KansaiWorkshop38's answer

stack.rb

class Stack

  class EmptyError < StandardError
  end

  class Node
    attr_accessor :prev, :data
    def initialize(prev, data)
      @prev = prev
      @data = data
    end
  end

  def initialize
    @cur = nil
  end

  def empty?
    @cur.nil?
  end

  def push(val)
    if @cur
      new = Node.new(@cur, val)
      @cur = new
    else
      @cur = Node.new(nil, val)
    end
    val
  end

  def pop
    raise EmptyError if empty?
    popped = @cur
    @cur = @cur.prev
    popped.data
  end

  def size
    count(@cur, 0)
  end

  private

  def count(node, memo)
    return memo if node.nil?
    if node.prev
      count(node.prev, memo+1)
    else
      memo + 1
    end
  end

end

test_stack.rb

require 'test/unit'
require 'stack'

class TestStack < Test::Unit::TestCase
  def setup
    @stack = Stack.new
  end

  def test_empty
    assert @stack.empty?
  end

  def test_size
    assert_equal 0, @stack.size
  end

  def test_empty_pop
    assert_raise(Stack::EmptyError){
      @stack.pop
    }
  end

  def test_push_and_pop
    @stack.push(3)
    assert_equal 3, @stack.pop
  end

  def test_push_and_size
    @stack.push(3)
    assert_equal 1, @stack.size
  end

  def test_push_push_and_size
    @stack.push(3)
    @stack.push(5)
    assert_equal 2, @stack.size
  end

  def test_push_and_empty
    @stack.push(3)
    assert !@stack.empty?
  end

  def test_push_push_pop_and_size
    @stack.push(3)
    @stack.push(5)
    @stack.pop
    assert_equal 1, @stack.size
  end

  def test_push_push_and_pop
    @stack.push(3)
    @stack.push(5)
    assert_equal 5, @stack.pop
  end

end