Linked List Implementation in Ruby

linked_list_example.rb

# create a new list
list = ListNode.new

# add some items to the head or tail
list.add_to_head(Node.new('Hello'))
list.add_to_tail(Node.new('World'))
list.to_a # => ["Hello", "World"]

# remove items from the head
list.head.remove
list.to_a # => ["World"]
list.head == list.tail # => true

# remove items from the tail
list.tail.remove
list.to_a # => []

# setup for enumeration example
list = ListNode.new
list.add_to_head(Node.new(1))
list.add_to_head(Node.new(2))
list.add_to_head(Node.new(3))
list.add_to_head(Node.new(4))

# list each value
list.each {|node| puts node.value }

# select all nodes with values greater than 2
list.each.select {|node| node.value > 2 }

# find the first node with a value of 4
list.each.find {|node| node.value == 4 }

list_node.rb

class ListNode < Node
  protected :remove # prevent incorrect access to Node methods

  def initialize
    @next = @prev = self
  end

  def head
    @next unless @next == self
  end

  def tail
    @prev unless @prev == self
  end

  def add_to_head(node)
    node.insert_after(self)
  end

  def add_to_tail(node)
    node.insert_after(self.prev)
  end

  def each(&block)
    return enum_for(:each) unless block_given?
    node = @next
    while node != self
      yield node
      node = node.next
    end
  end

  def to_a
    each.collect {|node| node.value }
  end
end

node.rb

class Node
  protected
  attr_writer :prev, :next

  public
  attr_reader :value, :prev, :next

  def initialize(value)
    @value = value
  end

  def remove
    @prev.next = @next if @prev
    @next.prev = @prev if @next
    @next = @prev = nil
  end

  def insert_after(node)
    remove

    @next = node.next
    @next.prev = self if @next
    @prev = node
    node.next = self
  end
end

node_example.rb

head = Node.new(1)

middle = Node.new(2)
middle.insert_after(head)

tail = Node.new(3)
tail.insert_after(middle)

middle.remove