how to get top N distinct elements from range?

[Andrea Fontana]

On Saturday, 9 March 2013 at 01:00:26 UTC, bearophile wrote:
> This is a shortened version of the filter(), it calls pred only 
> n times, but it calls _input.front n*2 times. If _input.front 
> is not deterministic (that means it's not pure, as in the case 
> of distinct filtering function) it gives wrong results:
>
>
> struct FilterResult(alias pred, Range) {
>     Range _input;
>
>     this(Range r) {
>         _input = r;
>         while (!_input.empty && !pred(_input.front)) {
>             _input.popFront();
>         }
>     }
>
>     @property bool empty() { return _input.empty; }
>
>     void popFront() {
>         do {
>             _input.popFront();
>         } while (!_input.empty && !pred(_input.front));
>     }
>
>     @property auto ref front() {
>         return _input.front;
>     }
> }
>
>
> This isn't exactly a bug of filter(), it's a design decision of 
> not caching _input.front.
>
> Bye,
> bearophile

Not funny btw :) It's not so easy to find out. And it's not easy 
to write a "cached map" function if you use classes or some 
pointers as ElementType...