def advect_timestep_donorcell(x,q,vi,dt):
    """
    Simple donor-cell advection scheme. First and last cell
    are ghost cells and are not affected.
    
    ARGUMENTS:
    x      The cell center locations (array of nx elements)
    q      The cell center q values (array of nx elements)
    vi     The velocities at the cell walls (array of nx-1 elements)
    dt     The time step

    RETURNS:
    qnew   The updated values of q at the next time step

    """
    xi   = 0.5*(x[1:]+x[:-1])    # Location of cell walls
    dx   = xi[1:]-xi[:-1]        # Array of cell widths
    dx   = np.hstack((0,dx,0))   # Get dx indices in line with cell indices
    fi   = np.zeros_like(vi)     # Prepare array of interface fluxes
    ipos = np.where(vi>=0.)[0]   # Indices of interfaces with positive vi
    ineg = np.where(vi<0.)[0]    # Indices of interfaces with negative vi

    <<< COMPLETE THE ALGORITHM HERE >>>