/* Creates a new node as a child of |dst| on side |dir|.
   Copies data from |src| into the new node, applying |copy()|, if non-null.
   Returns nonzero only if fully successful.
   Regardless of success, integrity of the tree structure is assured,
   though failure may leave a null pointer in a |tbst_data| member. */
static int
copy_node (struct tbst_table *tree,
           struct tbst_node *dst, int dir,
           const struct tbst_node *src, tbst_copy_func *copy)
{
  struct tbst_node *new =
    tree->tbst_alloc->libavl_malloc (tree->tbst_alloc, sizeof *new);
  if (new == NULL)
    return 0;

  new->tbst_link[dir] = dst->tbst_link[dir];
  new->tbst_tag[dir] = TBST_THREAD;
  new->tbst_link[!dir] = dst;
  new->tbst_tag[!dir] = TBST_THREAD;
  dst->tbst_link[dir] = new;
  dst->tbst_tag[dir] = TBST_CHILD;

  if (copy == NULL)
    new->tbst_data = src->tbst_data;
  else
    {
      new->tbst_data = copy (src->tbst_data, tree->tbst_param);
      if (new->tbst_data == NULL)
        return 0;
    }

  return 1;
}