Problem Statement

It is known that the set of finite lists of positive integers is   countable.
This means that there exists a subset of natural integers which can be mapped to the set of finite lists of positive integers.

Implement such a mapping:

Demonstrate your solution by:

There are many ways to do this.   Feel free to choose any one you like.

Make the   rank   function as a   bijection   and show   unrank(n)   for   n   varying from   0   to   10.

Let's start with the solution:

use bigint;
use ntheory qw(fromdigits todigitstring);
use feature 'say';

sub rank   { join   '', fromdigits(join('a',@_), 11) }
sub unrank { split 'a', todigitstring(@_[0],     11) }

say join ' ', @n = qw<12 11 0 7 9 15 15 5 7 13 5 5>;
say $n = rank(@n);
say join ' ', unrank $n;