pitrou commented on code in PR #48203:
URL: https://github.com/apache/arrow/pull/48203#discussion_r2556419213
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cpp/src/parquet/decoder.cc:
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@@ -2306,6 +2345,17 @@ class ByteStreamSplitDecoder<FLBAType> : public
ByteStreamSplitDecoderBase<FLBAT
const int num_decoded = this->DecodeRaw(decode_out, max_values);
DCHECK_EQ(num_decoded, max_values);
+#if !ARROW_LITTLE_ENDIAN
+ // On big-endian, ByteStreamSplitDecode (DoMergeStreams) reverses stream
positions
+ // to produce numeric values in native byte order. For FLBA (opaque byte
arrays),
+ // we need to undo this reversal to preserve the original byte sequence.
Review Comment:
Hmm, doing a second byteswap to undo the first one sounds entirely wasteful.
Perhaps the ByteStreamSplit routines should instead take an optional flag to do
the byte-swapping?
For example have something like:
```c++
// If fix_endianness is `true`, output will be converted from little endian
to native endian
inline void ByteStreamSplitDecode(const uint8_t* data, int width, int64_t
num_values,
int64_t stride, bool fix_endianness,
uint8_t* out) {
```
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