Add required methods to access inner builder for NullBufferBuilder

[Chen-Yuan-Lai]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

To replace NullBufferBuilder with BooleanBufferBuilder for optimization in DataFusion , some public methods are needed to be implemented in NullBufferBuilder with BooleanBufferBuilder to access inner builder.

Describe the solution you'd like

I would like to add methods like BooleanBufferBuilder :

• get_bit()
• capacity
• truncate()

Describe alternatives you've considered

Other strategies below also may access the inner builder:

• make the inner builder public
• add a method that can return the reference of the inner builder

Additional context

Related issue

Use NullBufferBuilder instead of BooleanBufferBuilder for creating Null masks #14115

[Chen-Yuan-Lai]

@jayzhan211

[jayzhan211]

@Chen-Yuan-Lai I think we need to add methods for NullBufferBuilder and not make the inner builder public.

NullBufferBuilder may not have bitmap_builder if all we only append true (materialize is not called), so we need if-else to handle non-materialized case

[tustvold]

I'd recommend following the naming on NullBuffer, e.g. is_valid vs get_bit.

[alamb]

FWIW I think the list of functions listed above are good and straightforward to add to NullBufferBuilder.

There are some function in Datafusion that I think might be too special purpose (aka only DataFusion would use them) such as take_n and thus probably don't belong in arrow-rs unless we have evidence others would use them

https://github.com/apache/datafusion/blob/63b94c8f9e128b938e81b7e867ce6256a94d67e6/datafusion/physical-plan/src/aggregates/group_values/null_builder.rs#L110C1-L111C1