linked_list.cr

module LinkedList(T)
  macro included
    include Enumerable(T)

    property _next, _prev : T | Nil
  end

  def append(node : T)
    node.next = @_next
    node.prev = self
    @_next = node
  end

  def prepend(node : T)
    node.next = self
    node.prev = @_next
    @_prev = node
  end

  def detach
    if prev = @_prev
      prev.next = @_next
    end
    if _next = @_next
      _next.prev = @_prev
    end

    @_next = @_prev = nil
    self
  end

  def each
    current = self
    yield current
    while current = current._next
      yield current
    end
    nil
  end

  def head?
    @_prev.nil?
  end

  def tail?
    @_next.nil?
  end

  def next : T | Nil
    _next
  end

  def prev : T | Nil
    _prev
  end

  protected def next=(value : T | Nil)
    @_next = value
  end

  protected def prev=(value : T | Nil)
    @_prev = value
  end
end

class Test
  property value : Int32

  include LinkedList(self)

  def initialize(@value)
  end
end

x = Test.new(5)
y = Test.new(6)
x.append(y)
p x.sum &.value