Simplify Tensor::unroll().

include/deal.II/base/tensor.h

@@ -424,13 +424,6 @@ class Tensor<0, dim, Number>
    */
   Number value;
 
-  /**
-   * Internal helper function for unroll.
-   */
-  template <typename Iterator>
-  Iterator
-  unroll_recursion(const Iterator current, const Iterator end) const;
-
   // Allow an arbitrary Tensor to access the underlying values.
   template <int, int, typename>
   friend class Tensor;
@@ -885,13 +878,6 @@ class Tensor
   // ... avoid a compiler warning in case of dim == 0 and ensure that the
   // array always has positive size.
 
-  /**
-   * Internal helper function for unroll.
-   */
-  template <typename Iterator>
-  Iterator
-  unroll_recursion(const Iterator current, const Iterator end) const;
-
   /**
    * This constructor is for internal use. It provides a way
    * to create constexpr constructors for Tensor<rank, dim, Number>
@@ -1255,24 +1241,6 @@ constexpr DEAL_II_HOST_DEVICE inline DEAL_II_ALWAYS_INLINE
 
 
 
-template <int dim, typename Number>
-template <typename Iterator>
-Iterator
-Tensor<0, dim, Number>::unroll_recursion(const Iterator current,
-                                         const Iterator end) const
-{
-  (void)end;
-  Assert(dim != 0,
-         ExcMessage("Cannot unroll an object of type Tensor<0,0,Number>"));
-  Assert(std::distance(current, end) >= 1,
-         ExcMessage("The provided iterator range must contain at least one "
-                    "element."));
-  *current = value;
-  return std::next(current);
-}
-
-
-
 template <int dim, typename Number>
 constexpr inline void
 Tensor<0, dim, Number>::clear()
@@ -1289,8 +1257,14 @@ template <class Iterator>
 inline void
 Tensor<0, dim, Number>::unroll(const Iterator begin, const Iterator end) const
 {
+  (void)end;
   AssertDimension(std::distance(begin, end), n_independent_components);
-  unroll_recursion(begin, end);
+  Assert(dim != 0,
+         ExcMessage("Cannot unroll an object of type Tensor<0,0,Number>"));
+  Assert(std::distance(begin, end) >= 1,
+         ExcMessage("The provided iterator range must contain at least one "
+                    "element."));
+  *begin = value;
 }
 
 
@@ -1775,25 +1749,32 @@ inline void
 Tensor<rank_, dim, Number>::unroll(const Iterator begin,
                                    const Iterator end) const
 {
-  AssertDimension(std::distance(begin, end), n_independent_components);
-  unroll_recursion(begin, end);
+  if constexpr (rank_ > 1)
+    {
+      // For higher-rank tensors, we recurse to the sub-tensors:
+      Iterator next = begin;
+      for (unsigned int i = 0; i < dim; ++i)
+        {
+          values[i].unroll(next, end);
+          std::advance(
+            next, Tensor<rank_ - 1, dim, Number>::n_independent_components);
+        }
+    }
+  else
+    {
+      // For rank-1 tensors, we can simply copy the current elements from
+      // our linear array into the output range, and then return the
+      // iterator moved forward by 'dim' elements:
+      Assert(std::distance(begin, end) >= dim,
+             ExcMessage(
+               "The provided iterator range must contain at least 'dim' "
+               "elements."));
+      std::copy(&values[0], &values[dim], begin);
+    }
 }
 
 
 
-template <int rank_, int dim, typename Number>
-template <typename Iterator>
-Iterator
-Tensor<rank_, dim, Number>::unroll_recursion(const Iterator current,
-                                             const Iterator end) const
-{
-  Iterator next = current;
-  for (unsigned int i = 0; i < dim; ++i)
-    next = values[i].unroll_recursion(next, end);
-  return next;
-}
-
-
 template <int rank_, int dim, typename Number>
 constexpr inline unsigned int
 Tensor<rank_, dim, Number>::component_to_unrolled_index(