Expand API surface and test cases

This adds a new abstraction for MPI_Group to be able to use the
post-start-complete-wait RMA cycle. Also adds documentation and more tests.

Co-authored-by: Mohamed Aziz Bellaaj <aziiizbelaj@gmail.com>

Author: hmenke

c++/mpi/group.hpp

@@ -0,0 +1,119 @@
+// Copyright (c) 2024 Simons Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0.txt
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Authors: Thomas Hahn, Alexander Hampel, Olivier Parcollet, Nils Wentzell
+
+/**
+ * @file
+ * @brief Provides a C++ wrapper class for an @p MPI_Group object.
+ */
+
+#pragma once
+
+#include "./communicator.hpp"
+#include "./environment.hpp"
+
+#include <mpi.h>
+
+#include <cstdlib>
+#include <unistd.h>
+
+namespace mpi {
+
+  /**
+   * @ingroup mpi_essentials
+   * @brief C++ wrapper around @p MPI_Group providing various convenience functions.
+   *
+   * @details It stores an @p MPI_Group object as its only member which by default is set to @p MPI_GROUP_NULL.
+   */
+  class group {
+    // Wrapped @p MPI_Group object.
+    MPI_Group _grp = MPI_GROUP_NULL;
+
+    public:
+    /// Construct a communicator with @p MPI_GROUP_NULL.
+    group() = default;
+
+    /// Deleted copy constructor.
+    group(group const &) = delete;
+
+    /// Deleted copy assignment operator.
+    group &operator=(group const &) = delete;
+
+    /// Move constructor leaves moved-from object with @p MPI_GROUP_NULL.
+    group(group &&other) noexcept : _grp{std::exchange(other._grp, MPI_GROUP_NULL)} {}
+
+    /// Move assignment operator leaves moved-from object with @p MPI_GROUP_NULL.
+    group &operator=(group &&rhs) noexcept {
+      if (this != std::addressof(rhs)) {
+        this->free();
+        this->_grp = std::exchange(rhs._grp, MPI_GROUP_NULL);
+      }
+      return *this;
+    }
+
+    /**
+     * @brief Take ownership of an existing @p MPI_Group object.
+     * @param grp The group to be handled.
+     */
+    explicit group(MPI_Group grp) : _grp(grp) {}
+
+    /**
+     * @brief Create a group from a communicator.
+     * @param c The communicator from which to create a group.
+     */
+    explicit group(communicator c) {
+      if (has_env) { MPI_Comm_group(c.get(), &_grp); }
+    }
+
+    /// Get the wrapped @p MPI_Group object.
+    [[nodiscard]] MPI_Group get() const noexcept { return _grp; }
+
+    /// Check if the contained @p MPI_Group is @p MPI_GROUP_NULL.
+    [[nodiscard]] bool is_null() const noexcept { return _grp == MPI_GROUP_NULL; }
+
+    /// Rank of the calling process in the given group.
+    [[nodiscard]] int rank() const {
+      int rank = 0;
+      if (has_env) { MPI_Group_rank(_grp, &rank); }
+      return rank;
+    }
+
+    /// Size of a group.
+    [[nodiscard]] int size() const {
+      int size = 1;
+      if (has_env) { MPI_Group_size(_grp, &size); }
+      return size;
+    }
+
+    /**
+     * @brief Produces a group by reordering an existing group and taking only listed members.
+     * @param ranks List of ranks to include in the new group.
+     * @returns New group containing only the listed members.
+     */
+    group incl(std::vector<int> const &ranks) const {
+      MPI_Group newgroup = MPI_GROUP_NULL;
+      if (has_env) { MPI_Group_incl(_grp, ranks.size(), ranks.data(), &newgroup); }
+      return group(newgroup);
+    }
+
+    /// Free the group.
+    void free() {
+      if (has_env) {
+        if (_grp != MPI_GROUP_NULL) { MPI_Group_free(&_grp); }
+      }
+    }
+  };
+
+} // namespace mpi