Support nested types for nth_element reduction

cpp/include/cudf/scalar/scalar_factories.hpp

@@ -121,6 +121,20 @@ std::unique_ptr<scalar> make_default_constructed_scalar(
   rmm::cuda_stream_view stream        = rmm::cuda_stream_default,
   rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
 
+/**
+ * @brief Creates an empty (invalid) scalar of the same type as the `input` column_view.
+ *
+ * @throw cudf::logic_error if the `input` column is struct type and empty
+ *
+ * @param input Immutable view of input column to emulate
+ * @param stream CUDA stream used for device memory operations.
+ * @param mr Device memory resource used to allocate the scalar's `data` and `is_valid` bool.
+ */
+std::unique_ptr<scalar> make_empty_scalar_like(
+  column_view const& input,
+  rmm::cuda_stream_view stream        = rmm::cuda_stream_default,
+  rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
+
 /**
  * @brief Construct scalar using the given value of fixed width type
  *

cpp/src/reductions/reductions.cpp

@@ -25,6 +25,7 @@
 #include <cudf/reduction.hpp>
 #include <cudf/scalar/scalar_factories.hpp>
 
+#include <cudf/structs/structs_column_view.hpp>
 #include <rmm/cuda_stream_view.hpp>
 
 namespace cudf {
@@ -112,15 +113,17 @@ std::unique_ptr<scalar> reduce(
   rmm::cuda_stream_view stream        = rmm::cuda_stream_default,
   rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource())
 {
-  std::unique_ptr<scalar> result = make_default_constructed_scalar(output_dtype, stream, mr);
-  result->set_valid_async(false, stream);
-
-  // check if input column is empty
-  if (col.size() <= col.null_count()) return result;
+  // Returns default scalar if input column is non-valid. In terms of nested columns, we need to
+  // handcraft the default scalar with input column.
+  if (col.size() <= col.null_count()) {
+    if (col.type().id() == type_id::EMPTY || col.type() != output_dtype) {
+      return make_default_constructed_scalar(output_dtype, stream, mr);
+    }
+    return make_empty_scalar_like(col, stream, mr);
+  }
 
-  result =
-    aggregation_dispatcher(agg->kind, reduce_dispatch_functor{col, output_dtype, stream, mr}, agg);
-  return result;
+  return aggregation_dispatcher(
+    agg->kind, reduce_dispatch_functor{col, output_dtype, stream, mr}, agg);
 }
 }  // namespace detail
 

cpp/src/scalar/scalar_factories.cpp

@@ -20,6 +20,7 @@
 #include <cudf/utilities/traits.hpp>
 #include <cudf/utilities/type_dispatcher.hpp>
 
+#include <cudf/detail/copy.hpp>
 #include <rmm/cuda_stream_view.hpp>
 
 namespace cudf {
@@ -165,4 +166,24 @@ std::unique_ptr<scalar> make_default_constructed_scalar(data_type type,
   return type_dispatcher(type, default_scalar_functor{}, stream, mr);
 }
 
+std::unique_ptr<scalar> make_empty_scalar_like(column_view const& column,
+                                               rmm::cuda_stream_view stream,
+                                               rmm::mr::device_memory_resource* mr)
+{
+  std::unique_ptr<scalar> result;
+  switch (column.type().id()) {
+    case type_id::LIST:
+      result = make_list_scalar(empty_like(column)->view(), stream, mr);
+      result->set_valid_async(false, stream);
+      break;
+    case type_id::STRUCT:
+      // The input column must have at least 1 row to extract a scalar (row) from it.
+      result = detail::get_element(column, 0, stream, mr);
+      result->set_valid_async(false, stream);
+      break;
+    default: result = make_default_constructed_scalar(column.type(), stream, mr);
+  }
+  return result;
+}
+
 }  // namespace cudf

cpp/tests/groupby/nth_element_tests.cpp

@@ -405,5 +405,101 @@ TYPED_TEST(groupby_nth_element_lists_test, EmptyInput)
     keys, values, expected_keys, expected_values, cudf::make_nth_element_aggregation(2));
 }
 
+struct groupby_nth_element_structs_test : BaseFixture {
+};
+
+TEST_F(groupby_nth_element_structs_test, Basics)
+{
+  using structs = cudf::test::structs_column_wrapper;
+  using ints    = cudf::test::fixed_width_column_wrapper<int>;
+  using doubles = cudf::test::fixed_width_column_wrapper<double>;
+  using strings = cudf::test::strings_column_wrapper;
+
+  auto keys   = ints{0, 0, 0, 1, 1, 1, 2, 2, 2, 3};
+  auto child0 = ints{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+  auto child1 = doubles{0.1, 1.2, 2.3, 3.4, 4.51, 5.3e4, 6.3231, -0.07, 832.1, 9.999};
+  auto child2 = strings{"", "a", "b", "c", "d", "e", "f", "g", "HH", "JJJ"};
+  auto values = structs{{child0, child1, child2}, {1, 0, 1, 0, 1, 1, 1, 1, 0, 1}};
+
+  auto expected_keys   = ints{0, 1, 2, 3};
+  auto expected_ch0    = ints{1, 4, 7, 0};
+  auto expected_ch1    = doubles{1.2, 4.51, -0.07, 0.0};
+  auto expected_ch2    = strings{"a", "d", "g", ""};
+  auto expected_values = structs{{expected_ch0, expected_ch1, expected_ch2}, {0, 1, 1, 0}};
+  test_single_agg(
+    keys, values, expected_keys, expected_values, cudf::make_nth_element_aggregation(1));
+
+  expected_keys   = ints{0, 1, 2, 3};
+  expected_ch0    = ints{0, 4, 6, 9};
+  expected_ch1    = doubles{0.1, 4.51, 6.3231, 9.999};
+  expected_ch2    = strings{"", "d", "f", "JJJ"};
+  expected_values = structs{{expected_ch0, expected_ch1, expected_ch2}, {1, 1, 1, 1}};
+  test_single_agg(keys,
+                  values,
+                  expected_keys,
+                  expected_values,
+                  cudf::make_nth_element_aggregation(0, null_policy::EXCLUDE));
+}
+
+TEST_F(groupby_nth_element_structs_test, NestedStructs)
+{
+  using structs = cudf::test::structs_column_wrapper;
+  using ints    = cudf::test::fixed_width_column_wrapper<int>;
+  using doubles = cudf::test::fixed_width_column_wrapper<double>;
+  using lists   = cudf::test::lists_column_wrapper<int>;
+
+  auto keys             = ints{0, 0, 0, 1, 1, 1, 2, 2, 2, 3};
+  auto child0           = ints{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+  auto child0_of_child1 = ints{0, -1, -2, -3, -4, -5, -6, -7, -8, -9};
+  auto child1_of_child1 = doubles{0.1, 1.2, 2.3, 3.4, 4.51, 5.3e4, 6.3231, -0.07, 832.1, 9.999};
+  auto child1           = structs{child0_of_child1, child1_of_child1};
+  auto child2           = lists{{0}, {1, 2, 3}, {}, {4}, {5, 6}, {}, {}, {7}, {8, 9}, {}};
+  auto values           = structs{{child0, child1, child2}, {1, 0, 1, 0, 1, 1, 1, 1, 0, 1}};
+
+  auto expected_keys       = ints{0, 1, 2, 3};
+  auto expected_ch0        = ints{1, 4, 7, 0};
+  auto expected_ch0_of_ch1 = ints{-1, -4, -7, 0};
+  auto expected_ch1_of_ch1 = doubles{1.2, 4.51, -0.07, 0.0};
+  auto expected_ch1        = structs{expected_ch0_of_ch1, expected_ch1_of_ch1};
+  auto expected_ch2        = lists{{1, 2, 3}, {5, 6}, {7}, {}};
+  auto expected_values     = structs{{expected_ch0, expected_ch1, expected_ch2}, {0, 1, 1, 0}};
+  test_single_agg(
+    keys, values, expected_keys, expected_values, cudf::make_nth_element_aggregation(1));
+
+  expected_keys       = ints{0, 1, 2, 3};
+  expected_ch0        = ints{0, 4, 6, 9};
+  expected_ch0_of_ch1 = ints{0, -4, -6, -9};
+  expected_ch1_of_ch1 = doubles{0.1, 4.51, 6.3231, 9.999};
+  expected_ch1        = structs{expected_ch0_of_ch1, expected_ch1_of_ch1};
+  expected_ch2        = lists{{0}, {5, 6}, {}, {}};
+  expected_values     = structs{{expected_ch0, expected_ch1, expected_ch2}, {1, 1, 1, 1}};
+  test_single_agg(keys,
+                  values,
+                  expected_keys,
+                  expected_values,
+                  cudf::make_nth_element_aggregation(0, null_policy::EXCLUDE));
+}
+
+TEST_F(groupby_nth_element_structs_test, EmptyInput)
+{
+  using structs = cudf::test::structs_column_wrapper;
+  using ints    = cudf::test::fixed_width_column_wrapper<int>;
+  using doubles = cudf::test::fixed_width_column_wrapper<double>;
+  using strings = cudf::test::strings_column_wrapper;
+
+  auto keys   = ints{};
+  auto child0 = ints{};
+  auto child1 = doubles{};
+  auto child2 = strings{};
+  auto values = structs{{child0, child1, child2}};
+
+  auto expected_keys   = ints{};
+  auto expected_ch0    = ints{};
+  auto expected_ch1    = doubles{};
+  auto expected_ch2    = strings{};
+  auto expected_values = structs{{expected_ch0, expected_ch1, expected_ch2}};
+  test_single_agg(
+    keys, values, expected_keys, expected_values, cudf::make_nth_element_aggregation(0));
+}
 }  // namespace test
 }  // namespace cudf