/src/io/parquet/reader_impl_preprocess_utils.cuh

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📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • Streamlined GPU processing for contiguous data splitting and Parquet file reading.
  • Simplified internal handling of row counts, page offsets, and string offsets while preserving existing results.
  • No changes to public APIs or user-facing functionality.

Walkthrough

The change replaces custom Thrust output iterators with indexed device functors used by tabulate output iterators. Contiguous split and Parquet preprocessing retain their existing scan and reduction data flow.

Changes

CUDA output writes

Layer / File(s)	Summary
Contiguous split output writes cpp/src/copying/contiguous_split.cu	Partition offsets and valid counts now use tabulate output iterators with device functors. Scan and reduction behavior remains unchanged.
Parquet preprocessing output writes cpp/src/io/parquet/reader_impl_preprocess_utils.cuh, cpp/src/io/parquet/reader_impl_preprocess.cu	Indexed functors write PageInfo::chunk_row and PageInfo::str_offset during list, subpass, and string-offset preprocessing scans.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 4925c

This is a localized iterator implementation cleanup with no actionable merge-blocking risk remaining after normal checks and review.

Suggested reviewers: nirandaperera, vyasr

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 14.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains that the pull request replaces hand-rolled output iterators with cuda::make_tabulate_output_iterator.
Title check	✅ Passed	The title accurately and concisely summarizes the main change: replacing hand-rolled output iterators with cuda::tabulate_output_iterator.
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.

Full details: Docstring Coverage

Explanation

Docstring coverage is 14.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files. (1 skipped: 1 unsupported.)

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✨ Finishing Touches

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[nirandaperera]

LGTM

[bdice]

This looks fine but I vaguely recall there being an issue that forced us to create custom output iterators. Maybe a historical compiler bug or Thrust performance issue that is now resolved? Let's check the history before merging.