/* Deletes from |tree| and returns an item matching |item|.
   Returns a null pointer if no matching item found. */
void *
avl_delete (struct avl_table *tree, const void *item)
{
  /* Stack of nodes. */
  struct avl_node *pa[AVL_MAX_HEIGHT]; /* Nodes. */
  unsigned char da[AVL_MAX_HEIGHT];    /* |avl_link[]| indexes. */
  int k;                               /* Stack pointer. */

  struct avl_node *p;   /* Traverses tree to find node to delete. */
  int cmp;              /* Result of comparison between |item| and |p|. */

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

  k = 0;
  p = (struct avl_node *) &tree->avl_root;
  for (cmp = -1; cmp != 0;
       cmp = tree->avl_compare (item, p->avl_data, tree->avl_param))
    {
      int dir = cmp > 0;

      pa[k] = p;
      da[k++] = dir;

      p = p->avl_link[dir];
      if (p == NULL)
        return NULL;
    }
  item = p->avl_data;

  if (p->avl_link[1] == NULL)
    pa[k - 1]->avl_link[da[k - 1]] = p->avl_link[0];
  else
    {
      struct avl_node *r = p->avl_link[1];
      if (r->avl_link[0] == NULL)
        {
          r->avl_link[0] = p->avl_link[0];
          r->avl_balance = p->avl_balance;
          pa[k - 1]->avl_link[da[k - 1]] = r;
          da[k] = 1;
          pa[k++] = r;
        }
      else
        {
          struct avl_node *s;
          int j = k++;

          for (;;)
            {
              da[k] = 0;
              pa[k++] = r;
              s = r->avl_link[0];
              if (s->avl_link[0] == NULL)
                break;

              r = s;
            }

          s->avl_link[0] = p->avl_link[0];
          r->avl_link[0] = s->avl_link[1];
          s->avl_link[1] = p->avl_link[1];
          s->avl_balance = p->avl_balance;

          pa[j - 1]->avl_link[da[j - 1]] = s;
          da[j] = 1;
          pa[j] = s;
        }
    }

  tree->avl_alloc->libavl_free (tree->avl_alloc, p);

  assert (k > 0);
  while (--k > 0)
    {
      struct avl_node *y = pa[k];

      if (da[k] == 0)
        {
          y->avl_balance++;
          if (y->avl_balance == +1)
            break;
          else if (y->avl_balance == +2)
            {
              struct avl_node *x = y->avl_link[1];
              if (x->avl_balance == -1)
                {
                  struct avl_node *w;
                  assert (x->avl_balance == -1);
                  w = x->avl_link[0];
                  x->avl_link[0] = w->avl_link[1];
                  w->avl_link[1] = x;
                  y->avl_link[1] = w->avl_link[0];
                  w->avl_link[0] = y;
                  if (w->avl_balance == +1)
                    x->avl_balance = 0, y->avl_balance = -1;
                  else if (w->avl_balance == 0)
                    x->avl_balance = y->avl_balance = 0;
                  else /* |w->avl_balance == -1| */
                    x->avl_balance = +1, y->avl_balance = 0;
                  w->avl_balance = 0;
                  pa[k - 1]->avl_link[da[k - 1]] = w;
                }
              else
                {
                  y->avl_link[1] = x->avl_link[0];
                  x->avl_link[0] = y;
                  pa[k - 1]->avl_link[da[k - 1]] = x;
                  if (x->avl_balance == 0)
                    {
                      x->avl_balance = -1;
                      y->avl_balance = +1;
                      break;
                    }
                  else
                    x->avl_balance = y->avl_balance = 0;
                }
            }
        }
      else
        {
          y->avl_balance--;
          if (y->avl_balance == -1)
            break;
          else if (y->avl_balance == -2)
            {
              struct avl_node *x = y->avl_link[0];
              if (x->avl_balance == +1)
                {
                  struct avl_node *w;
                  assert (x->avl_balance == +1);
                  w = x->avl_link[1];
                  x->avl_link[1] = w->avl_link[0];
                  w->avl_link[0] = x;
                  y->avl_link[0] = w->avl_link[1];
                  w->avl_link[1] = y;
                  if (w->avl_balance == -1)
                    x->avl_balance = 0, y->avl_balance = +1;
                  else if (w->avl_balance == 0)
                    x->avl_balance = y->avl_balance = 0;
                  else /* |w->avl_balance == +1| */
                    x->avl_balance = -1, y->avl_balance = 0;
                  w->avl_balance = 0;
                  pa[k - 1]->avl_link[da[k - 1]] = w;
                }
              else
                {
                  y->avl_link[0] = x->avl_link[1];
                  x->avl_link[1] = y;
                  pa[k - 1]->avl_link[da[k - 1]] = x;
                  if (x->avl_balance == 0)
                    {
                      x->avl_balance = +1;
                      y->avl_balance = -1;
                      break;
                    }
                  else
                    x->avl_balance = y->avl_balance = 0;
                }
            }
        }
    }

  tree->avl_count--;
  tree->avl_generation++;
  return (void *) item;
}