[REVIEW] Add cudf::dictionary::make_dictionary_pair_iterator

CHANGELOG.md

@@ -52,6 +52,7 @@
 - PR #6614 Add support for conversion to Pandas nullable dtypes and fix related issue in `cudf.to_json`
 - PR #6622 Update `to_pandas` api docs
 - PR #6623 Add operator overloading to column and clean up error messages
+- PR #6651 Add cudf::dictionary::make_dictionary_pair_iterator
 - PR #6635 Add cudf::test::dictionary_column_wrapper class
 
 ## Bug Fixes

cpp/include/cudf/dictionary/detail/iterator.cuh

@@ -15,6 +15,7 @@
  */
 
 #include <cudf/column/column_device_view.cuh>
+#include <cudf/dictionary/dictionary_column_view.hpp>
 
 namespace cudf {
 namespace dictionary {
@@ -61,6 +62,59 @@ auto make_dictionary_iterator(column_device_view const& dictionary_column)
                                          dictionary_access_fn<KeyType>{dictionary_column});
 }
 
+/**
+ * @brief Accessor functor for returning a dictionary pair iterator.
+ *
+ * @tparam KeyType The type of the dictionary's key element.
+ * @tparam has_nulls Set to true if `d_dictionary` has nulls.
+ *
+ * @throw cudf::logic_error if `has_nulls==true` and `d_dictionary` is not nullable.
+ */
+template <typename KeyType, bool has_nulls>
+struct dictionary_access_pair_fn {
+  dictionary_access_pair_fn(column_device_view const& d_dictionary) : d_dictionary{d_dictionary}
+  {
+    if (has_nulls) { CUDF_EXPECTS(d_dictionary.nullable(), "unexpected non-nullable column"); }
+  }
+
+  __device__ thrust::pair<KeyType, bool> operator()(size_type idx) const
+  {
+    if (has_nulls && d_dictionary.is_null(idx)) return {KeyType{}, false};
+    auto keys = d_dictionary.child(1);
+    return {keys.element<KeyType>(static_cast<size_type>(d_dictionary.element<dictionary32>(idx))),
+            true};
+  };
+
+ private:
+  column_device_view const d_dictionary;
+};
+
+/**
+ * @brief Create dictionary iterator that produces key and valid element pair.
+ *
+ * The iterator returns a pair where the `first` value is
+ * `dictionary_column.keys[dictionary_column.indices[i]]`
+ * The `second` pair member is a `bool` which is set to
+ * `dictionary_column.is_valid(i)`.
+ *
+ * @throw cudf::logic_error if `dictionary_column` is not a dictionary column.
+ *
+ * @tparam KeyType The type of the dictionary's key element.
+ * @tparam has_nulls Set to true if the dictionary_column has nulls.
+ *
+ * @param dictionary_column The dictionary device view to iterate.
+ * @return Pair iterator with `{value,valid}`
+ */
+template <typename KeyType, bool has_nulls>
+auto make_dictionary_pair_iterator(column_device_view const& dictionary_column)
+{
+  CUDF_EXPECTS(dictionary_column.type().id() == type_id::DICTIONARY32,
+               "Dictionary iterator is only for dictionary columns");
+  return thrust::make_transform_iterator(
+    thrust::make_counting_iterator<size_type>(0),
+    dictionary_access_pair_fn<KeyType, has_nulls>{dictionary_column});
+}
+
 }  // namespace detail
 }  // namespace dictionary
 }  // namespace cudf

cpp/include/cudf/utilities/traits.hpp

@@ -136,27 +136,6 @@ struct is_numeric_impl {
   }
 };
 
-/**
- * @brief Indicates whether the type `T` is a unsigned numeric type.
- *
- * @tparam T  The type to verify
- * @return true `T` is unsigned numeric
- * @return false  `T` is signed numeric
- **/
-template <typename T>
-constexpr inline bool is_unsigned()
-{
-  return std::is_unsigned<T>::value;
-}
-
-struct is_unsigned_impl {
-  template <typename T>
-  bool operator()()
-  {
-    return is_unsigned<T>();
-  }
-};
-
 /**
  * @brief Indicates whether `type` is a numeric `data_type`.
  *
@@ -214,6 +193,26 @@ constexpr inline bool is_index_type(data_type type)
   return cudf::type_dispatcher(type, is_index_type_impl{});
 }
 
+/**
+ * @brief Indicates whether the type `T` is a unsigned numeric type.
+ *
+ * @tparam T  The type to verify
+ * @return true `T` is unsigned numeric
+ * @return false  `T` is signed numeric
+ **/
+template <typename T>
+constexpr inline bool is_unsigned()
+{
+  return std::is_unsigned<T>::value;
+}
+
+struct is_unsigned_impl {
+  template <typename T>
+  bool operator()()
+  {
+    return is_unsigned<T>();
+  }
+};
 /**
  * @brief Indicates whether `type` is a unsigned numeric `data_type`.
  *
@@ -435,6 +434,38 @@ constexpr inline bool is_chrono(data_type type)
   return cudf::type_dispatcher(type, is_chrono_impl{});
 }
 
+/**
+ * @brief Indicates whether the type `T` is a dictionary type.
+ *
+ * @tparam T  The type to verify
+ * @return true `T` is a dictionary-type
+ * @return false  `T` is not dictionary-type
+ **/
+template <typename T>
+constexpr inline bool is_dictionary()
+{
+  return std::is_same<dictionary32, T>::value;
+}
+
+struct is_dictionary_impl {
+  template <typename T>
+  bool operator()()
+  {
+    return is_dictionary<T>();
+  }
+};
+
+/**
+ * @brief Indicates whether `type` is a dictionary `data_type`.
+ *
+ * @param type The `data_type` to verify
+ * @return true `type` is a dictionary type
+ * @return false `type` is not a dictionary type
+ **/
+constexpr inline bool is_dictionary(data_type type)
+{
+  return cudf::type_dispatcher(type, is_dictionary_impl{});
+}
 /**
  * @brief Indicates whether elements of type `T` are fixed-width.
  *