pitrou commented on code in PR #50629:
URL: https://github.com/apache/arrow/pull/50629#discussion_r3656474359
##########
cpp/src/parquet/column_reader.cc:
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@@ -222,6 +254,161 @@ auto LevelDecoder::CountUpTo(int16_t value, int
batch_size) -> CountUpToResult {
};
}
+/**************************
+ * LevelToBitmapDecoder *
+ **************************/
+
+/// Decoder for definition levels that writes directly into a validity bitmap.
+///
+/// This is the bitmap counterpart of ``LevelDecoder``, specialized for levels
+/// encoded on a single bit (a max level of 1), such as the definition levels
of a
+/// flat, nullable column. Rather than decoding into an ``int16_t`` array and
+/// re-encoding into an Arrow validity bitmap, it decodes straight into the
bitmap.
+///
+/// @see LevelDecoder
+class LevelToBitmapDecoder {
Review Comment:
> The issue is that `Decode` takes fundamentally different arguments:
> * In the general case, an `int16_t*` array where the data type
correspond to one value
> * In the bitmap case, a `uint8_t*`array + bit offset to find the exact
pointed bit.
Ideally the RecordReader would not see levels values anymore, so this
discrepancy would disappear.
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