Implement StridedArrayView

include/deal.II/base/array_view.h

@@ -633,6 +633,143 @@ ArrayView<ElementType, MemorySpaceType>::operator[](const std::size_t i) const
 
 
 
+/**
+ * A variation of @p ArrayView which allows strided access into the view.
+ * This is particularly useful when you want to access only one lane of a
+ * VectorizedArray.
+ */
+template <typename ElementType, std::size_t stride = 1>
+class StridedArrayView
+{
+public:
+  /**
+   * An alias that denotes the "value_type" of this container-like class,
+   * i.e., the type of the element it "stores" or points to.
+   */
+  using value_type = ElementType;
+
+  /**
+   * Constructor.
+   *
+   * @param[in] starting_element A pointer to the first element of the array
+   * this object should represent.
+   * @param[in] n_elements The length (in elements) of the chunk of memory
+   * this object should represent.
+   *
+   * @note The object that is constructed from these arguments has no
+   * knowledge how large the object into which it points really is. As a
+   * consequence, whenever you call ArrayView::operator[], the array view can
+   * check that the given index is within the range of the view, but it can't
+   * check that the view is indeed a subset of the valid range of elements of
+   * the underlying object that allocated that range. In other words, you need
+   * to ensure that the range of the view specified by the two arguments to
+   * this constructor is in fact a subset of the elements of the array into
+   * which it points. The appropriate way to do this is to use the
+   * make_array_view() functions.
+   */
+  StridedArrayView(value_type *starting_element, const std::size_t n_elements);
+
+  /**
+   * Return the size (in elements) of the view of memory this object
+   * represents.
+   */
+  std::size_t
+  size() const;
+
+  /**
+   * Return a bool whether the array view is empty.
+   */
+  bool
+  empty() const;
+
+  /**
+   * Return a pointer to the underlying array serving as element storage.
+   * In case the container is empty a nullptr is returned.
+   */
+  value_type *
+  data() const noexcept;
+
+  /**
+   * Return a reference to the $i$th element of the range represented by the
+   * current object.
+   *
+   * This function is marked as @p const because it does not change the
+   * <em>view object</em>. It may however return a reference to a non-@p const
+   * memory location depending on whether the template type of the class is @p
+   * const or not.
+   *
+   * This function is only allowed to be called if the underlying data is indeed
+   * stored in CPU memory.
+   */
+  value_type &
+  operator[](const std::size_t i) const;
+
+protected:
+  /**
+   * A pointer to the first element of the range of locations in memory that
+   * this object represents.
+   */
+  value_type *starting_element;
+
+  /**
+   * The length of the array this object represents.
+   */
+  std::size_t n_elements;
+};
+
+
+
+template <typename ElementType, std::size_t stride>
+typename StridedArrayView<ElementType, stride>::value_type &
+StridedArrayView<ElementType, stride>::operator[](const std::size_t i) const
+{
+  AssertIndexRange(i, this->n_elements);
+
+  return *(this->starting_element + stride * i);
+}
+
+
+
+template <typename ElementType, std::size_t stride>
+typename StridedArrayView<ElementType, stride>::value_type *
+StridedArrayView<ElementType, stride>::data() const noexcept
+{
+  if (this->n_elements == 0)
+    return nullptr;
+  else
+    return this->starting_element;
+}
+
+
+
+template <typename ElementType, std::size_t stride>
+bool
+StridedArrayView<ElementType, stride>::empty() const
+{
+  return this->n_elements == 0;
+}
+
+
+
+template <typename ElementType, std::size_t stride>
+std::size_t
+StridedArrayView<ElementType, stride>::size() const
+{
+  return this->n_elements;
+}
+
+
+
+template <typename ElementType, std::size_t stride>
+StridedArrayView<ElementType, stride>::StridedArrayView(
+  value_type       *starting_element,
+  const std::size_t n_elements)
+  : starting_element(starting_element)
+  , n_elements(n_elements)
+{}
+
+
+
 #ifndef DOXYGEN
 namespace internal
 {