Vectorizing closest match

Frank,

Apologies if I'm being very stupid, but I'd guess that reducing the overall
complexity would help for large datasets. The following is not vectorised,
but should be linear apart from the order() operations, which are hopefully
O(nlogn):

<snip actual code>

It is all based on sorting the data first so that only m+n comparisons are
needed rather than mn.

Looking at how findInterval is defined it seems that my solution is
roughly O(m logn) if n>>m but roughly O(m+n) if n<=m, so there's not that
much to choose between them in asymptotic complexity.

	-thomas

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