Tree depth in Perl 6

dfs.pl

use v6;
# inspired by http://www.perlmonks.org/?node_id=959260

my $graphspec = '1 -> 2 2 -> 0 3 -> 4 4 -> 2 5 -> 10 6 -> 10 7 -> 9 8 -> 9
9 -> 6 10 -> 4 11 -> 10 12 -> 10 13 -> 12 14 -> 10 15 -> 2';


my %graph;
for $graphspec.match(:g, rx:s/(\d+) '->' (\d+)/) -> $/ {
    %graph{$0} = +$1;
}

sub depth($source) {
    ($source, { %graph{$_} } ... 0) - 1;
}

say max %graph.keys.map: &depth;