how to get top N distinct elements from range?

[bearophile]

> auto firstDistinct(Range)(Range r, in size_t n)
> if (isInputRange!Range) {
>     bool[ForeachType!Range] mySet;
>
>     return r.filter!((k) {
>         if (k in mySet)
>             return false;
>         mySet[k] = true;
>         return true;
>     }).take(n);
> }

I think the standard library of Ada2012 has bounded containers, 
so you can use them to stack-allocate a hash set that can contain 
up to n items (plus some more free cells to keep its load factor 
low enough).

In Rust there are fully safe stack-allocated closures. Combining 
the two things it's possible to create a function like that 
firstDistinct() with zero heap allocations, that is probably fast.

Bye,
bearophile