how to get top N distinct elements from range?

[bearophile]

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