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

["Niranda Perera (Jira)" <ji...@apache.org>]

Niranda Perera created ARROW-13312:
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             Summary: [C++] Bitmap::VisitWordAndWrite epilogue needs to work on Words (not bytes)
                 Key: ARROW-13312
                 URL: https://issues.apache.org/jira/browse/ARROW-13312
             Project: Apache Arrow
          Issue Type: Improvement
          Components: C++
            Reporter: Niranda Perera


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

This could lead to incorrect results in the client code.

ex:
{code:java}
// 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);
}{code}
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 prologue.

 

 



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