Index finite lists of positive integers: Difference between revisions

'''Extra credit'''

To get a bijection we need to shift the encoding of the length by two:

<lang perl6>sub rank(@n) { compress compress(@n), +@n - 2}

sub unrank(Int $n) { my ($a, $b) = expand($n, 2); expand $a, $b + 2 }

for ^20 {

my @unrank = unrank $_;

say @unrank, " <-> ", rank @unrank;

}</lang>

{{out}}

<pre>0 0 <-> 0

1 0 <-> 1

0 0 0 <-> 2

1 0 0 <-> 3

0 1 <-> 4

1 1 <-> 5

2 0 0 <-> 6

0 1 0 <-> 7

0 0 0 0 <-> 8

1 0 0 0 <-> 9

0 0 0 0 0 <-> 10</pre>

This does not deal with the degenerate case of lists of length one, though.