Re: how to get top N distinct elements from range?

[bearophile]

With this modified filter (that caches _input.front) it seems to 
work:

import std.stdio, std.range, std.algorithm, std.traits, 
std.random;

struct FilterResult2(alias pred, Range) {
    import std.traits: ForeachType;

    Range _input;
    ForeachType!Range rFront;

    this(Range r) {
        _input = r;

        while (!_input.empty) {
            this.rFront = _input.front;
            if (pred(this.rFront))
                break;
            _input.popFront();
        }
    }

    @property bool empty() { return _input.empty; }

    void popFront() {
        do {
            _input.popFront();
            if (_input.empty)
                break;
            this.rFront = _input.front;
            if (pred(this.rFront))
                break;
        } while (true);
    }

    @property auto ref front() {
        return this.rFront;
    }
}

FilterResult2!(pred, Range) filter2(alias pred, Range)(Range rs) {
    return FilterResult2!(pred, Range)(rs);
}


auto distinct(Range)(Range r)
if (isInputRange!Range) {
    bool[ForeachType!Range] mySet;

    return r.filter2!((k) {
        if (k in mySet)
            return false;
        mySet[k] = true;
        return true;
    });
}

void main() {
    100.iota.map!(_ => uniform(0, 
10)).distinct.array.sort.writeln;
}


Is it a good idea to replace std.algorithm.filter with something 
like that filter2 (plus some missing methods)?

Bye,
bearophile