Ranges/algorithms for aggregation

["Luís]

On Saturday, 22 March 2014 at 01:08:11 UTC, bearophile wrote:
> how is it supposed to know where to group items? Usually you 
> build an associative array for that.

It would swap elements, like sort, so it doesn't need to put them 
anywhere, just permute them. The advantage is this:

Input: [7, 3, 7, 1, 1, 1, 1]
Output sort: [1, 1, 1, 1, 3, 7, 7]
Output groupSort: [3, 7, 7, 1, 1, 1, 1]

groupSort (or whatever it would be called) only makes one swap, 
while sort makes a lot of them. So groupSort is a lot cheaper. 
I'm not sure what the asymptotic time complexity of groupSort is, 
at this moment's notice (I guess it would depend on what strategy 
it would use).