Handle empty results with nested types in copy_if_else (#8359)

Fixes:  #8322

The code was calling `make_empty_column()` to produce empty results, which does not work for nested types. Fix was to use `empty_like()` instead.

As part of this, I implemented a new public function:

`std::unique_ptr<column> empty_like(scalar const& input);`

Authors:
  - https://github.com/nvdbaranec

Approvers:
  - Jake Hemstad (https://github.com/jrhemstad)
  - Nghia Truong (https://github.com/ttnghia)
  - Paul Taylor (https://github.com/trxcllnt)
  - MithunR (https://github.com/mythrocks)

URL: #8359

cpp/include/cudf/copying.hpp

@@ -182,6 +182,14 @@ enum class mask_allocation_policy {
  */
 std::unique_ptr<column> empty_like(column_view const& input);
 
+/**
+ * @brief Initializes and returns an empty column of the same type as the `input`.
+ *
+ * @param[in] input Scalar to emulate
+ * @return std::unique_ptr<column> An empty column of same type as `input`
+ */
+std::unique_ptr<column> empty_like(scalar const& input);
+
 /**
  * @brief Creates an uninitialized new column of the same size and type as the `input`.
  * Supports only fixed-width types.

cpp/src/copying/copy.cpp

@@ -21,6 +21,7 @@
 #include <cudf/detail/null_mask.hpp>
 #include <cudf/detail/nvtx/ranges.hpp>
 #include <cudf/lists/lists_column_view.hpp>
+#include <cudf/strings/detail/utilities.hpp>
 #include <cudf/table/table.hpp>
 #include <cudf/utilities/traits.hpp>
 
@@ -44,6 +45,79 @@ inline mask_state should_allocate_mask(mask_allocation_policy mask_alloc, bool m
   }
 }
 
+/**
+ * @brief Functor to produce an empty column of the same type as the
+ * input scalar.
+ *
+ * In the case of nested types, full column hierarchy is preserved.
+ */
+template <typename T>
+struct scalar_empty_like_functor_impl {
+  std::unique_ptr<column> operator()(scalar const& input)
+  {
+    return cudf::make_empty_column(input.type());
+  }
+};
+
+template <>
+struct scalar_empty_like_functor_impl<cudf::string_view> {
+  std::unique_ptr<column> operator()(scalar const& input)
+  {
+    return cudf::strings::detail::make_empty_strings_column(rmm::cuda_stream_default,
+                                                            rmm::mr::get_current_device_resource());
+  }
+};
+
+template <>
+struct scalar_empty_like_functor_impl<cudf::list_view> {
+  std::unique_ptr<column> operator()(scalar const& input)
+  {
+    auto ls = static_cast<list_scalar const*>(&input);
+
+    // TODO:  add a manual constructor for lists_column_view.
+    column_view offsets{cudf::data_type{cudf::type_id::INT32}, 0, nullptr};
+    std::vector<column_view> children;
+    children.push_back(offsets);
+    children.push_back(ls->view());
+    column_view lcv{cudf::data_type{cudf::type_id::LIST}, 0, nullptr, nullptr, 0, 0, children};
+
+    return empty_like(lcv);
+  }
+};
+
+template <>
+struct scalar_empty_like_functor_impl<cudf::struct_view> {
+  std::unique_ptr<column> operator()(scalar const& input)
+  {
+    auto ss = static_cast<struct_scalar const*>(&input);
+
+    // TODO: add a manual constructor for structs_column_view
+    // TODO: add cudf::get_element() support for structs
+    cudf::table_view tbl = ss->view();
+    std::vector<column_view> children(tbl.begin(), tbl.end());
+    column_view scv{cudf::data_type{cudf::type_id::STRUCT}, 0, nullptr, nullptr, 0, 0, children};
+
+    return empty_like(scv);
+  }
+};
+
+template <>
+struct scalar_empty_like_functor_impl<cudf::dictionary32> {
+  std::unique_ptr<column> operator()(scalar const& input)
+  {
+    CUDF_FAIL("Dictionary scalars not supported");
+  }
+};
+
+struct scalar_empty_like_functor {
+  template <typename T>
+  std::unique_ptr<column> operator()(scalar const& input)
+  {
+    scalar_empty_like_functor_impl<T> func;
+    return func(input);
+  }
+};
+
 }  // namespace
 
 /*
@@ -91,6 +165,15 @@ std::unique_ptr<column> empty_like(column_view const& input)
     input.type(), 0, rmm::device_buffer{}, rmm::device_buffer{}, 0, std::move(children));
 }
 
+/*
+ * Initializes and returns an empty column of the same type as the `input`.
+ */
+std::unique_ptr<column> empty_like(scalar const& input)
+{
+  CUDF_FUNC_RANGE();
+  return type_dispatcher(input.type(), detail::scalar_empty_like_functor{}, input);
+};
+
 /*
  * Creates a table of empty columns with the same types as the `input_table`
  */

cpp/src/copying/copy.cu

@@ -289,7 +289,7 @@ std::unique_ptr<column> copy_if_else(Left const& lhs,
   CUDF_EXPECTS(boolean_mask.type() == data_type(type_id::BOOL8),
                "Boolean mask column must be of type type_id::BOOL8");
 
-  if (boolean_mask.is_empty()) { return cudf::make_empty_column(lhs.type()); }
+  if (boolean_mask.is_empty()) { return cudf::empty_like(lhs); }
 
   auto bool_mask_device_p             = column_device_view::create(boolean_mask);
   column_device_view bool_mask_device = *bool_mask_device_p;

cpp/tests/copying/copy_tests.cu

@@ -285,6 +285,85 @@ TYPED_TEST(CopyTest, CopyIfElseBadInputLength)
   }
 }
 
+struct CopyEmptyNested : public cudf::test::BaseFixture {
+};
+
+TEST_F(CopyEmptyNested, CopyIfElseTestEmptyNestedColumns)
+{
+  // lists
+  {
+    cudf::test::lists_column_wrapper<cudf::string_view> col{{{"abc", "def"}, {"xyz"}}};
+    auto lhs = cudf::empty_like(col);
+    auto rhs = cudf::empty_like(col);
+    cudf::test::fixed_width_column_wrapper<bool> mask{};
+
+    auto expected = empty_like(col);
+
+    auto out = cudf::copy_if_else(*lhs, *rhs, mask);
+    CUDF_TEST_EXPECT_COLUMNS_EQUAL(out->view(), *expected);
+  }
+
+  // structs
+  {
+    cudf::test::lists_column_wrapper<cudf::string_view> _col0{{{"abc", "def"}, {"xyz"}}};
+    auto col0 = cudf::empty_like(_col0);
+    cudf::test::fixed_width_column_wrapper<int> col1;
+
+    std::vector<std::unique_ptr<cudf::column>> cols;
+    cols.push_back(std::move(col0));
+    cols.push_back(col1.release());
+    cudf::test::structs_column_wrapper struct_col(std::move(cols));
+    auto lhs = cudf::empty_like(struct_col);
+    auto rhs = cudf::empty_like(struct_col);
+
+    cudf::test::fixed_width_column_wrapper<bool> mask{};
+
+    auto expected = cudf::empty_like(struct_col);
+
+    auto out = cudf::copy_if_else(*lhs, *rhs, mask);
+    CUDF_TEST_EXPECT_COLUMNS_EQUAL(out->view(), *expected);
+  }
+}
+
+TEST_F(CopyEmptyNested, CopyIfElseTestEmptyNestedScalars)
+{
+  // lists
+  {
+    cudf::test::lists_column_wrapper<cudf::string_view> _col{{{"abc", "def"}, {"xyz"}}};
+    std::unique_ptr<cudf::scalar> lhs = cudf::get_element(_col, 0);
+    std::unique_ptr<cudf::scalar> rhs = cudf::get_element(_col, 0);
+
+    cudf::test::fixed_width_column_wrapper<bool> mask{};
+
+    auto expected = empty_like(_col);
+
+    auto out = cudf::copy_if_else(*lhs, *rhs, mask);
+    CUDF_TEST_EXPECT_COLUMNS_EQUAL(out->view(), *expected);
+  }
+
+  // structs
+  {
+    cudf::test::lists_column_wrapper<cudf::string_view> col0{{{"abc", "def"}, {"xyz"}}};
+    cudf::test::fixed_width_column_wrapper<int> col1{1};
+
+    cudf::table_view tbl({col0, col1});
+    cudf::struct_scalar lhs(tbl);
+    cudf::struct_scalar rhs(tbl);
+
+    std::vector<std::unique_ptr<cudf::column>> cols;
+    cols.push_back(col0.release());
+    cols.push_back(col1.release());
+    cudf::test::structs_column_wrapper struct_col(std::move(cols));
+
+    cudf::test::fixed_width_column_wrapper<bool> mask{};
+
+    auto expected = cudf::empty_like(struct_col);
+
+    auto out = cudf::copy_if_else(lhs, rhs, mask);
+    CUDF_TEST_EXPECT_COLUMNS_EQUAL(out->view(), *expected);
+  }
+}
+
 template <typename T>
 struct CopyTestNumeric : public cudf::test::BaseFixture {
 };

cpp/tests/copying/utility_tests.cpp

@@ -72,6 +72,84 @@ TEST_F(EmptyLikeStringTest, ColumnStringTest)
   check_empty_string_columns(got->view(), strings);
 }
 
+template <typename T>
+struct EmptyLikeScalarTest : public cudf::test::BaseFixture {
+};
+
+TYPED_TEST_CASE(EmptyLikeScalarTest, cudf::test::FixedWidthTypes);
+
+TYPED_TEST(EmptyLikeScalarTest, FixedWidth)
+{
+  // make a column
+  auto input = make_fixed_width_column(
+    cudf::data_type{cudf::type_to_id<TypeParam>()}, 1, rmm::device_buffer{});
+  // get a scalar out of it
+  std::unique_ptr<cudf::scalar> sc = cudf::get_element(*input, 0);
+
+  // empty_like(column) -> column
+  auto expected = cudf::empty_like(*input);
+  // empty_like(scalar) -> column
+  auto result = cudf::empty_like(*sc);
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(*expected, *result);
+}
+
+struct EmptyLikeScalarStringTest : public EmptyLikeScalarTest<std::string> {
+};
+
+TEST_F(EmptyLikeScalarStringTest, String)
+{
+  // make a column
+  cudf::test::strings_column_wrapper input{"abc"};
+
+  // get a scalar out of it
+  std::unique_ptr<cudf::scalar> sc = cudf::get_element(input, 0);
+
+  // empty_like(column) -> column
+  auto expected = cudf::empty_like(input);
+  // empty_like(scalar) -> column
+  auto result = cudf::empty_like(*sc);
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(*expected, *result);
+}
+
+struct EmptyLikeScalarListTest : public EmptyLikeScalarTest<cudf::list_view> {
+};
+
+TEST_F(EmptyLikeScalarListTest, List)
+{
+  // make a column
+  cudf::test::lists_column_wrapper<cudf::string_view> input{{{"abc", "def"}, {"h", "ijk"}},
+                                                            {{"123", "456"}, {"78"}}};
+  // get a scalar out of it
+  std::unique_ptr<cudf::scalar> sc = cudf::get_element(input, 0);
+
+  // empty_like(column) -> column
+  auto expected = cudf::empty_like(input);
+  // empty_like(scalar) -> column
+  auto result = cudf::empty_like(*sc);
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(*expected, *result);
+}
+
+struct EmptyLikeScalarStructTest : public EmptyLikeScalarTest<cudf::struct_view> {
+};
+
+TEST_F(EmptyLikeScalarStructTest, Struct)
+{
+  cudf::test::lists_column_wrapper<cudf::string_view> col0{{{"abc", "def"}, {"h", "ijk"}}};
+  cudf::test::strings_column_wrapper col1{"abc"};
+  cudf::test::fixed_width_column_wrapper<float> col2{1.0f};
+  // scalar. TODO:  make cudf::get_element() work for struct scalars
+  cudf::table_view tbl({col0, col1, col2});
+  cudf::struct_scalar sc(tbl);
+  // column
+  cudf::test::structs_column_wrapper input({col0, col1, col2});
+
+  // empty_like(column) -> column
+  auto expected = cudf::empty_like(input);
+  // empty_like(scalar) -> column
+  auto result = cudf::empty_like(sc);
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(*expected, *result);
+}
+
 std::unique_ptr<cudf::table> create_table(cudf::size_type size, cudf::mask_state state)
 {
   auto num_column_1 = make_numeric_column(cudf::data_type{cudf::type_id::INT64}, size, state);