/**
# The tree-grid iterator

![The page draws the cell-iterator movie](itmov/s.mp4)

We use Bview to draw the process of grid-iteration
 */
#include "view.h"

/**
We visualize scalar field `s` as the iterator progresses.
 */
int base_lev = 4, n, jj, nmin;
scalar s[];
/**
Each iteration we will draw a frame with all the cells and the `s`
field.
 */
void draw_frame(char * fname){
  cells();
  squares ("s", min = 0, max = n);
  save (fname);
}

int main(){
  /**
     We define a camera-centered grid with some refinement features
   */
  X0 = Y0 = -L0/2;
  init_grid (1 << base_lev);
  refine (fabs(x + y) < 0.2 && level <= base_lev);
  refine (sq(x - 0.25) + sq(y - 0.25) < sq(0.1) && level < base_lev + 2);
  /**
     We *unset* the data stored in `s` and count the number of cells
     for each thread.
   */
  foreach(){
    s[] = nodata;
    n++;
  }
  /**
For `_MPI`, each thread should help draw the same number of frames.
therefore, the maximum number of cells is communicated between the
processor ranks.
   */
#if _MPI
  MPI_Allreduce (&n, &nmin, 1, MPI_INT, MPI_MIN, MPI_COMM_WORLD);
  MPI_Allreduce (MPI_IN_PLACE, &n, 1, MPI_INT, MPI_MAX, MPI_COMM_WORLD);
  printf("%d %d %d\n", pid(), n, nmin);
  //balance(); //<- This is already in `refine()`
#endif
  /**
We loop over the cells and per iteration we increase the value of `s`.
   */
  view (fov = 20);
  draw_frame("s.mp4");
  foreach(){
    s[] = jj;
    draw_frame("s.mp4");
    if (jj++ == nmin - 1 && nmin != 0)
      draw_frame("unbalanced.png");
  }
  /**
and continue untill all threads are finished. 
   */
  if (jj < n){
    draw_frame("s.mp4");
    jj++;
  }
}
/**
![White cell(s) indicate an unbalance](itmov/unbalanced.png)
 */