class Async::List

A double linked list.

def delete(item) 

def delete(item)
	if @head.equal?(item)
		@head = @head.tail
	else
		item.head.tail = item.tail
	end
	
	if @tail.equal?(item)
		@tail = @tail.head
	else
		item.tail.head = item.head
	end
	
	item.head = nil
	item.tail = nil
	
	@size -= 1
	
	return self
end

def each 

def each
	return to_enum unless block_given?
	
	item = @head
	while item
		# We store the tail pointer so we can remove the current item from the linked list:
		tail = item.tail
		yield item
		item = tail
	end
end

def empty? 

def empty?
	@head.nil?
end

def first 

def first
	@head
end

def include?(needle) 

def include?(needle)
	self.each do |item|
		return true if needle.equal?(item)
	end
	
	return false
end

def initialize 

def initialize
	@head = nil
	@tail = nil
	@size = 0
end

def insert(item)

Inserts an item at the end of the list. 
def insert(item)
	unless @head
		@head = item
		@tail = item
		
		# Consistency:
		item.head = nil
		item.tail = nil
	else
		@tail.tail = item
		item.head = @tail
		
		# Consistency:
		item.tail = nil
		
		@tail = item
	end
	
	@size += 1
	
	return self
end

def last 

def last
	@tail
end

def nil? 

def nil?
	@head.nil?
end