Linked List Queue

queue.rb

class Queue
  def initialize
    @first = nil
    @last = nil
    @length = 0
  end

  def push(element)
    node = Node.new
    node.value = element
    @length += 1

    if @first.nil?
      @first = node
    else
      @last.next = node
    end
    @last = node
  end

  def pop
    node = @first
    @length -= 1

    @first = @first.next

    node.value
  end

  def peek
    @first.value
  end

  def size
    @length
  end

  def empty?
    @first.nil?
  end
end

class Node
  attr_accessor :next
  attr_accessor :value
end

queue_spec.rb

require File.join(File.dirname(__FILE__), 'queue')

describe Queue do
  before(:each) do
    @queue = Queue.new
  end

  it "should be able to push things on the queue" do
    @queue.push(1)

    @queue.size.should == 1
  end

  it "should pop of the first item on the queue" do
    @queue.push(1)
    @queue.push(2)

    @queue.pop.should == 1
    @queue.size.should == 1
  end

  it "should be able to get the next element without removing it" do
    @queue.push(1)

    @queue.peek.should == 1
    @queue.size.should == 1
  end

  it "should know if it's empty" do
    @queue.should be_empty

    @queue.push(1)
    @queue.pop

    @queue.should be_empty

    @queue.push(1)

    @queue.should_not be_empty
  end

  it "should not adjust the length of the internal array if not needed" do
    @queue.push(1)
    @queue.push(2)
    @queue.push(3)
    @queue.push(4)
    @queue.push(5)

    @queue.elements.size.should == 5

    @queue.pop.should == 1
    @queue.pop.should == 2

    @queue.elements.size.should == 3

    @queue.push(6)
    @queue.push(7)
    @queue.push(8)

    @queue.elements.size.should == 6

    @queue.pop.should == 3
    @queue.pop.should == 4
    @queue.pop.should == 5
    @queue.pop.should == 6
    @queue.pop.should == 7
    @queue.pop.should == 8

    @queue.push(9)
    @queue.pop.should == 9

    @queue.elements.size.should == 0
  end
end

Not sure if all these specs work for this, I may have changed them since this implementation.