function step = X_1way(t,y)

global c dx

%  finite difference: -c(dy/dx)
%    - shifted values to left & right
%
%           yL  y  yR
%    --------+--+--+------
%           xL  x  xR

yR = [y(2:end) ; y(1)];
yL = [y(end) ; y(1:end-1)];

%  1st-order upwind diff
step = -c*(y - yL)/dx;

%  2nd-order centered diff
%step = -c*(yR - yL)/dx/2;

%  1st-order downwind diff
%step = -c*(yR - y)/dx;

return