how to find the index of the adjacent cell of one cell?
 

Thread *tf;
face_t f;
cell_t c0;
real x[ND_ND], xf[ND_ND];
int n;
real T;

C_CENTROID(x, c, t);
c_face_loop(c, t, n) /* loops over all faces of a cell */
{
tf = C_FACE_THREAD(c,t,n);
f = C_FACE(c,t,n);
F_CENTROID(xf, f, tf);
if is_left_to_the_cell(x, xf) /* is the face left to the cell? */
{
c0 = F_C0(f, tf);
if (c0 == c) /* is itself */
c0 = F_C1(f, tf);
T = C_T(c0,t);
break;
}
}

Than you very much !