Added null_type and implemented null_layout

CMakeLists.txt

@@ -104,6 +104,7 @@ set(SPARROW_HEADERS
     ${SPARROW_INCLUDE_DIR}/sparrow/iterator.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/memory.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/mp_utils.hpp
+    ${SPARROW_INCLUDE_DIR}/sparrow/null_layout.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/sparrow_version.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/typed_array.hpp
     ${SPARROW_INCLUDE_DIR}/sparrow/variable_size_binary_layout.hpp

include/sparrow/array_data_factory.hpp

@@ -39,6 +39,24 @@
 
 namespace sparrow
 {
+    /*
+     * \brief Creates an array_data object for a null layout.
+     *
+     * This function creates an array_data object.
+     */
+    inline array_data make_array_data_for_null_layout(std::size_t size = 0u)
+    {
+        return {
+            .type = data_descriptor(arrow_type_id<null_type>()),
+            .length = static_cast<std::int64_t>(size),
+            .offset = 0,
+            .bitmap = {},
+            .buffers = {},
+            .child_data = {},
+            .dictionary = nullptr
+        };
+    }
+
     /**
      * \brief Creates an array_data object for a fixed-size layout.
      *

include/sparrow/data_traits.hpp

@@ -29,10 +29,10 @@ namespace sparrow
     };
 
     template <>
-    struct arrow_traits<std::nullopt_t>
+    struct arrow_traits<null_type>
     {
         static constexpr data_type type_id = data_type::NA;
-        using value_type = std::nullopt_t;
+        using value_type = null_type;
         using default_layout = fixed_size_layout<value_type>;  // TODO: replace this by a special layout
                                                                // that's always empty
     };

include/sparrow/data_type.hpp

@@ -25,7 +25,6 @@ namespace date = std::chrono;
 
 #include <climits>
 #include <cstdint>
-#include <optional>
 #include <string>
 #include <vector>
 
@@ -136,10 +135,13 @@ namespace sparrow
         TIMESTAMP = 18,
     };
 
+    struct null_type {};
+    inline bool operator==(const null_type&, const null_type&) { return true; }
+
     /// C++ types value representation types matching Arrow types.
     // NOTE: this needs to be in sync-order with `data_type`
     using all_base_types_t = mpl::typelist<
-        std::nullopt_t,
+        null_type,
         bool,
         std::uint8_t,
         std::int8_t,

include/sparrow/null_layout.hpp

@@ -0,0 +1,256 @@
+// Copyright 2024 Man Group Operations Limited
+//
+// 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 mplied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include <cstddef>
+#include <functional>
+#include <optional>
+#include <ranges>
+
+#include "sparrow/data_type.hpp"
+#include "sparrow/iterator.hpp"
+
+namespace sparrow
+{
+
+    /*
+     * @class empty_iterator
+     *
+     * @brief Iterator used by the null_layout class.
+     *
+     * @tparam T the value_type of the iterator
+     */
+    template <class T>
+    class empty_iterator : public iterator_base<
+                               empty_iterator<T>,
+                               T,
+                               std::contiguous_iterator_tag,
+                               T>
+    {
+    public:
+
+        using self_type = empty_iterator<T>;
+        using base_type = iterator_base<
+            self_type,
+            T,
+            std::contiguous_iterator_tag,
+            T>;
+        using reference = typename base_type::reference;
+        using difference_type = typename base_type::difference_type;
+
+        explicit empty_iterator(difference_type index = difference_type()) noexcept;
+
+    private:
+
+        reference dereference() const;
+        void increment();
+        void decrement();
+        void advance(difference_type n);
+        difference_type distance_to(const self_type& rhs) const;
+        bool equal(const self_type& rhs) const;
+        bool less_than(const self_type& rhs) const;
+
+        difference_type m_index;
+
+        friend class iterator_access;
+    };
+
+    /*
+     * @class null_layout
+     *
+     * @brief Memory-efficient layout for the Null data type.
+     *
+     * This layout is a memory-efficient layout for the Null data type where
+     * all values are null. In this case, no memory buffers are allocated.
+     */
+    class null_layout
+    {
+    public:
+
+        using inner_value_type = null_type;
+        using value_type = std::optional<inner_value_type>;
+        using iterator = empty_iterator<value_type>;
+        using const_iterator = empty_iterator<value_type>;
+        using reference = iterator::reference;
+        using const_reference = const_iterator::reference;
+        using size_type = std::size_t;
+        using difference_type = iterator::difference_type;
+        using iterator_tag = std::contiguous_iterator_tag;
+
+
+        using const_value_iterator = empty_iterator<int>;
+        using const_bitmap_iterator = empty_iterator<bool>;
+
+        using const_value_range = std::ranges::subrange<const_value_iterator>;
+        using const_bitmap_range = std::ranges::subrange<const_bitmap_iterator>;
+
+        explicit null_layout(array_data& data);
+        void rebind_data(array_data& data);
+
+        size_type size() const;
+
+        reference operator[](size_type i);
+        const_reference operator[](size_type i) const;
+
+        iterator begin();
+        iterator end();
+
+        const_iterator cbegin() const;
+        const_iterator cend() const;
+
+        const_value_range values() const;
+        const_bitmap_range bitmap() const;
+
+    private:
+
+        difference_type ssize() const;
+
+        array_data& data_ref();
+        const array_data& data_ref() const;
+
+        std::reference_wrapper<array_data> m_data;
+    };
+
+    /*********************************
+     * empty_iterator implementation *
+     *********************************/
+
+    template <class T>
+    empty_iterator<T>::empty_iterator(difference_type index) noexcept
+        : m_index(index)
+    {
+    }
+
+    template <class T>
+    auto empty_iterator<T>::dereference() const -> reference
+    {
+        return T();
+    }
+
+    template <class T>
+    void empty_iterator<T>::increment()
+    {
+        ++m_index;
+    }
+
+    template <class T>
+    void empty_iterator<T>::decrement()
+    {
+        --m_index;
+    }
+
+    template <class T>
+    void empty_iterator<T>::advance(difference_type n)
+    {
+        m_index += n;
+    }
+
+    template <class T>
+    auto empty_iterator<T>::distance_to(const self_type& rhs) const -> difference_type
+    {
+        return rhs.m_index - m_index;
+    }
+
+    template <class T>
+    bool empty_iterator<T>::equal(const self_type& rhs) const
+    {
+        return m_index == rhs.m_index;
+    }
+
+    template <class T>
+    bool empty_iterator<T>::less_than(const self_type& rhs) const
+    {
+        return m_index < rhs.m_index;
+    }
+
+    /******************************
+     * null_layout implementation *
+     ******************************/
+
+    inline null_layout::null_layout(array_data& data)
+        : m_data(data)
+    {
+        SPARROW_ASSERT_TRUE(data_ref().buffers.size() == 0u);
+    }
+
+    inline void null_layout::rebind_data(array_data& data)
+    {
+        SPARROW_ASSERT_TRUE(data_ref().buffers.size() == 0u);
+        m_data = data;
+    }
+
+    inline auto null_layout::size() const -> size_type
+    {
+        return static_cast<size_type>(data_ref().length);
+    }
+
+    inline auto null_layout::operator[](size_type i) -> reference
+    {
+        SPARROW_ASSERT_TRUE(i < size());
+        return *(begin());
+    }
+
+    inline auto null_layout::operator[](size_type i) const -> const_reference
+    {
+        SPARROW_ASSERT_TRUE(i < size());
+        return *(cbegin());
+    }
+
+    inline auto null_layout::begin() -> iterator
+    {
+        return iterator(0);
+    }
+
+    inline auto null_layout::end() -> iterator
+    {
+        return iterator(ssize());
+    }
+
+    inline auto null_layout::cbegin() const -> const_iterator
+    {
+        return const_iterator(0);
+    }
+
+    inline auto null_layout::cend() const -> const_iterator
+    {
+        return const_iterator(ssize());
+    }
+
+    inline auto null_layout::values() const -> const_value_range
+    {
+        return std::ranges::subrange(const_value_iterator(0), const_value_iterator(ssize()));
+    }
+
+    inline auto null_layout::bitmap() const -> const_bitmap_range
+    {
+        return std::ranges::subrange(const_bitmap_iterator(0), const_bitmap_iterator(ssize()));
+    }
+
+    inline auto null_layout::ssize() const -> difference_type
+    {
+        return static_cast<difference_type>(size());
+    }
+
+    inline array_data& null_layout::data_ref()
+    {
+        return m_data.get();
+    }
+
+    inline const array_data& null_layout::data_ref() const
+    {
+        return m_data.get();
+    }
+}
+

test/CMakeLists.txt

@@ -50,6 +50,7 @@ set(SPARROW_TESTS_SOURCES
     test_iterator.cpp
     test_memory.cpp
     test_mpl.cpp
+    test_null_layout.cpp
     test_traits.cpp
     test_typed_array.cpp
     test_typed_array_timestamp.cpp