unit class LinkedList:ver<2>;

class ListNode {
    has $.next     is rw;
    has $.previous is rw;
    has $.value;
}

has $!first;
has $!last;

method push-list($value) {
    my $next = ListNode.new( value => $value, previous => $!last );
    if ( $!last ) {
        $!last = $!last.next= $next ;
    }
    else {
        $!first = $!last = $next;
    }
}

method unshift-list($value) {
    my $next = ListNode.new( value => $value, next => $!first );
    if ( $!first ) {
        $!first = $!first.previous = $next ;
    }
    else {
        $!first = $!last = $next;
    }
}

method shift-list() {
    my $f = $!first;

    $!first = $!first.next;

    unless $!first  { $!last = $!first }

    return $f.value;
}

method pop-list() {
    my $f = $!last;

    $!last = $!last.previous;

    unless $!.last  { $!first = $!last }

    return $f.value;
}