AntoinePrv commented on code in PR #50422:
URL: https://github.com/apache/arrow/pull/50422#discussion_r3552460615
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cpp/src/arrow/util/rle_encoding_internal.h:
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@@ -515,6 +573,85 @@ class RleBitPackedDecoder {
int64_t valid_bits_offset, rle_size_t
null_count);
};
+/// Minimal decoder for legacy bit packed encoding (BIT_PACKED = 4).
+///
+/// The number of values that the decoder can represent is up to 2^31 - 1.
+template <typename T>
+class BitPackedDecoder : private BitPackedRunDecoder<T> {
+ private:
+ using Base = BitPackedRunDecoder<T>;
+
+ public:
+ /// The type in which the data should be decoded.
+ using value_type = T;
+
+ using Base::Advance;
+
+ BitPackedDecoder() noexcept = default;
+
+ /// Create a decoder object.
+ ///
+ /// Unless explicitly given, then number of values is deduced from the data
size,
+ /// in which case it may read more values from the input stream than the user
+ /// intended (to reach final byte alignment). This option is mandatory if the
+ /// `value_bit_width` is zero.
Review Comment:
I added this from a copilot review: if the value_bit_width is 0, we have no
idea of the number of values even though we may be calling `GetBatch`, so we
need to set one.
Ideally we'd manage to move the known number of values into these decoder so
they have a better-defined API.
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