From cbbbbba621150e001429bc0a35c9619e872be7c4 Mon Sep 17 00:00:00 2001
From: Peter Munch <peterrmuench@gmail.com>
Date: Tue, 1 Aug 2023 19:09:53 +0200
Subject: [PATCH] Introduce Triangulation::as_dof_handler_level_iterator()

---
 doc/news/changes/minor/20230801Munch_2 |  4 ++++
 include/deal.II/grid/tria_accessor.h   | 13 +++++++++++++
 source/grid/tria_accessor.cc           | 19 +++++++++++++++++++
 3 files changed, 36 insertions(+)
 create mode 100644 doc/news/changes/minor/20230801Munch_2

diff --git a/doc/news/changes/minor/20230801Munch_2 b/doc/news/changes/minor/20230801Munch_2
new file mode 100644
index 0000000000..8025fb633c
--- /dev/null
+++ b/doc/news/changes/minor/20230801Munch_2
@@ -0,0 +1,4 @@
+New: The new function Triangulation::as_dof_handler_level_iterator()
+allows to create level iterators based on other cell iterators.
+<br>
+(Peter Munch, 2023/08/01)
diff --git a/include/deal.II/grid/tria_accessor.h b/include/deal.II/grid/tria_accessor.h
index 65e1a02047..8d9265b90c 100644
--- a/include/deal.II/grid/tria_accessor.h
+++ b/include/deal.II/grid/tria_accessor.h
@@ -3184,6 +3184,19 @@ public:
   TriaActiveIterator<DoFCellAccessor<dim, spacedim, false>>
   as_dof_handler_iterator(const DoFHandler<dim, spacedim> &dof_handler) const;
 
+  /**
+   * A function similar to as_dof_handler_iterator(). It converts
+   * a Triangulation active/level cell iterator to a
+   * DoFHandler active level cell iterator, or a DoFHandler level active/cell
+   * iterator to a level cell iterator of another DoFHandler. The
+   * @p iterator must be associated with the triangulation of the
+   * @p dof_handler.
+   */
+  TriaIterator<DoFCellAccessor<dim, spacedim, true>>
+  as_dof_handler_level_iterator(
+    const DoFHandler<dim, spacedim> &dof_handler) const;
+
+
   /**
    * @}
    */
diff --git a/source/grid/tria_accessor.cc b/source/grid/tria_accessor.cc
index 98602affe2..3267aae53e 100644
--- a/source/grid/tria_accessor.cc
+++ b/source/grid/tria_accessor.cc
@@ -2071,6 +2071,25 @@ CellAccessor<dim, spacedim>::as_dof_handler_iterator(
 }
 
 
+
+template <int dim, int spacedim>
+TriaIterator<DoFCellAccessor<dim, spacedim, true>>
+CellAccessor<dim, spacedim>::as_dof_handler_level_iterator(
+  const DoFHandler<dim, spacedim> &dof_handler) const
+{
+  Assert(&this->get_triangulation() == &dof_handler.get_triangulation(),
+         ExcMessage("The triangulation associated with the iterator does not "
+                    "match that of the DoFHandler."));
+
+  return typename DoFHandler<dim, spacedim>::level_cell_iterator(
+    &dof_handler.get_triangulation(),
+    this->level(),
+    this->index(),
+    &dof_handler);
+}
+
+
+
 // For codim>0 we proceed as follows:
 // 1) project point onto manifold and
 // 2) transform to the unit cell with a Q1 mapping
-- 
2.39.5