[GitHub] [spark] zhengruifeng commented on issue #27758: [SPARK-31007][ML] KMeans optimization based on triangle-inequality

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zhengruifeng commented on issue #27758: [SPARK-31007][ML] KMeans optimization based on triangle-inequality
URL: https://github.com/apache/spark/pull/27758#issuecomment-593323571
 
 
   In current impl, following Lemma is used in KMeans:
   
   0, Let x be a point, let b be a center and o be the origin, then d(x,c) >= |(d(x,o) - d(c,o))| = |norm-norm(c)|
   this can be applied in `EuclideanDistance`, but not in `CosineDistance`
   
   According to [Using the Triangle Inequality to Accelerate K-Meanswe](https://www.aaai.org/Papers/ICML/2003/ICML03-022.pdf) can go futher, and there are another two Lemmas can be used:
   
   1, Let x be a point, and let b and c be centers. If d(b,c)>=2d(x,b) then d(x,c) >= d(x,b);
   
   this can be applied in `EuclideanDistance`, but not in `CosineDistance`.
   However, luckily for `CosineDistance` we can get a variant in the space of radian/angle.
   
   2, Let x be a point, and let b and c be centers. Then d(x,c) >= max{0, d(x,b)-d(b,c)};
   
   this can be applied in EuclideanDistance, but not in CosineDistance
   
   The application of Lemma 2 is a little complex: It need to cache/update the distance/lower bounds to previous centers, and thus can be only applied in training, not usable in predction.
   
   So this PR is mainly for Lemma 1. Its idea is quite simple, if point x is close to center b enough (less than a pre-computed radius), then we can say point x belong to center c without computing the distances between x and other centers. It can be used in both training and predction.

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