[ARROW-13312] [C++] Bitmap::VisitWordAndWrite epilogue needs to work on Words (not bytes)

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: C++
• External issue URL: https://github.com/apache/arrow/issues/28988
• Assignee: Unassigned
• Reporter: Niranda Perera
• Created: 12/Jul/21 15:25
• Updated: 11/Jan/23 08:32

Description

In recently added `Bitmap::VisitWordAndWrite` method, translates the `visitor` lambda (that works on a `Word`) to a byte-visitor while handling the epilogue.

This could lead to incorrect results in the client code.

ex:

// code placeholder
// N readers, M writers 
int64_t bits_written = 0;
auto visitor = [&](std::array<Word, N> in, std::array<Word, M>* out){
 ...
 bits_written += (sizeof(Word) * 8);
}

At the end of the Visit, bits_written would have an incorrect sum because in the prologue, it adds 64 to bits_written for each trailing byte, whereas it should've been 8.

 

Possible solution:

Needs to add ReadTrailingWord and WriteTrailingWord functionality to BitmapWordReader and BitmapWordWriter respectively and call visitor with the words in the epilogue.