<p>In <a href="https://github.com/mlpack/mlpack/pull/747#discussion_r74316245">src/mlpack/core/tree/spill_tree/spill_tree_impl.hpp</a>:</p>
<pre style='color:#555'>> + }
> +
> + std::vector<size_t> leftPoints, rightPoints;
> + // Split the node.
> + overlappingNode = SplitPoints(tau, rho, points, leftPoints, rightPoints);
> +
> + // We don't need the information in points, so lets clean it.
> + std::vector<size_t>().swap(points);
> +
> + // Now we will recursively split the children by calling their constructors
> + // (which perform this splitting process).
> + left = new SpillTree(this, leftPoints, tau, maxLeafSize, rho);
> + right = new SpillTree(this, rightPoints, tau, maxLeafSize, rho);
> +
> + // Update count number, to represent the number of descendant points.
> + count = left->NumDescendants() + right->NumDescendants();
</pre>
<p>I see what you mean; I did not understand what you wrote the first time. Sorry about that. I don't think holding a list of unique points in each node is a good solution, and I can't think of any other easy solution to this problem. Any way I can think of to organize the points and still get access to unique descendants from arbitrary levels requires caching potentially very much. So I think unfortunately we should go with your second solution---disable this type of tree for rank-approximate nearest neighbor search or other algorithms that depend on <code>NumDescendants()</code> being correct. Let's do that like this:</p>
<ul>
<li>add a <code>TreeTraits</code> element indicating whether <code>NumDescendants()</code> returns the number of unique descendants</li>
<li>add a <code>static_assert</code> in <code>RASearch</code> to ensure that with the given tree type, the number of unique descendants is correctly returned by <code>NumDescendants()</code>, using that new <code>TreeTraits</code> element</li>
</ul>
<p>What do you think? I think that's the best we can do here, unfortunately.</p>
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