struct traverser
  {
    struct bst_table *table;                  /* Tree being traversed. */
    struct bst_node *node;                    /* Current node in tree. */
    struct bst_node *stack[BST_MAX_HEIGHT];   /* Parent nodes to revisit. */
    size_t height;                            /* Number of nodes in |stack|. */
  };

/* Initializes |trav| for |tree|.
   Returns data item in |tree| with the smallest value,
   or |NULL| if |tree| is empty.
   In the former case, |next_item()| may be called with |trav|
   to retrieve additional data items. */
void *
first_item (struct bst_table *tree, struct traverser *trav)
{
  assert (tree != NULL && trav != NULL);
  trav->table = tree;
  trav->node = tree->bst_root;
  trav->height = 0;
  return next_item (trav);
}

/* Returns the next data item in inorder
   within the tree being traversed with |trav|,
   or if there are no more data items returns |NULL|.
   In the former case |next_item()| may be called again
   to retrieve the next item. */
void *
next_item (struct traverser *trav)
{
  struct bst_node *node;

  assert (trav != NULL);
  node = trav->node;
  while (node != NULL)
    {
      if (trav->height >= BST_MAX_HEIGHT)
        {
          fprintf (stderr, "tree too deep\n");
          exit (EXIT_FAILURE);
        }

      trav->stack[trav->height++] = node;
      node = node->bst_link[0];
    }

  if (trav->height == 0)
    return NULL;

  node = trav->stack[--trav->height];
  trav->node = node->bst_link[1];
  return node->bst_data;
}