Fix StackOverflowError in HotPixelIndex

[FObermaier]

Add nested CoordinateShuffler class and wire it add(Coordinate[] pts) in order to get a more balanced KdTree. The insertion speed for geometries with many coordinates greatly improves!

relates to #602

Signed-off-by: Felix Obermaier felix.obermaier@netcologne.de

[dr-jts]

Nice fix, @FObermaier ! I was starting to think about this myself, but this is a very elegant implementation. And it provides a dramatic performance boost.

[dr-jts]

I wonder if it would be even better to skip shuffling if there are only a small number of points (say <= 100) ?

[dr-jts]

Or another idea: instead of randomly shuffling the array indices, step through the coordinate array with a stride length which is coprime to the array size. That will visit every array entry once, but in a jumbled order.

This may not make a big peformance difference, but will use less memory.

The trick is finding a stride length which is relatively prime. One way to do it is given here (and a Java implementation of GCD here)

[dr-jts]

Further testing indicates that:

• Gating the shuffling to only use it for larger arrays sizes doesn't make an appreciable difference in performance (this is often the case for indexes - there's not much advantage in not using them)
• Using a coprime stride to step through the array is slightly better, except for some cases when it is MUCH worse. So that is too risky to use

Based on this, I'll leave this approach as it is.