[Issue 15553] topN very inefficient [slower than sort, even for topN(0)] but should be O(n)

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15553

--- Comment #3 from Andrei Alexandrescu <andrei at erdani.com> ---
BTW uniform() isn't the problem; replacing it with r.length / 2 keeps things
slow.

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