#include <stdio.h>
#include <stdlib.h>

struct point {int x; int y;};
typedef struct point point;

void prnt_point(point p){
  printf("Point: %i, %i\n", p.x, p.y);
}   

struct cons_cell {
  void             *data;
  struct cons_cell *next;
};

typedef struct cons_cell cons_cell;

/* Returns a pointer to a new cons cell, 
   which refers data and next via pointers*/
cons_cell *cons(void *data, cons_cell *next){
 cons_cell *result;
 result = malloc(sizeof(cons_cell));
 result->data = data;
 result->next = next;
 return result;
}

/* Return the head reference of the cons cell */
void *head(cons_cell *cell){
  return cell->data;
}

/* Return the tail refererence f the cons cell */
cons_cell *tail(cons_cell *cell){
  return cell->next;
}   

/* Change the data position of cell to new_head */
void set_head(cons_cell *cell, void *new_head){
  cell->data = new_head;
}

/* Change the next position of cell to new_tail */
void set_tail(cons_cell *cell, cons_cell *new_tail){
  cell->next = new_tail;
}   

/* Insert new_element after the cons-cell ptr_list */
void insert_after(void *new_element, cons_cell *ptr_list){
  cons_cell *new_cons_cell, *ptr_after;
 
  /* Make a new cons-cell around new_element */
  new_cons_cell= cons(new_element,NULL);

  /* Remember pointer to cons-cell after ptr_list */
  ptr_after = tail(ptr_list);

  /* Chain in the new_cons_cell */
  set_tail(ptr_list, new_cons_cell);

  /* ... and connect to rest of list */
  set_tail(new_cons_cell, ptr_after);
}
  
/* Delete the element after ptr_list. Assume as a
   precondition that there exists an element after ptr_list */
void delete_after(cons_cell *ptr_list){  
  cons_cell *ptr_after, *ptr_dispose;

  /* cons-cell to delete later */
  ptr_dispose = tail(ptr_list);

  /* The element to follow ptr_list */
  ptr_after = tail(ptr_dispose);

  /* Mutate the tail of ptr_list */
  set_tail(ptr_list, ptr_after);

  /* Free storage - only one cons-cell */
  free(ptr_dispose);
}


int main(void) {

  cons_cell *points, *pl;

  point p1 = {1,2}, p2 = {3,4}, p3 = {5,6}, p_new = {11,12};

  points = cons(&p1,
                cons(&p2,
                     cons(&p3,
                          NULL)));


  insert_after(&p_new, tail(points));  

  pl = points;    
  while (pl != NULL) {
    prnt_point(*((point*)(head(pl))));
    pl = tail(pl);
  }

  printf("\n\n");

  delete_after(points);

  pl = points;    
  while (pl != NULL) {
    prnt_point(*((point*)(head(pl))));
    pl = tail(pl);
  }

  return 0;
}