Intersecting Number Wheels

A succinct Perl 6 example using a few additional language features. Wheels are implemented as infinite repeating sequences, allowing a single iterator to keep track of the current position. This means the code contains no position tracking whatsoever.

#| advance rotates a named wheel $n by consuming an item from an infinite sequence. It is called
#| from within a gather block and so can use take in order to construct an infinite, lazy sequence
#| of result values
sub advance($g, $n) {
    given $g{$n}.pull-one {
        when /\d/ { take $_ }
        default   { samewith $g, $_ } # samewith re-calls this function with new parameters
    }
}

#| Input groups are a hash containing each wheel name as the key, and a list of values constructed
#| using <> to split on whitespace. They are transformed using xx * to repeat the list infinitely.
#| We then retrieve the underlying iterator in order for wheel position to be persistent. Each group
#| is then aggregated into a lazy output sequence using an infinite loop inside a gather block.
[
    {A => <1 2 3>},
    {A => <1 B 2>, B => <3 4>},
    {A => <1 D D>, D => <6 7 8>},
    {A => <1 B C>, B => <3 4>, C => <5 B>},
]
    #| %() converts a list of pairs produced by map into a hash. $^k and $^v are implicit variables.
    #| They are processed in alphabetical order and make the block arity 2, called with two vars.
    #| .kv gets the list of wheel names and wheel values from the input entry
    ==> map({ %(.kv.map: { $^k => (|$^v xx *).iterator }) })
    #| gather constructs a lazy sequence, in which we infinitely loop advancing wheel A
    ==> map({ gather { loop { advance $_, 'A' }} })
    #| state variables are only initialised once, and are kept between invocations.
    ==> map({ state $i = 1; say "Group {$i++}, First 20 values: $_[^20]" })

Output:

Group 1, First 20 values: 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2
Group 2, First 20 values: 1 3 2 1 4 2 1 3 2 1 4 2 1 3 2 1 4 2 1 3
Group 3, First 20 values: 1 6 7 1 8 6 1 7 8 1 6 7 1 8 6 1 7 8 1 6
Group 4, First 20 values: 1 3 5 1 4 3 1 4 5 1 3 4 1 3 5 1 4 3 1 4