scott-routledge2 commented on code in PR #48171:
URL: https://github.com/apache/arrow/pull/48171#discussion_r2585769433
##########
cpp/src/arrow/compute/kernels/scalar_cast_string.cc:
##########
@@ -304,8 +305,21 @@ BinaryToBinaryCastExec(KernelContext* ctx, const ExecSpan&
batch, ExecResult* ou
}
}
- // Start with a zero-copy cast, but change indices to expected size
- RETURN_NOT_OK(ZeroCopyCastExec(ctx, batch, out));
+ std::shared_ptr<ArrayData> input_arr = input.ToArrayData();
+ ArrayData* output = out->array_data().get();
+ output->length = input_arr->length;
+ output->SetNullCount(input_arr->null_count);
+ output->buffers = std::move(input_arr->buffers);
+ output->child_data = std::move(input_arr->child_data);
+
+ if (output->buffers[0]) {
+ // If reusing the null bitmap, ensure offset into the first byte is the
same as input.
+ output->offset = input_arr->offset % 8;
+ output->buffers[0] = SliceBuffer(output->buffers[0], input_arr->offset /
8);
+ } else {
+ output->offset = 0;
+ }
Review Comment:
Sorry for the late reply. What you are saying makes sense, however, I think
I am still a little confused about specifics of the slicing here. Wouldn't we
want to slice the offset buffer with a different value than the validity
buffer?
For example, if we are casting a slice that has length 8 and offset 8, we
would slice the
validity buffer with a value of 1, and be left with a buffer of length 1
representing the 8 null bits for the elements in our casted slice. We would
also slice the offsets buffer by 1, which would mean the buffer will have
length 17*offset_size - 1 and be out of alignment.
Similarly in the case where we have `output->null_count == 0` (and buffer[0]
!= nullptr), we would slice the offset buffer by 8, leaving a buffer of size
17*offset_size - 8 and we would also slice the validity buffer by 8 , which
would go out of bounds.
Wouldn't we want to slice the offsets buffer by `input_arr->offset*
sizeof(typename I::offset_type)` and the validity buffer by `input_arr->offset
/ 8`
Edit: I think the "offset" in SliceBuffer is the byte-offset as opposed to
the logical offset.
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