[jira] [Commented] (ARROW-7767) [C++] Add a facility to create a Bitmap buffer from an data pointer with a specified sentinel

["Wes McKinney (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-7767?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17030150#comment-17030150 ] 

Wes McKinney commented on ARROW-7767:
-------------------------------------

Seems useful. Bonus points for vectorizing (pack 8 values into a byte at a time). 

> [C++] Add a facility to create a Bitmap buffer from an data pointer with a specified sentinel
> ---------------------------------------------------------------------------------------------
>
>                 Key: ARROW-7767
>                 URL: https://issues.apache.org/jira/browse/ARROW-7767
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++, R
>            Reporter: Francois Saint-Jacques
>            Priority: Major
>
> This is a special case for R and other cases where the null value is represented by a sentinel. This would read the data pointer and return a null bitmap buffer where bits are activate for every row where the value is not the sentinel value. If no sentinel is encountered, return nullptr. 
> {code:c++}
> template <typename ArrowType, typename CType = ArrowType::c_type>
> Result<std::shared_ptr<Buffer>> NullBitmapFromSentinelData(MemoryPool* pool, const CType* data, size_t n_values, CType sentinel_value>();
> {code}



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