Lazy range, extract only Nth element, set range size constraint?

[ag0aep6g via Digitalmars-d-learn]

On 07/09/2017 11:51 PM, biocyberman wrote:
> Following is the code for a more generalized Fibonacci range.
> 
> Questions:
> 
> 1. How do I get only the value of the Nth (i.e. N = 25) element in an 
> idiomatic way?

As you've only got an input range, you have to popFront the 24 values 
that come before. You can use std.range.drop:

----
import std.range : drop;
auto twentyfifth = fib.drop(24).front;
----

But if you're not sure that the range actually has 25 elements, you 
should check `empty`, of course:

----
import std.range : popFrontN;
fib.popFrontN(24); /* or fib = fib.drop(24); */
if (!fib.empty)
{
     auto twentyfifth = fib.front;
}
----

> 2. Can I set constraints in the range so that user gets warning if he 
> asks for Nth element greater than a limit, say N> 30;

You can keep track of N in FibonacciRange and when it hits 30 you throw 
an exception or print a message or just set `empty` to true.

You can't make it a compilation warning/error as far as I can tell.

> or if the actually 
> range value at N is greater than datatype limit (e.g. max long)?

You can use std.experimental.checkedint to detect it at run time:

----
private bool _empty = false;
bool empty() const @property { return _empty; }

void popFront()
{
     import std.experimental.checkedint : checked, Throw;
     long tmp = 0;
     try tmp = (checked!Throw(first)*multifactor + second).get;
     catch (Throw.CheckFailure e) _empty = true;
     first = second;
     second = tmp;
}
----

(There may be a smarter way than making the operation throw an exception 
and catching that.)