Add decomposition factory to decouple from mpi

Author: ahojukka5

CMakeLists.txt

@@ -102,6 +102,7 @@ add_library(OpenPFC
     src/openpfc/core/world.cpp
     src/openpfc/core/box3d.cpp
     src/openpfc/core/decomposition.cpp
+    src/openpfc/factory/decomposition_factory.cpp
     # Add more .cpp files as you go
 )
 

include/openpfc/core/decomposition.hpp

@@ -7,21 +7,41 @@
 #include "openpfc/core/world.hpp"
 #include <array>
 #include <heffte.h>
-#include <mpi.h>
 #include <ostream>
 #include <stdexcept>
 #include <vector>
 
 namespace pfc {
 
 /**
- * @brief Class representing the domain decomposition for parallel Fast Fourier
- * Transform.
+ * @brief Represents a partitioning of the global simulation domain into local
+ * subdomains.
  *
- * The Decomposition class defines the domain decomposition for parallel FFT. It
- * splits the domain into multiple parts based on the number of processors and
- * the grid setup. It provides information about the local domain and
- * communication patterns.
+ * The Decomposition class describes how the World is split among multiple
+ * processes. Each instance provides access to the local subdomain owned by the
+ * current process, including size, offset, and basic mappings between global
+ * and local indices.
+ *
+ *
+ * ## Responsibilities
+ *
+ * - Split the World into non-overlapping local boxes.
+ * - Provide access to local box information.
+ * - Support global-local coordinate mapping.
+ *
+ *
+ * ## Design Justification
+ *
+ * - Keeps World (global domain) and local process view separate.
+ * - Decouples decomposition logic from communication (MPI, FFT backends).
+ * - Enables different decomposition strategies if needed (block, slab, pencil).
+ *
+ *
+ * ## Relations to Other Components
+ *
+ * - Used by Fields and Arrays to allocate local data.
+ * - Used by Communication components to know where data lives.
+ * - Passed to FFT backends as a description of local domains.
  */
 class Decomposition {
 private:
@@ -33,20 +53,6 @@ class Decomposition {
   const std::array<int, 3> proc_grid;                              ///< Processor grid dimensions.
   const std::vector<heffte::box3d<int>> real_boxes, complex_boxes; ///< Local domain boxes.
 
-  /**
-   * @brief Get the rank of the current MPI communicator.
-   * @param comm The MPI communicator.
-   * @return The rank of the current process.
-   */
-  int get_comm_rank(MPI_Comm comm);
-
-  /**
-   * @brief Get the size of the current MPI communicator.
-   * @param comm The MPI communicator.
-   * @return The size of the communicator (total number of processes).
-   */
-  int get_comm_size(MPI_Comm comm);
-
 public:
   const heffte::box3d<int> inbox, outbox; ///< Local communication boxes.
   const int r2c_direction = 0;            ///< Real-to-complex symmetry direction.
@@ -64,15 +70,6 @@ class Decomposition {
    */
   Decomposition(const World &world, int rank, int num_domains);
 
-  /**
-   * @brief Construct a new Decomposition object using MPI communicator. In this
-   * case, the total number of domains equals the communicator size.
-   *
-   * @param world Reference to the World object.
-   * @param comm The MPI communicator (default: MPI_COMM_WORLD).
-   */
-  Decomposition(const World &world, MPI_Comm comm = MPI_COMM_WORLD);
-
   /**
    * @brief Get the size of the inbox.
    *

include/openpfc/factory/decomposition_factory.hpp

@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: 2025 VTT Technical Research Centre of Finland Ltd
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+#ifndef PFC_DECOMPOSITION_FACTORY_HPP
+#define PFC_DECOMPOSITION_FACTORY_HPP
+
+#include "openpfc/core/decomposition.hpp"
+#include "openpfc/core/world.hpp"
+#include <mpi.h>
+
+namespace pfc {
+
+/**
+ * @brief Factory function to create a Decomposition from MPI communicator.
+ *
+ * @param world The World object.
+ * @param comm The MPI communicator (defaults to MPI_COMM_WORLD).
+ * @return Decomposition
+ */
+Decomposition make_decomposition(const World &world, MPI_Comm comm = MPI_COMM_WORLD);
+
+} // namespace pfc
+
+#endif // PFC_DECOMPOSITION_FACTORY_HPP

src/openpfc/core/decomposition.cpp

@@ -9,18 +9,6 @@
 
 namespace pfc {
 
-int Decomposition::get_comm_rank(MPI_Comm comm) {
-  int rank;
-  MPI_Comm_rank(comm, &rank);
-  return rank;
-}
-
-int Decomposition::get_comm_size(MPI_Comm comm) {
-  int size;
-  MPI_Comm_size(comm, &size);
-  return size;
-}
-
 Decomposition::Decomposition(const World &world, int rank, int num_domains)
     : m_world(world),
       m_rank(rank < num_domains ? rank
@@ -35,10 +23,6 @@ Decomposition::Decomposition(const World &world, int rank, int num_domains)
   assert(real_indexes.r2c(r2c_direction) == complex_indexes);
 }
 
-Decomposition::Decomposition(const World &world, MPI_Comm comm)
-    : Decomposition(world, get_comm_rank(comm), get_comm_size(comm)) {
-}
-
 const std::array<int, 3> &Decomposition::get_inbox_size() const {
   return inbox.size;
 }

src/openpfc/factory/decomposition_factory.cpp

@@ -0,0 +1,16 @@
+// SPDX-FileCopyrightText: 2025 VTT Technical Research Centre of Finland Ltd
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+#include "openpfc/factory/decomposition_factory.hpp"
+
+namespace pfc {
+
+Decomposition make_decomposition(const World &world, MPI_Comm comm) {
+  int rank;
+  int size;
+  MPI_Comm_rank(comm, &rank);
+  MPI_Comm_size(comm, &size);
+  return Decomposition(world, rank, size);
+}
+
+} // namespace pfc