cwr.p6

use v6;
use Test;

sub combinations_with_replacement(@iterable, $r)  {
    gather {
        cwr(@iterable, [], $r);
    }
}
sub cwr(@iterable, @state, $r) {
    my $place = @state.elems;
    @state.push(Nil);
    for @iterable {
        @state[$place] = $_[];
        if $r > 1 {
            cwr(@iterable, @state, $r-1);
            @state.pop;
        } else {
            take @state[];
        }
    }
}

say combinations_with_replacement(('a','b','c'), 2);
#Output: ([a a] [a b] [a c] [b a] [b b] [b c] [c a] [c b] [c c])
is combinations_with_replacement(('a','b','c'), 2).List, (('a','a'), ('a','b'), ('a','c'), ('b','a'), ('b','b'), ('b','c'), ('c','a'), ('c','b'), ('c','c'));
# Failed test at t/itertools.t line 39
# expected: 'a a a b a c b a b b b c c a c b c c'
#      got: 'c c c c c c c c c'