Add mixin class for an indexed iterator

dune/common/CMakeLists.txt

@@ -62,6 +62,7 @@ install(FILES
         hash.hh
         hybridutilities.hh
         indent.hh
+        indexediterator.hh
         indices.hh
         integersequence.hh
         interfaces.hh

dune/common/indexediterator.hh

+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+// SPDX-FileCopyrightInfo: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
+#ifndef DUNE_COMMON_INDEXEDITERATOR_HH
+#define DUNE_COMMON_INDEXEDITERATOR_HH
+
+#include <iterator>
+#include <type_traits>
+
+namespace Dune
+{
+  /**
+   * \brief An iterator mixin that adds an `index()` method returning an
+   * enumeration index for the traversal.
+   *
+   * This is a mixin class that derives from its template parameter `Iter` and
+   * adds the method `index()` to retrieve an enumeration index during traversal.
+   * Only the increment and decrement operators are adapted to also increment
+   * (decrement) the accompanying index in the same way the iterator is incremented
+   * or decremented. Especially the dereference and comparison operators are not
+   * modified by this mixin class.
+   *
+   * \tparam Iter  An iterator type the `IndexedIterator` will inherit from.
+   */
+  template<class Iter>
+  class IndexedIterator
+      : public Iter
+  {
+    using Traits = std::iterator_traits<Iter>;
+    static_assert(std::is_base_of_v<std::forward_iterator_tag, typename Traits::iterator_category>);
+
+  public:
+    using size_type = typename Traits::difference_type;
+
+    //! Default constructor default-constructs the `Iter` base type and the index by 0.
+    IndexedIterator () = default;
+
+    //! Construct the `Iter` base type by `it` and the index by the given starting index.
+    IndexedIterator (Iter it, size_type index = 0)
+      : Iter(it)
+      , index_(index)
+    {}
+
+    //! Return the enumeration index.
+    size_type index () const
+    {
+      return index_;
+    }
+
+    //! Increment the iterator and the index.
+    IndexedIterator& operator++ ()
+    {
+      Iter::operator++();
+      ++index_;
+      return *this;
+    }
+
+    //! Increment the iterator and the index.
+    IndexedIterator operator++ (int)
+    {
+      IndexedIterator tmp(*this);
+      this->operator++();
+      return tmp;
+    }
+
+    //! Decrement the iterator and the index.
+    template <class I = Iter,
+      decltype(--std::declval<I&>(), bool{}) = true>
+    IndexedIterator& operator-- ()
+    {
+      Iter::operator--();
+      --index_;
+      return *this;
+    }
+
+    //! Decrement the iterator and the index.
+    template <class I = Iter,
+      decltype(std::declval<I&>()--, bool{}) = true>
+    IndexedIterator operator-- (int)
+    {
+      IndexedIterator tmp(*this);
+      this->operator--();
+      return tmp;
+    }
+
+    //! Increment the iterator and the index.
+    template <class I = Iter,
+      decltype(std::declval<I&>()+=1, bool{}) = true>
+    IndexedIterator& operator+= (typename Iter::difference_type n)
+    {
+      Iter::operator+=(n);
+      index_ += n;
+      return *this;
+    }
+
+    //! Decrement the iterator and the index.
+    template <class I = Iter,
+      decltype(std::declval<I&>()-=1, bool{}) = true>
+    IndexedIterator& operator-= (typename Iter::difference_type n)
+    {
+      Iter::operator-=(n);
+      index_ -= n;
+      return *this;
+    }
+
+  private:
+    size_type index_ = 0;
+  };
+
+} // end namespace Dune
+
+#endif // DUNE_COMMON_INDEXEDITERATOR_HH

dune/common/test/CMakeLists.txt

@@ -208,6 +208,9 @@ dune_add_test(SOURCES hybridutilitiestest.cc
               LINK_LIBRARIES dunecommon
               LABELS quick)
 
+dune_add_test(SOURCES indexediteratortest.cc
+              LABELS quick)
+
 dune_add_test(SOURCES indicestest.cc
               LABELS quick)
 

dune/common/test/indexediteratortest.cc

+// SPDX-FileCopyrightInfo: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
+#include <config.h>
+
+#include <numeric>
+#include <vector>
+
+#include <dune/common/indexediterator.hh>
+#include <dune/common/iteratorrange.hh>
+#include <dune/common/rangeutilities.hh>
+#include <dune/common/test/testsuite.hh>
+
+auto testSparseRange()
+{
+  using namespace Dune;
+  TestSuite suite("Check sparseRange()");
+  std::vector<int> vec(10);
+  std::iota(vec.begin(), vec.end(), 0);
+
+  auto indexedRange = IteratorRange{IndexedIterator{vec.begin()}, IndexedIterator{vec.end()}};
+  for (auto&& [vi,i] : Dune::sparseRange(indexedRange))
+    suite.check(vi == i);
+
+  return suite;
+}
+
+int main()
+{
+  Dune::TestSuite suite;
+  suite.subTest(testSparseRange());
+
+  return suite.exit();
+}