/**
 * Intrusive doubly-linked list.
 *
 * The list is looped, thus when empty its head node must have both
 * 'prev' and 'next' field point the same value pointing to the head.
 */
struct list
{
  struct list *prev;
  struct list *next;
};

#define make_list_head(head)                                                  \
  {                                                                           \
    .prev = &head, .next = &head                                              \
  }

/**
 * Inserts new element into the list.
 * @return new list head.
 */
struct list *list_insert (struct list *, struct list *);
/**
 * Remove element from the list.
 * @return new list head or NULL.
 */
struct list *list_remove (struct list *);

struct list *
list_insert (struct list *head, struct list *node)
{
  node->prev = head;
  node->next = head->next;
  head->next->prev = node;
  return head->next = node;
}

struct list *
list_remove (struct list *node)
{
  struct list *prev = node->prev;
  struct list *next = node->next;
  next->prev = prev;
  return prev->next = next;
}