feat(python): Implement bitmap unpacking #450
Merged
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In prototyping a real-world use case, I remembered that unpacking bits is exceedingly difficult to get right if you need to support an arbitrary offset/length. The math for this is very fiddly and we spent a few rounds getting it right in the C function
ArrowBitsUnpackInt(8|32)
. This PR makes that available so that we can do things like (1) convert bool arrays to numpy and (2) convert null masks to something that somebody else can work with (e.g., a numpy mask).This seems to be relatively performant (thanks to @WillAyd's work optimizing this!)