scott-routledge2 commented on code in PR #48171:
URL: https://github.com/apache/arrow/pull/48171#discussion_r2586541152
##########
cpp/src/arrow/compute/kernels/scalar_cast_string.cc:
##########
@@ -304,10 +305,33 @@ 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));
- return CastBinaryToBinaryOffsets<typename I::offset_type, typename
O::offset_type>(
- ctx, input, out->array_data().get());
+ if constexpr (sizeof(typename I::offset_type) != sizeof(typename
O::offset_type)) {
+ std::shared_ptr<ArrayData> input_arr = input.ToArrayData();
+ ArrayData* output = out->array_data().get();
+ output->length = input_arr->length;
+ // output->offset is set below
+ output->SetNullCount(input_arr->null_count);
+ output->buffers = std::move(input_arr->buffers);
+
+ // Slice buffers to reduce allocation when casting the offsets buffer
+ int64_t offset = input_arr->offset;
+ size_t input_offset_type_size = sizeof(typename I::offset_type);
+ if (output->null_count != 0 && output->buffers[0]) {
+ // Avoid reallocation of the validity buffer by allowing some padding
bits
+ output->offset = input_arr->offset % 8;
+ } else {
+ output->offset = 0;
+ }
+ if (output->buffers[0]) {
+ output->buffers[0] = SliceBuffer(output->buffers[0], offset / 8);
+ }
+ output->buffers[1] = SliceBuffer(output->buffers[1], offset *
input_offset_type_size);
Review Comment:
I believe in order for this slicing logic to be fully correct it needs to
take into account `output->offset` when doing the slicing as well as the
minimum size requirements. So it would look something like
``` c++
int64_t offset_buffer_offset = (input_offset - output_offset) *
offset_type_size
int64_t offset_buffer_length = (length + output_offset + 1) *
offset_type_size
if (offset_buffer_length < minimum_size) {
// update the output->offset so that it's now == minimum size
}
```
?
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