cross combinations

[jql]

Darren Duncan (Rosetta author) asked on a p6 mailing-list if there would be a standard function to sort of 'cross join' data. I wanted to exercise my perl6 skills with a stab at it.

(quoted from Darren)

One thing I would like to be able to do is this:

  @baz = cross([1,2],[3,4]); # yields ([1,3],[1,4],[2,3],[2,4])

And alternately, this:

  for cross([1,2],[3,4]) -> $foo,$bar { ... } # loop has 4 iterations

More examples:

  cross() # yields ()
  cross([],[1]) # yields ()
  cross([1,2]) # yields ([1],[2])
  cross([1,2],[3]); # yields ([1,3],[2,3])
  cross([1],[2],[3]) # yields ([1,2,3])
  cross([1],[2],[3,4]) # yields ([1,2,3],[1,2,4])

Warning: Perl6 code ahead.

sub cross {
    my @list = @_.reverse or return;
    gather {
        my $count = [*] 1, @_.map:{ .elems };
        for 0 .. $count - 1 -> $idx is copy {
            take [
                @_.map:{ ($idx, my $elem) = divmod($idx, .elems); .[$e
+lem] }
            ].reverse;
        }
    }
}

Update: the default 1 goes first, I guess