incorporate review suggestions

examples/usage/allgather_example.cpp

@@ -20,7 +20,6 @@
 #include "helpers_for_examples.hpp"
 #include "kamping/checking_casts.hpp"
 #include "kamping/collectives/allgather.hpp"
-#include "kamping/collectives/alltoall.hpp"
 #include "kamping/communicator.hpp"
 #include "kamping/data_buffer.hpp"
 #include "kamping/environment.hpp"
@@ -30,21 +29,67 @@ int main() {
     using namespace kamping;
     kamping::Environment  e;
     kamping::Communicator comm;
-    std::vector<int>      input(comm.size(), comm.rank_signed());
+    // std::vector<int>      input(comm.size(), comm.rank_signed());
 
-    { // Basic form: Provide a send buffer and let KaMPIng allocate the receive buffer.
-        auto output = comm.allgather(send_buf(input));
-        print_result_on_root(output, comm);
-    }
+    //{ // Basic form: Provide a send buffer and let KaMPIng allocate the receive buffer.
+    //    auto output = comm.allgather(send_buf(input));
+    //    print_result_on_root(output, comm);
+    //}
+
+    // print_on_root("------", comm);
 
-    print_on_root("------", comm);
+    //{ // We can also send only parts of the input and specify an explicit receive buffer.
+    //    std::vector<int> output;
 
-    { // We can also send only parts of the input and specify an explicit receive buffer.
-        std::vector<int> output;
+    //    // this can also be achieved with `kamping::Span`
+    //    comm.allgather(send_buf(Span(input.begin(), 2)), recv_buf<resize_to_fit>(output));
+    //    print_result_on_root(output, comm);
+    //    return 0;
+    //}
 
-        // this can also be achieved with `kamping::Span`
-        comm.allgather(send_buf(Span(input.begin(), 2)), recv_buf<resize_to_fit>(output));
-        print_result_on_root(output, comm);
-        return 0;
+    MPI_Comm         mpi_graph_comm;
+    int              in_degree = -1, out_degree = -1;
+    std::vector<int> input{comm.rank_signed()};
+    std::vector<int> recv_buf(10, -1);
+    int              send_count = -1, recv_count = -1;
+    std::vector<int> in_ranks, out_ranks;
+    if (comm.rank() == 0) {
+        in_degree  = 0;
+        out_degree = 0;
+        send_count = 1;
+        recv_count = 1;
+    } else if (comm.rank() == 1) {
+        in_degree  = 0;
+        out_degree = 1;
+        out_ranks.push_back(2);
+        send_count = 1;
+        recv_count = 1;
+    } else if (comm.rank() == 2) {
+        in_degree  = 1;
+        out_degree = 1;
+        in_ranks.push_back(1);
+        out_ranks.push_back(3);
+        send_count = 1;
+        recv_count = 1;
+    } else if (comm.rank() == 3) {
+        in_degree  = 1;
+        out_degree = 0;
+        in_ranks.push_back(2);
+        send_count = 1;
+        recv_count = 1;
     }
+    MPI_Dist_graph_create_adjacent(
+        comm.mpi_communicator(),
+        in_degree,
+        in_ranks.data(),
+        MPI_UNWEIGHTED,
+        out_degree,
+        out_ranks.data(),
+        MPI_UNWEIGHTED,
+        MPI_INFO_NULL,
+        false,
+        &mpi_graph_comm
+    );
+
+    MPI_Neighbor_alltoall(input.data(), send_count, MPI_INT, recv_buf.data(), recv_count, MPI_INT, mpi_graph_comm);
 }

include/kamping/collectives/neighborhood/alltoall.hpp

@@ -43,14 +43,16 @@
 
 /// @brief Wrapper for \c MPI_Neighbor_alltoall.
 ///
-/// This wrapper for \c MPI_Neighbor_alltoall sends the same amount of data from a rank i to each of its neighbour j for
-/// which an edge (i,j) in the communication graph exists. The following buffers are required:
+/// @todo check again once the concrete semantics (potential differing number of send/recv counts) of
+/// MPI_Neighbor_alltoall has been clarified. This wrapper for \c MPI_Neighbor_alltoall sends the same amount of data
+/// from a rank i to each of its neighbour j for which an edge (i,j) in the communication graph exists. The following
+/// buffers are required:
 /// - \ref kamping::send_buf() containing the data that is sent to each neighbor. This buffer has to be divisible by the
-/// size of the communicator unless a send_count or a send_type is explicitly given as parameter.
+/// out degree unless a send_count or a send_type is explicitly given as parameter.
 ///
 /// The following parameters are optional:
 /// - \ref kamping::send_count() specifying how many elements are sent. If
-/// omitted, the size of send buffer divided by number of neighbors is used.
+/// omitted, the size of send buffer divided by number of outgoing neighbors is used.
 /// This has to be the same on all ranks.
 /// This parameter is mandatory if \ref kamping::send_type() is given.
 ///
@@ -59,17 +61,14 @@
 /// This parameter is mandatory if \ref kamping::recv_type() is given.
 ///
 /// - \ref kamping::recv_buf() specifying a buffer for the output. A buffer of at least
-/// `recv_count * communicator size` is required.
+/// `recv_count * in degree` is required.
 ///
 /// - \ref kamping::send_type() specifying the \c MPI datatype to use as send type. If omitted, the \c MPI datatype is
 /// derived automatically based on send_buf's underlying \c value_type.
 ///
 /// - \ref kamping::recv_type() specifying the \c MPI datatype to use as recv type. If omitted, the \c MPI datatype is
 /// derived automatically based on recv_buf's underlying \c value_type.
 ///
-/// Inplace alltoall is supported by passing send_recv_buf as parameter. This changes the requirements for the other
-/// parameters, see \ref Communicator::alltoall_inplace.
-///
 /// @tparam Args Automatically deduced template parameters.
 /// @param args All required and any number of the optional buffers described above.
 /// @return Result type wrapping the output parameters to be returned by value.