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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