module imports.std15030algo;

template filter(alias pred)
{
    auto filter(R)(R r)
    {
        return FilterResult!(pred, R)(r);
    }
}

private struct FilterResult(alias pred, R)
{
    R _input;

    this(R r)
    {
        _input = r;
        while (_input.length != 0 && !pred(_input[0]))
        {
            _input = _input[1..$];
        }
    }

    @property bool empty() { return _input.length == 0; }

    @property auto ref front() { return _input[0]; }

    void popFront()
    {
        do
        {
            _input = _input[1..$];
        } while (_input.length != 0 && !pred(_input[0]));
    }
}