## random-sided Riffle shuffle
sub riffle {
    if (rand() < 0.5) {
        splice @_, @_/2 - $_, 0, pop
            for 0 .. (@_/2)-1;
    } else {
        splice @_, @_/2 + $_, 0, shift
            for 0 .. (@_/2)-1;
    }
    return @_;
}