[GitHub] [arrow] WillAyd commented on a change in pull request #12248: ARROW-1888: [C++] Implement Struct Casts

[GitBox <gi...@apache.org>]

WillAyd commented on a change in pull request #12248:
URL: https://github.com/apache/arrow/pull/12248#discussion_r796311267



##########
File path: cpp/src/arrow/compute/kernels/scalar_cast_nested.cc
##########
@@ -150,6 +150,73 @@ void AddListCast(CastFunction* func) {
   DCHECK_OK(func->AddKernel(SrcType::type_id, std::move(kernel)));
 }
 
+struct CastStruct {
+  static Status Exec(KernelContext* ctx, const ExecBatch& batch, Datum* out) {
+    const CastOptions& options = CastState::Get(ctx);
+    const auto in_field_count =
+        checked_cast<const StructType&>(*batch[0].type()).num_fields();
+    const auto out_field_count =
+        checked_cast<const StructType&>(*out->type()).num_fields();
+
+    if (in_field_count != out_field_count) {
+      ARROW_RETURN_NOT_OK(
+          Status(StatusCode::TypeError, "struct field sizes do not match"));
+    }
+
+    for (int64_t i = 0; i < in_field_count; ++i) {
+      const auto in_field_name =
+          checked_cast<const StructType&>(*batch[0].type()).field(i)->name();
+      const auto out_field_name =
+          checked_cast<const StructType&>(*out->type()).field(i)->name();
+      if (in_field_name != out_field_name) {
+        ARROW_RETURN_NOT_OK(
+            Status(StatusCode::TypeError, "struct field names do not match"));
+      }
+    }
+
+    if (out->kind() == Datum::SCALAR) {
+      const auto& in_scalar = checked_cast<const StructScalar&>(*batch[0].scalar());
+      auto out_scalar = checked_cast<StructScalar*>(out->scalar().get());
+
+      for (int64_t i = 0; i < in_field_count; i++) {
+        auto values = in_scalar.value[i];
+        auto target_type = out->type()->field(i)->type();
+        ARROW_ASSIGN_OR_RAISE(Datum cast_values,
+                              Cast(values, target_type, options, ctx->exec_context()));
+        DCHECK_EQ(Datum::SCALAR, cast_values.kind());
+        out_scalar->value.push_back(cast_values.scalar());
+      }
+
+      out_scalar->is_valid = true;
+      return Status::OK();
+    }
+
+    const ArrayData& in_array = *batch[0].array();
+    ArrayData* out_array = out->mutable_array();
+
+    for (int64_t i = 0; i < in_field_count; ++i) {
+      auto values = in_array.child_data[0];

Review comment:
       I think I set up a minimal case for this in the tests but it did not fail - let me know if I am overlooking something




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