/* Inserts |item| into |tree| and returns a pointer to |item|'s address.
   If a duplicate item is found in the tree,
   returns a pointer to the duplicate without inserting |item|.
   Returns |NULL| in case of memory allocation failure. */
void **
bst_probe (struct bst_table *tree, void *item)
{
  struct bst_node **q;
  int cmp;

  assert (tree != NULL && item != NULL);

  for (q = &tree->bst_root; *q != NULL; q = &(*q)->bst_link[cmp > 0])
    {
      cmp = tree->bst_compare (item, (*q)->bst_data, tree->bst_param);
      if (cmp == 0)
        return &(*q)->bst_data;
    }

  *q = tree->bst_alloc->libavl_malloc (tree->bst_alloc, sizeof **q);
  if (*q == NULL)
    return NULL;

  (*q)->bst_link[0] = (*q)->bst_link[1] = NULL;
  (*q)->bst_data = item;
  tree->bst_count++;
  return &(*q)->bst_data;
}