sorting, O(n^2) and O(n log(n))

[Stephen R. van den Berg]

This recent patch:

    Added quick_sort_locations().
    This replaces some O(n^2) algorithms with some that are O(n log(n))

Tries to optimise an O(n^2) operation to O(n log(n)), yet uses quicksort
in the process which can be O(n^2) all by itself.
Why not use (my personal favourite): heapsort?
Heapsort is O(n log(n)) as the *worst* case.
-- 
Sincerely,
           Stephen R. van den Berg.
Our four weapons are: fear, surprise, a ruthless efficiency, an almost
fanatical devotion to the Pope... and nice red uniforms.