/// <summary>
/// returns all external rows of <paramref name="M"/>
/// corresponding to ghost cells of <paramref name="map"/>,
/// which are located on other Mpi-ranks.
/// </summary>
/// <param name="map">Multigrid mapping</param>
/// <param name="M">matrix distributed according to <paramref name="map"/></param>
/// <returns></returns>
public static BlockMsrMatrix GetAllExternalRows(MultigridMapping map, BlockMsrMatrix M)
{
var extcells = map.AggGrid.iLogicalCells.NoOfExternalCells.ForLoop(i => i + map.LocalNoOfBlocks);
var SBS = new SubBlockSelector(map);
SBS.CellSelector(extcells, false);
var AllExtMask = new BlockMaskExt(SBS, 0);
var ExternalRows_BlockI0 = AllExtMask.GetAllSubMatrixCellOffsets();
var ExternalRows_BlockN = AllExtMask.GetAllSubMatrixCellLength();
var ExternalRowsIndices = AllExtMask.m_GlobalMask;
BlockPartitioning PermRow = new BlockPartitioning(ExternalRowsIndices.Count, ExternalRows_BlockI0, ExternalRows_BlockN, M.MPI_Comm, i0isLocal: true);
BlockMsrMatrix Perm = new BlockMsrMatrix(PermRow, M._RowPartitioning);
for (int iRow = 0; iRow < ExternalRowsIndices.Count; iRow++)
{
Debug.Assert(M._RowPartitioning.IsInLocalRange(ExternalRowsIndices[iRow]) == false);
Perm[iRow + PermRow.i0, ExternalRowsIndices[iRow]] = 1;
}
#if TEST
Perm.SaveToTextFileSparseDebug("Perm");
#endif
return(BlockMsrMatrix.Multiply(Perm, M));
}