felipecrv commented on code in PR #41827:
URL: https://github.com/apache/arrow/pull/41827#discussion_r1623650513
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cpp/src/arrow/compute/kernels/scalar_cast_string.cc:
##########
@@ -510,6 +510,60 @@ void AddBinaryToFixedSizeBinaryCast(CastFunction* func) {
AddBinaryToFixedSizeBinaryCast<FixedSizeBinaryType>(func);
}
+// ----------------------------------------------------------------------
+// Union to String
+
+template <typename O>
+struct UnionToStringCastFunctor {
+ using BuilderType = typename TypeTraits<O>::BuilderType;
+
+ static Status Exec(KernelContext* ctx, const ExecSpan& batch, ExecResult*
out) {
+ const ArraySpan& input = batch[0].array;
+ const auto& union_type = checked_cast<const UnionType&>(*input.type);
+ const auto type_ids = input.GetValues<int8_t>(1);
+ const auto& offsets = input.GetValues<int32_t>(2);
+
+ BuilderType builder(input.type->GetSharedPtr(), ctx->memory_pool());
+ RETURN_NOT_OK(builder.Reserve(input.length));
+
+ for (int64_t i = 0; i < input.length; ++i) {
Review Comment:
A string formatter for unions should allocate a vector of string formatters
that can do virtual dispatching (and deal with nesting themselves as well).
`StringFormatter<T>` performs static dispatch which allows loop-specialization
for the non-nested types. But for nested types we will need to setup a vector
of `VirtualStringFormatter` (which is actually a tree) so that all the
"switching on the type" happens at construction time (beginning of the loop)
and invocations inside the loop are following the same function pointers from
the vtables.
```cpp
// in header
class VirtualStringFormatter {
virtual ... = 0;
};
Result<std::unique_ptr<VirtualStringFormatter>> MakeFormatter(
const std::shared_ptr<DataType>& type);
// in an anon namespace of the .cpp
// one sub-class per `Type::type`
class <T>StringFormatter : public VirtualStringFormatter {
}
// you can use templates to cover most cases delegating to StringFormatter<T>
class UnionStringFormatter : ...
```
This hierarchy would be similar to the builder class hierarchy.
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