GH-42245: [Swift] Ensure map behavior is the same for all key types

[abandy]

Rationale for this change

Behavior of decoding a map currently changes based on the key type (String or not String)

What changes are included in this PR?

Added method for handling map to ensure that all maps are decoded the same way.

Are these changes tested?

Yes

• GitHub Issue: [Swift] Ensure map behavior is the same between all data types #42245

[kou]

Can we call doDecode<T>(_ col: Int) instead of adding a similar code?

[abandy]

Will do. My goal was to make it clear that the index for the single column was being handled a bit different but sending in the singleRBCol to the doDecode method works.

[kou]

Ah, defining doDecodeSingleValue() and isNullSingleValue() is OK. I just wanted to say that changing the implementation something like:

    func doDecodeSingleValue<T>() throws -> T? {
        return doDecode(self.singleRBCol)
    }

[abandy]

Aha, I see! Thanks for clarifying! We do remove the overhead of another func call by removing the SingleValue methods. Let me know what you think.

[kou]

If there is a performance overhead, I'm OK with the current approach.

[kou]

Can we call isNull(_ col: Int) instead?

[kou]

+1

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 7 benchmarking runs that have been run so far on merge-commit d28078d.

There were 12 benchmark results with an error:

• Commit Run on ec2-r5-4xlarge-us-east-2 at 2024-06-22 04:19:18Z
  • partitioned-dataset-filter (R) with dataset=dataset-taxi-parquet, language=R, query=payment_type_3
  • partitioned-dataset-filter (R) with dataset=dataset-taxi-parquet, language=R, query=payment_type_3
• and 10 more (see the report linked below)

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 9 possible false positives for unstable benchmarks that are known to sometimes produce them.