Set null-count in linked_column_view conversion operator (#13121)

Removes the `UNKNOWN_NULL_COUNT` usage in the `linked_column_view::column_view()` conversion operator. The null-count is copied from the parent instance. The `linked_column_view` class was reworked to move the C++ function definitions from the header file to a new .cpp file.

Contributes to: #11968

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Nghia Truong (https://github.com/ttnghia)

URL: #13121

cpp/CMakeLists.txt

@@ -605,6 +605,7 @@ add_library(
   src/unary/nan_ops.cu
   src/unary/null_ops.cu
   src/utilities/default_stream.cpp
+  src/utilities/linked_column.cpp
   src/utilities/logger.cpp
   src/utilities/traits.cpp
   src/utilities/type_checks.cpp

cpp/include/cudf/detail/utilities/linked_column.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, NVIDIA CORPORATION.
+ * Copyright (c) 2022-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,9 +16,11 @@
 
 #pragma once
 
+#include <cudf/column/column_view.hpp>
 #include <cudf/table/table_view.hpp>
 
-#include <thrust/iterator/transform_iterator.h>
+#include <memory>
+#include <vector>
 
 namespace cudf::detail {
 
@@ -28,40 +30,34 @@ using LinkedColPtr    = std::shared_ptr<linked_column_view>;
 using LinkedColVector = std::vector<LinkedColPtr>;
 
 /**
- * @brief column_view with the added member pointer to the parent of this column.
- *
+ * @brief A column_view class with pointer to parent's column_view
  */
 struct linked_column_view : public column_view_base {
   linked_column_view(linked_column_view const&) = delete;
   linked_column_view& operator=(linked_column_view const&) = delete;
 
-  linked_column_view(column_view const& col) : linked_column_view(nullptr, col) {}
+  /**
+   * @brief Construct from column_view
+   *
+   * @param col column_view to wrap
+   */
+  linked_column_view(column_view const& col);
 
-  linked_column_view(linked_column_view* parent, column_view const& col)
-    : column_view_base(col), parent(parent)
-  {
-    children.reserve(col.num_children());
-    std::transform(
-      col.child_begin(), col.child_end(), std::back_inserter(children), [&](column_view const& c) {
-        return std::make_shared<linked_column_view>(this, c);
-      });
-  }
+  /**
+   * @brief Construct from column_view with it's parent
+   *
+   * @param parent Pointer to the column_view's parent column_view
+   * @param col column_view to wrap
+   */
+  linked_column_view(linked_column_view* parent, column_view const& col);
 
-  operator column_view() const
-  {
-    auto child_it = thrust::make_transform_iterator(
-      children.begin(), [](auto const& c) { return static_cast<column_view>(*c); });
-    return column_view(this->type(),
-                       this->size(),
-                       this->head(),
-                       this->null_mask(),
-                       UNKNOWN_NULL_COUNT,
-                       this->offset(),
-                       std::vector<column_view>(child_it, child_it + children.size()));
-  }
+  /**
+   * @brief Conversion operator to cast this instance to it's column_view
+   */
+  operator column_view() const;
 
-  linked_column_view* parent;  //!< Pointer to parent of this column. Nullptr if root
-  LinkedColVector children;
+  linked_column_view* parent;  ///< Pointer to parent of this column; nullptr if root
+  LinkedColVector children;    ///< Vector of children of this instance
 };
 
 /**
@@ -70,11 +66,6 @@ struct linked_column_view : public column_view_base {
  * @param table table of columns to convert
  * @return Vector of converted linked_column_views
  */
-inline LinkedColVector table_to_linked_columns(table_view const& table)
-{
-  auto linked_it = thrust::make_transform_iterator(
-    table.begin(), [](auto const& c) { return std::make_shared<linked_column_view>(c); });
-  return LinkedColVector(linked_it, linked_it + table.num_columns());
-}
+LinkedColVector table_to_linked_columns(table_view const& table);
 
 }  // namespace cudf::detail

cpp/src/utilities/linked_column.cpp

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2022-2023, NVIDIA CORPORATION.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <cudf/detail/utilities/linked_column.hpp>
+
+#include <thrust/iterator/transform_iterator.h>
+
+namespace cudf::detail {
+
+linked_column_view::linked_column_view(column_view const& col) : linked_column_view(nullptr, col) {}
+
+linked_column_view::linked_column_view(linked_column_view* parent, column_view const& col)
+  : column_view_base(col), parent(parent)
+{
+  children.reserve(col.num_children());
+  std::transform(
+    col.child_begin(), col.child_end(), std::back_inserter(children), [&](column_view const& c) {
+      return std::make_shared<linked_column_view>(this, c);
+    });
+}
+
+linked_column_view::operator column_view() const
+{
+  auto child_it = thrust::make_transform_iterator(
+    children.begin(), [](auto const& c) { return static_cast<column_view>(*c); });
+  return column_view(this->type(),
+                     this->size(),
+                     this->head(),
+                     this->null_mask(),
+                     this->null_count(),
+                     this->offset(),
+                     std::vector<column_view>(child_it, child_it + children.size()));
+}
+
+LinkedColVector table_to_linked_columns(table_view const& table)
+{
+  auto linked_it = thrust::make_transform_iterator(
+    table.begin(), [](auto const& c) { return std::make_shared<linked_column_view>(c); });
+  return LinkedColVector(linked_it, linked_it + table.num_columns());
+}
+
+}  // namespace cudf::detail