[mapleFU]

Rationale for this change

Optimize level counting in parquet column writer.

See: https://godbolt.org/z/qhEGa418x

What changes are included in this PR?

Some naive changes, this separate some dependencies, thus the code would be easy to vectorize.

Are these changes tested?

Covered by existing

Are there any user-facing changes?

No

Was AI used for this PR?

PR code and description written by:

• Human

Reviewed before submission by:

• Human
• AI

• GitHub Issue: [C++][Parquet] Minor: optimize level counting in parquet writer #55307

[github-actions]

⚠️ GitHub issue #55307 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟡 Changes recommended

The newly cached level variables remain unused, causing warnings that break compilation under Arrow’s CHECKIN debug configuration with -Wall -Werror.

1 open issue

• Use cached level variables to avoid unused-variable build errors · New

What changed in this PR

This PR optimizes level counting in the C++ Parquet column writer to make the counting loops easier to vectorize, with no intended user-facing changes.

Changes:

• Replace repeated-row counting loops with std::count.
• Accumulate definition-level counts locally before updating output parameters.

File	Description
cpp/​src/​parquet/​column_writer.cc	Optimize repetition- and definition-level counting in the writer.

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

No active review issues are reported for the column writer’s level-counting changes.

No open issues 🎉

1 resolved since last review

• Use cached level variables to avoid unused-variable build errors

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

No active review issues were identified for the Parquet level-counting optimization.

No open issues 🎉

_{Review effort: Balanced}