viirya opened a new pull request, #5741:
URL: https://github.com/apache/arrow-rs/pull/5741
# Which issue does this PR close?
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Closes #.
# Rationale for this change
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Encountered an issue when importing empty variable-size binary layout array
(e.g., string) from Java Arrow.
There is difference between Java Arrow and arrow-rs when computing the
length of data buffer:
https://github.com/apache/arrow/pull/41610#issuecomment-2103964293
This is how Java Arrow imports an Utf8 array:
```
try (ArrowBuf offsets = importOffsets(type, VarCharVector.OFFSET_WIDTH)) {
final int start = offsets.getInt(0);
final int end = offsets.getInt(fieldNode.getLength() * (long)
VarCharVector.OFFSET_WIDTH);
final int len = end - start;
...
}
```
So even the offset buffer is not initialized, for empty array with one
element offset buffer, `end - start` is always 0 that is the length of data
buffer. That is why the added roundtrip tests are passed.
But in arrow-rs, it takes the last value of the offset buffer as the length
of data buffer, i.e., `end`. If the value is not initialized to zero, the
computed length of data buffer is incorrect.
That is what I found for the first offset value from the
[spec](https://arrow.apache.org/docs/format/Columnar.html#variable-size-binary-layout):
```
Generally the first slot in the offsets array is 0, and the last slot is the
length of the values array.
When serializing this layout, we recommend normalizing the offsets to start
at 0.
```
It looks like the first value doesn't have to be 0, although generally it
is. So seems Java Arrow's approach is (more) correct.
# What changes are included in this PR?
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sometimes worth providing a summary of the individual changes in this PR.
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Changing the way to compute the length of data buffer when importing an
empty variable-size binary layout array.
# Are there any user-facing changes?
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No
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