[C++] Implement Bitmap class to modularize handling of bitmaps

[asfimport]

This could be a simple view or it could own a shared_ptr<Buffer>. In either case, it would greatly simplify situations where a pointer, offset, length are currently passed

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] Provide bulk method on TypedBufferBuilder for appending a bitmap (is related to)

_{Note: This issue was originally created as ARROW-4722. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I have run into this issue in ARROW-3772 where Parquet decoders currently pass around bitmaps in pieces (see eg https://github.com/apache/arrow/blob/master/cpp/src/parquet/encoding.h#L56). Currently we allow passing null/not-null markers using a bytemap in the builder classes, but not a bitmap. I propose that we have a simple bitmap object with structure like:

class Bitmap {
  const uint8_t* data_;
  int64_t length_;
  int64_t offset_;
};

I'm not sure how to best handle non-const bitmaps, though. We might have to have

class MutableBitmap : public Bitmap {
  ...
};

A first place where this would be useful to have is as an alternative to any ArrayBuilder method that currently takes valid_bytes, like

https://github.com/apache/arrow/blob/master/cpp/src/arrow/array/builder_primitive.h#L119

Bitmap can have helper methods GetReader and GetWriter for constructing BitmapReader and BitmapWriter respectively

[asfimport]

Antoine Pitrou / @pitrou:
ARROW-6396 added a Bitmap class to bit_util.h that more or less fits the bill, should we close this?