void barrier (int rank, int size)
{
    MPI_Status status;
    int test;
    int layers = ceil (log2 (size));
    int pow2 = 1;
    for (int p = 1; p <= layers; p++)
    {
        pow2 *= 2;
        if (rank % pow2 == 0)
        {
            if (rank + p < size)
            {
                MPI_Recv(&test, 1, MPI_INT, rank + pow2 / 2, 0,  MPI_COMM_WORLD, &status);
            }
        }
        else if (rank % pow2 == pow2 / 2)
        {
            MPI_Send (&test, 1, MPI_INT, rank - pow2 / 2, 0, MPI_COMM_WORLD);
        }
    }
    for (int p = layers; p > 0; p--)
    {
        if (rank % pow2 == 0)
        {
            if (rank + p < size)
                MPI_Send (&test, 1, MPI_INT, rank + pow2 / 2, 1, MPI_COMM_WORLD);
        }
        else if (rank % pow2 == pow2 / 2)
            MPI_Recv(&test, 1, MPI_INT, rank - pow2 / 2, 1,  MPI_COMM_WORLD, &status);
        pow2 = pow2 / 2;
    }
}