Re: geometry: knn search of point to many linestrings?

From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2021-07-14 02:02:02

W dniu 13.07.2021 o 17:04, Jens Thiele via Boost-users pisze:
> Another problem I face now is that each run produces different results
> which is still correct (multiple linestrings might have the same
> distance to some point) but I would prefer a reproducible result.
>
> I guess the cause for this is that
> rtree_t::const_query_iterator it=t->qbegin(idx::nearest(pt, tree_size));
> returns an iterator where the order isn't stable.
> Would it be difficult to fix the iteration order?
The rtree can have different internal structure depending on balancing 
algorithm and the order of inserts. So unless you know that values are 
always inserted in the same order it's impossible to guarantee that the 
structure is going to be the same. Then the order is also determined by 
the internals of the iterator, how exactly the rtree is traversed, how 
nodes and values are prioritized based on distance. And yes, currently 
heap and sort is used and I plan to replace sort with minmax heap for 
performance. Neither of these is stable.

But I don't understand what do you mean by "each run" and "stable 
order"? What the stability refers to? Could you give an example?

Adam