/* Linked lists
 * 15-122 Principles of Imperative Programming, Fall 2010
 * Frank Pfenning
 */

// type elem must be defined
typedef int elem;

struct list {
  elem data;
  struct list* next;
};

typedef struct list* list;

bool is_segment(list start, list end)
{ list p = start;
  while (p != end) {
    if (p == NULL) return false;
    p = p->next;
  }
  return true;
}

bool is_circular(list l)
{ if (l == NULL) return false;
  { list t = l;       // tortoise
    list h = l->next; // hare
    while (t != h)
      //@loop_invariant is_segment(t, h);
      { if (h == NULL || h->next == NULL) return false;
	t = t->next;
	h = h->next->next;
      }
    return true;
  }
}