best way to memoize a range?

[John Colvin via Digitalmars-d-learn]

On Friday, 11 September 2015 at 13:09:33 UTC, Laeeth Isharc wrote:
> obviously it's trivial to do with a little aa cache.  and I 
> know I can memoize a function, and turn the memoized version 
> into an infinite range.  but suppose I have a lazy function 
> that returns a finite range, and its expensive to calculate.
>
> can I use Phobos to produce a memoized range?  So it will 
> calculate on access to an element if it needs to, but will 
> return the cached value if it has been calculated before.

perhaps 
http://dlang.org/phobos/std_algorithm_iteration.html#.cache would 
do what you need. An AA based cache (with a normal range 
interface if you want) is probably the sensible way to get true 
random-access here, unless you have reasonably dense and 
monotonic accesses in which case an appender-based linear cache 
could work, either using Nullable!T or a separate list of bools 
(or bits) marking whether a result is cached yet.