Feature: Add adapt function for mesh handle

mesh_handle/adapt.hxx

@@ -0,0 +1,211 @@
+/*
+  This file is part of t8code.
+  t8code is a C library to manage a collection (a forest) of multiple
+  connected adaptive space-trees of general element classes in parallel.
+
+  Copyright (C) 2026 the developers
+
+  t8code is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+  t8code is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with t8code; if not, write to the Free Software Foundation, Inc.,
+  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+*/
+
+/** \file adapt.hxx
+ * This file provides helper functionality to adapt a \ref t8_mesh_handle::mesh
+ * according to a user defined callback.
+ */
+
+#pragma once
+
+#include <t8.h>
+#include <t8_forest/t8_forest_general.h>
+#include "mesh.hxx"
+#include <vector>
+#include <memory>
+
+namespace t8_mesh_handle
+{
+
+/** Namespace detail to hide implementation details from the user. */
+namespace detail
+{
+
+/** Virtual base class for mesh adaptation contexts.
+ * We need this base class and not only \ref MeshAdaptContext for the \ref AdaptRegistry. 
+ * AdaptRegistry should not be templated because we need to access registered contexts in \ref mesh_adapt_callback_wrapper,
+ * where we do not know the type of the mesh. Therefore, we work with a map of forests to instances of this base class to remain template free.
+ */
+struct MeshAdaptContextBase
+{
+  /** Virtual destructor for safe polymorphic deletion.
+   */
+  virtual ~MeshAdaptContextBase () = default;
+
+  /** Pure virtual callback for mesh adaptation.
+   * \param[in] lelement_handle_id Local element ID in the mesh handle.
+   * \param [in] is_family         If 1, the entries in \a elements form a family. If 0, they do not.
+   * \param [in] num_elements      The number of entries in \a elements.
+   * \param [in] elements          Pointers to members of a family or, if \a is_family is zero, pointer to one element.
+   *  \return 1 if the first entry in \a elements should be refined,
+   *         -1 if the family \a elements shall be coarsened,
+   *          0 else.
+   */
+  virtual int
+  adapt_mesh (const t8_locidx_t lelement_handle_id, const int is_family, const int num_elements,
+              t8_element_t* elements[])
+    = 0;
+};
+
+/** Templated mesh adaptation context holding the mesh handle and the user defined callback.
+ * Class inherits from \ref MeshAdaptContextBase and implements the virtual adapt callback using the mesh and the callback.
+ * \tparam TMesh The mesh handle class.
+ */
+template <typename TMesh>
+struct MeshAdaptContext final: MeshAdaptContextBase
+{
+  /** Constructor of the context with the mesh handle and the user defined callback.
+   * \param [in] mesh_handle      The mesh handle to adapt.
+   * \param [in] adapt_callback   The adapt callback.
+   */
+  MeshAdaptContext (TMesh& mesh_handle, typename TMesh::adapt_callback_type adapt_callback)
+    : m_mesh_handle (mesh_handle), m_adapt_callback (std::move (adapt_callback))
+  {
+  }
+
+  /** Callback for mesh adaptation using the user defined adapt callback.
+   * \param [in] lelement_handle_id Local flat element ID in the mesh handle.
+   * \param [in] is_family          If 1, the entries in \a elements form a family. If 0, they do not.
+   * \param [in] num_elements       The number of entries in \a elements.
+   * \param [in] elements           Pointers to members of a family or, if \a is_family is zero, pointer to one element.
+   * \return 1 if the first entry in \a elements should be refined,
+   *        -1 if the family \a elements shall be coarsened,
+   *         0 else.
+   */
+  int
+  adapt_mesh (const t8_locidx_t lelement_handle_id, const int is_family, const int num_elements,
+              t8_element_t* elements[]) override
+  {
+    // Check if adapt callback is set and call it using the correct mesh handle function arguments.
+    T8_ASSERTF (m_adapt_callback, "No adapt callback set.");
+    std::vector<typename TMesh::element_class> element_vec;
+    if (is_family) {
+      for (int i = 0; i < num_elements; i++) {
+        element_vec.push_back (m_mesh_handle[lelement_handle_id + i]);
+      }
+    }
+    else {
+      element_vec.push_back (m_mesh_handle[lelement_handle_id]);
+    }
+    return m_adapt_callback (m_mesh_handle, element_vec);
+  }
+
+ private:
+  TMesh& m_mesh_handle;                                 /**< The mesh handle to adapt. */
+  typename TMesh::adapt_callback_type m_adapt_callback; /**< The adapt callback. */
+};
+
+/** Registry pattern is used to register contexts, which provides access to the adapt callback and the mesh handle.
+ * This globally accessible static class is required to get the handle and the callback in the forest callback, 
+ * as the predefined header permits to give these as function arguments. 
+ */
+class AdaptRegistry {
+ public:
+  /** Static function to register \a context using \a forest as identifier. 
+   * This makes the context publicly available using the Registry.
+   * \param [in] forest  The forest identifier. In our case, this is the forest to be adapted 
+   *                     and not the forest from which we adapt.
+   * \param [in] context The context to register. Use unique pointer to ensure proper memory management and ownership.
+   */
+  static void
+  register_context (t8_forest_t forest, std::unique_ptr<MeshAdaptContextBase> context)
+  {
+    auto& map = get_map ();
+    auto [it, inserted] = map.emplace (forest, std::move (context));
+    if (!inserted) {
+      throw std::logic_error ("Context already registered");
+    }
+  }
+
+  /** Static function to unregister a context using \a forest as identifier. 
+   * \param [in] forest The forest identifier. In our case, this is the forest to be adapted.
+   */
+  static void
+  unregister_context (t8_forest_t forest)
+  {
+    auto& map = get_map ();
+    [[maybe_unused]] const auto erased = map.erase (forest);
+    T8_ASSERT (erased == 1);
+  }
+
+  /** Getter for a context using \a forest as identifier. 
+   * \param [in] forest The forest identifier. In our case, this is the forest to be adapted.
+   * \return Pointer to the context registered with the id \a forest if found, nullptr otherwise.
+   */
+  static MeshAdaptContextBase*
+  get (t8_forest_t forest)
+  {
+    auto& map = get_map ();
+    auto it = map.find (forest);
+    return it != map.end () ? it->second.get () : nullptr;
+  }
+
+ private:
+  /** Get the static map associating t8_forest_t with MeshAdaptContextBase references.
+   * We use a getter instead of private member variable to ensure single initialization.
+   * \return Reference to the static unordered map of t8_forest_t to MeshAdaptContextBase references.
+   */
+  static std::unordered_map<t8_forest_t, std::unique_ptr<MeshAdaptContextBase>>&
+  get_map ()
+  {
+    static std::unordered_map<t8_forest_t, std::unique_ptr<MeshAdaptContextBase>> map;
+    return map;
+  }
+};
+
+/** Wrapper around the mesh handle adapt functionality to be able to pass the callback to the classic adapt routine of a forest. 
+ * The function header fits the definition of \ref t8_forest_adapt_t.
+ * \param [in] forest       Unused; forest to which the new elements belong.
+ * \param [in] forest_from  Forest that is adapted.
+ * \param [in] which_tree   Local tree containing \a elements.
+ * \param [in] tree_class   Unused; eclass of \a which_tree.
+ * \param [in] lelement_id  The local element id in the tree of the first element in elements.
+ * \param [in] scheme       Unused; scheme of the forest.
+ * \param [in] is_family    If 1, the entries in \a elements form a family. If 0, they do not.
+ * \param [in] num_elements The number of entries in \a elements.
+ * \param [in] elements     Pointers to a family or, if \a is_family is zero, pointer to one element.
+ * \return 1 if the first entry in \a elements should be refined,
+ *        -1 if the family \a elements shall be coarsened,
+ *         0 else.
+ */
+int
+mesh_adapt_callback_wrapper ([[maybe_unused]] t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree,
+                             [[maybe_unused]] t8_eclass_t tree_class, t8_locidx_t lelement_id,
+                             [[maybe_unused]] const t8_scheme* scheme, const int is_family, const int num_elements,
+                             t8_element_t* elements[])
+{
+  // Get static adapt context from the registry.
+  // Via this, we can access the mesh handle and the user defined adapt callback that uses mesh handle functionality.
+  auto* context = AdaptRegistry::get (forest);
+  if (!context) {
+    t8_global_infof (
+      "Something went wrong while registering the adaptation callbacks. Please check your implementation.");
+    return 0;  // No adaptation as default.
+  }
+  // Convert to index used in the mesh handle.
+  const t8_locidx_t mesh_index = t8_forest_get_tree_element_offset (forest_from, which_tree) + lelement_id;
+  // Call the actual adapt callback stored in the context.
+  return context->adapt_mesh (mesh_index, is_family, num_elements, elements);
+}
+
+}  // namespace detail
+}  // namespace t8_mesh_handle