linked_list.rb

class LinkedList
  include Enumerable

  def initialize(options)
    @sorted = options.delete(:sorted) || false
    @double = options.delete(:double) || false

    @behaviour = options.delete(:behaviour)
    @behaviour = :stack unless [:stack, :queue].include?(@behaviour)

    @head = nil
    @tail = nil
  end

  def each
    curr = @head
    begin yield curr.data end until (curr = curr.nxt).nil?
  end

  def unsorted_add(item)
    @head = Node.new(item,@head)
  end
    
  def sorted_add(item)
    if @head.nil? or (item < @head.data)
      @head = Node.new(item, @head)
    else 
      curr = @head
      until (curr.nxt.nil? or (item < curr.nxt.data)) do
        curr = curr.nxt
      end
      curr.nxt = Node.new(item, curr.nxt)
    end
  end

  def double_unsorted_add(item)
    if @behaviour == :stack
      @head = Node.new(item,@head,nil)
      @head.nxt.prv = @head if @head.nxt
    elsif @behaviour == :queue
      @tail = Node.new(item, nil, @tail)
      @tail.prv.nxt = @tail if @tail
    end
  end

  def double_sorted_add(item)
    raise NotImplementedError, "Double Sorted Linked Lists are not supported"
  end
  
  def add(item)
    if @double
      @sorted ? double_sorted_add(item) : double_unsorted_add(item)
    else
      @sorted ? sorted_add(item) : unsorted_add(item)
    end
  end

  def pop(side=:front)
    if side == :front
      value = @head.data
      @head = @head.nxt
      @head.prv = nil if @head
      return value
    elsif side == :back
      raise TypeError, "Won't remove from tail of a single LL" unless @double
      value = @tail.data
      @tail = @tail.prv
      @tail.nxt = nil if @tail
      return value
    end
    
  end
    
end