Made bitmap not cache null count

[jorgecarleitao]

This PR removes the cached null_count on Bitmap, making the computation only happen on request.

This makes the struct simpler and removes the need to compute the count even if the count will not be used.

[jorgecarleitao]

what do you think @ritchie46 @houqp @Dandandan ?

[codecov]

Codecov Report

Merging #563 (296838f) into main (5fc843d) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #563      +/-   ##
==========================================
+ Coverage   78.92%   78.98%   +0.06%     
==========================================
  Files         382      385       +3     
  Lines       24276    24704     +428     
==========================================
+ Hits        19160    19513     +353     
- Misses       5116     5191      +75     

Impacted Files	Coverage Δ
src/bitmap/immutable.rs	84.61% <100.00%> (-1.50%)	⬇️
src/io/ipc/read/array/union.rs	70.49% <0.00%> (-4.51%)	⬇️
src/array/union/mod.rs	79.27% <0.00%> (-0.34%)	⬇️
tests/it/array/mod.rs	100.00% <0.00%> (ø)
tests/it/compute/bitwise.rs	100.00% <0.00%> (ø)
benches/bitwise.rs	0.00% <0.00%> (ø)
src/compute/bitwise.rs	100.00% <0.00%> (ø)
src/array/display.rs	53.47% <0.00%> (+2.37%)	⬆️
src/array/utf8/mutable.rs	88.17% <0.00%> (+2.91%)	⬆️
src/datatypes/mod.rs	81.65% <0.00%> (+4.03%)	⬆️
... and 3 more

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[ritchie46]

What do you think about a lazy cache? Same to init_validities(). That we only set the null count at first request. I can imagine that we have long living arrays that are base for many computations.

For instance in Polars, almost every operation starts with a null_count check to see if we can use a fast path.

[jorgecarleitao]

Doesn't lazy initialization require &mut self?

For instance in Polars, almost every operation starts with a null_count check to see if we can use a fast path.

shouldn't it start with if let Some(bitmap) = array.validity()? Or is it common to have arrays with only null values?

[ritchie46]

Doesn't lazy initialization require &mut self?

For instance in Polars, almost every operation starts with a null_count check to see if we can use a fast path.

shouldn't it start with if let Some(bitmap) = array.validity()? Or is it common to have arrays with only null values?

The series has potentially multiple chunks, so we check null_count. But indeed, I could add the branch check for the validity and return 0 if it is missing. Thanks, now it should not matter that much.

[ritchie46]

Just for clarity, I think it is a good enhancement. 👍 :)

Certainly when iterating lists, this saves quite some compute, I think.

[jorgecarleitao]

Reverted because it triggered a major regression in polars, so more care is needed here. Thanks to @ritchie46 for pointing it out.