I am also having trouble making sense of this structure and converting it.

 /* Mark cells containing particles as fluid cells (loop over particle
+s) */
 /*-------------------------------------------------------------------
+---*/
 for (n=1;n<=N;n++)
    for(part=Particlelines[n].Particles; part->next != NULL; part=part
+->next)
      {
       x = part->next->x;
       y = part->next->y;

       i = (int)(x/delx)+1;
       j = (int)(y/dely)+1;

       if (FLAG[i][j] < C_F) 
         {     /* delete particles in obstacle cells */
          help = part->next->next;
          free(part->next);
          part->next = help;
          Particlelines[n].length--;
     }
       else
          FLAG[i][j] = FLAG[i][j] & ~C_E;
      }
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