Ruby Stack Implementation With Linked List

linked_list_stack_impl.rb

# Mustafa Turan
# coded at 30.11.2015 with NO LICENSE
# Feel free to use/modify/distribute/reimplement

# node.rb
class Node
  attr_accessor :node_ref
  attr_reader :val

  def initialize(node_ref, val)
    @node_ref = node_ref
    @val = val
  end
end

# stack.rb
class Stack
  attr_reader :init_node
  def push(val)
    @init_node = Node.new(@init_node, val) 
  end

  def pop
    if @init_node
      @init_node, pop_val = @init_node.node_ref, @init_node.val
      return pop_val
    end
    raise StandardError, 'Stack is empty!'
  end

  def top
    @init_node.val
  end
end


# stack_spec.rb
require 'spec_helper'

describe Stack do
  let(:stack) { Stack.new }

  describe '#push' do
    it 'adds new node to stack' do
      expect(stack.init_node).to eql nil
      node_1 = Node.new(nil, 1)
      node_2 = Node.new(node_1, 2)
      node_3 = Node.new(node_2, 3)
      expect(stack.push(1).val).to eql node_1.val
      expect(stack.push(2).node_ref.val).to eql node_2.node_ref.val
      expect(stack.push(3).val).to eql node_3.val
    end
  end

  describe '#pop' do
    it 'pops always last element' do
      stack.push 1
      stack.push 2
      stack.push 3
      expect(stack.pop).to eql 3
      expect(stack.pop).to eql 2
      expect(stack.pop).to eql 1
    end

    it 'can not pop when stack empty' do
      expect { stack.pop }.to raise_error(StandardError)
      stack.push 1
      expect(stack.pop).to eql 1
      expect { stack.pop }.to raise_error(StandardError)
    end
  end
  
  describe '#top' do
    it 'return always last element' do
      stack.push 1
      expect(stack.top).to eql 1
      stack.push 2
      expect(stack.top).to eql 2
      stack.pop
      expect(stack.top).to eql 1
    end
  end
end

hey @mustafaturan, how about slightly different naming for variables and also using Struct to define Node's structure rather than class?

# MIT license
Node = Struct.new(:next, :data)

class Stack
  def push(data)
    @head = Node.new(@head, data)
  end

  def pop
    if @head.nil?
      return
    end

    data = @head.data
    @head = @head.next
    return data
  end

  def head
    @head.data
  end
end

require 'minitest/autorun'
class StackTest < Minitest::Test
  def setup
    @stack = Stack.new
  end

  def test_stack_push_1
    @stack.push('first')
    assert_equal 'first', @stack.head
  end

  def test_stack_push_2
    @stack.push('first')
    @stack.push('second')
    assert_equal 'second', @stack.head
  end

  def test_stack_pop_1
    assert_nil @stack.pop
  end

  def test_stack_pop_2
    @stack.push('first')
    assert_equal 'first', @stack.pop
  end

  def test_stack_pop_3
    @stack.push('first')
    @stack.push('second')
    assert_equal 'second', @stack.pop
  end
end