[mlpack-git] [mlpack/mlpack] Spill trees (#747)

MarcosPividori notifications at github.com
Sun Aug 7 23:59:28 EDT 2016

@rcurtin  Thanks for your feedback! 

I implemented Hybrid Spill Search. It does:
 + backtracking in non-overlapping node.
 + defeatist search on overlapping nodes.

So, it is different to pure defeatist search.

Because of that, I think we should continue using the Tree Traversers for the implementation.

I agree that code could be improved to avoid calculating the score of both child nodes (which means calculating the projection 2 times). It can make a difference, especially with non axis-orthogonal splitting hyperplanes, because the projection takes O(d) time.

So, I can implement a new method as you suggested:  "GetNearestChild()" and use it when working with a overlapping node in the `SingleTreeTraverser` and `DualTreeTraverser`.

That method is not strictly necessary. We could avoid including a new method, implementing it in a simply way:
If I have a overlapping node I calculate the score of one child, and decide based on that score without calculating the score of the second child:
- If the score of the first child is `DBL_MAX`, I have to traverse the second child (its score is `0`).
- If the score of the first child is `0`, I have to traverse the first child (the score of the second child is `DBL_MAX`).

Also, we could implement a pure `DefeatistTreeTraverser`, for every kind of tree (not only spill trees) that always considers the score of all the child nodes, chose the one with the lowest score and doesn't do backtracking.


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