#!/usr/bin/env perl
package Ring;
# return an empty ring structure
sub new {
 my $class = shift;
 my $node = { };
 $node->{NEXT} = $node->{PREV} = $node;
 my $self = { DUMMY => $node, COUNT => 0 };
 bless $self, $class;
 return $self;
}

# $node = $ring->search( $value ) : find $value in the ring
# structure in $node
sub search {
    my ($ring, $value) = @_;
    my $node = $ring->{DUMMY}->{NEXT};
    while ($node != $ring->{DUMMY} && $node->{VALUE} != $value) {
          $node = $node->{NEXT};
    }
    return $node;
} 

# $ring->insert( $value ) : insert $value into the ring structure
sub insert_value {
    my ($ring, $value) = @_;
    my $node = { VALUE => $value };
    $node->{NEXT} = $ring->{DUMMY}->{NEXT};
    $ring->{DUMMY}->{NEXT}->{PREV} = $node;
    $ring->{DUMMY}->{NEXT} = $node;
    $node->{PREV} = $ring->{DUMMY};
    ++$ring->{COUNT};
} 

# $ring->delete_value( $value ) : delete a node from the ring
# structure by value
sub delete_value {
    my ($ring, $value) = @_;
    my $node = $ring->search($value);
    return if $node == $ring->{DUMMY};
    $ring->delete_node($node);
}

1;