refactor(parquet): introduce string_offset_scan_state for preprocess_string_offsets_kernel

[vyasr]

Description

This PR builds on #23479 to continue narrowing the shared state used by Parquet preprocessing kernels. It extracts page_decode_progress_state from page_state_s, grouping the decode-position/progress fields that are needed by string-offset preprocessing.

The primary addition is string_offset_scan_state, composed from page_decode_setup_state, page_decode_stream_state, and page_decode_progress_state. preprocess_string_offsets_kernel now uses this smaller composed state instead of the full page_state_s, which drops unrelated output-conversion, nesting, and level-scratch fields from that kernel's shared-memory state. This follows the same pattern as #23479: pull one coherent sub-state out of page_state_s, migrate the target kernel to the minimal composed state it actually needs, and leave the remaining full decode state for a later PR.

A fresh cuobjdump -res-usage comparison against upstream/main shows preprocess_string_offsets_kernel saves 512 bytes of shared memory on every generated architecture, with no register-count changes:

Arch	shmem before	shmem after	delta	regs before	regs after
sm_70	2048	1536	-512	40	40
sm_75	2048	1536	-512	64	64
sm_80	2048	1536	-512	37	37
sm_86	2048	1536	-512	38	38
sm_90a	3072	2560	-512	32	32
sm_100f	3072	2560	-512	32	32
sm_120a	3072	2560	-512	40	40
sm_120	3072	2560	-512	40	40

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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Reviewing files that changed from the base of the PR and between 43ca9a2 and 03f9c78.

📒 Files selected for processing (1)

• cpp/src/io/parquet/page_state_composed.cuh

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

• cpp/src/io/parquet/page_state_composed.cuh

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved Parquet decoding reliability for fixed-width, string, dictionary-encoded, boolean, delta-encoded, and nested data.
  • Improved handling of page boundaries, row limits, validity information, and list data.
  • Improved string offset processing and page containment checks for more consistent results.
  • Improved source-position tracking and value-count handling during dictionary and delta decoding.

Walkthrough

Parquet decoding now stores cumulative counters and row-bound state in page_state_s::progress. Page-boundary helpers use page state directly. Fixed-width, page, delta, and string decoders were updated. String offset preprocessing now uses a composed state.

Changes

Parquet progress-state migration

Layer / File(s)	Summary
Progress state and page-boundary contracts cpp/src/io/parquet/page_decode.cuh	Adds page_decode_progress_state, nests progress in page_state_s, updates boundary helpers, and initializes progress fields.
Fixed-width decoding and preprocessing cpp/src/io/parquet/decode_fixed.cu, cpp/src/io/parquet/decode_preprocess.cu	Routes validity, row, value, leaf, dictionary, and row-bound counters through s->progress.
Page and delta decoder progress cpp/src/io/parquet/page_data.cu, cpp/src/io/parquet/page_delta_decode.cu	Updates page and delta decoders to use nested source-position, dictionary-position, and input counters.
String offset scan state cpp/src/io/parquet/page_state_composed.cuh, cpp/src/io/parquet/page_string_decode.cu	Adds string_offset_scan_state and updates string offset readers and page-boundary calls to use composed state.

Estimated code review effort: 3 (Moderate) | ~30 minutes

Possibly related PRs

• rapidsai/cudf#23241: Refactors the same Parquet page-boundary helpers and preprocessing and string-decoding call sites.
• rapidsai/cudf#23374: Overlaps in the Parquet page-state and decoder paths modified by this change.
• rapidsai/cudf#23471: Refactors page_state_s access across the same Parquet decoder kernels.

Suggested reviewers: bdice, mythrocks

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: introducing string_offset_scan_state for Parquet string-offset preprocessing.
Description check	✅ Passed	The description accurately explains the state refactoring, kernel change, and shared-memory reduction.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@cpp/src/io/parquet/page_decode.cuh`:
- Around line 204-207: Update the Doxygen blocks for is_bounds_page and the
other affected helper declarations to document the actual s parameter, replacing
the obsolete page and chunk_start_row entries. Ensure every documented parameter
matches its corresponding signature, including start_row, num_rows, and
has_repetition.

In `@cpp/src/io/parquet/page_state_composed.cuh`:
- Around line 47-50: Update the static_assert involving string_offset_scan_state
to compare its size directly against sizeof(page_state_s), removing the
additional page_decode_* size terms so the assertion detects shared-memory size
regressions.

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Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 7f58752 and 4f6848d.

📒 Files selected for processing (7)

• cpp/src/io/parquet/decode_fixed.cu
• cpp/src/io/parquet/decode_preprocess.cu
• cpp/src/io/parquet/page_data.cu
• cpp/src/io/parquet/page_decode.cuh
• cpp/src/io/parquet/page_delta_decode.cu
• cpp/src/io/parquet/page_state_composed.cuh
• cpp/src/io/parquet/page_string_decode.cu

[bdice]

Is this meant to be left in?

[vyasr]

It can be removed, but since CI already passed here unless another reviewer requests changes I'll just leave it for now and clean it up in a follow-up PR. We'll be either renaming/removing page_state_s so this'll have to change then.