Fibonacci Range Example

// A solution (modified to return 11 instead of 10 in the range) from
// http://tour.dlang.org/tour/basics/13

import std.stdio;

struct FibonacciRange
{
    auto lastButTwo = 0;
    auto lastButOne = 0;
    auto last = 0;

    @property empty() const
    {
        // So when does the Fibonacci sequence
        // end?!
        return false;
    }

    void popFront()
    {
        lastButTwo = lastButOne;
        lastButOne = last;
    }

    int front()
    {
        if (lastButTwo == 0)
        {
            last = 1;
        }
        else
        {
            last = lastButOne + lastButTwo;
        }
        return last;
    }
}

void main() {
    import std.range: take;
    import std.array: array;

    // Let's test this FibonacciRange!
    FibonacciRange fib;

    // The magic function take creates another
    // range which just will return N elements
    // at maximum. This range is _lazy_ and just
    // touches the original range if actually
    // needed (iteration)!
    auto fib11 = take(fib, 11);

    // But we do want to touch all elements and
    // convert the range to an array of integers.
    int[] the11Fibs = array(fib11);

    writeln("Your first 11 Fibonacci numbers: ",
        the11Fibs);
    assert(the11Fibs ==
        [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]);
}