pitrou commented on code in PR #41565:
URL: https://github.com/apache/arrow/pull/41565#discussion_r1592639889
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cpp/src/parquet/encoding.h:
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@@ -255,6 +255,10 @@ class Decoder {
// Sets the data for a new page. This will be called multiple times on the
same
// decoder and should reset all internal state.
+ //
+ // In current implementations, `num_values` is the `num_values` field in the
+ // data page header, which may greater than the number of values in the data
+ // buffer.
Review Comment:
```suggestion
// `num_values` comes from the data page header, and may be greater than
the number of
// physical values in the data buffer if there are some omitted (null)
values.
// `len`, on the other hand, is the size in bytes of the data buffer and
// directly relates to the number of physical values.
```
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