#rubykansai 第71回のハンズオンでやった Stackを実装する課題をなんとなくイミュータブルに実装してみた

Gemfile

ruby "2.3.0"

gem "test-unit"

mystack.rb

class MyStack
  def self.empty
    MyStack.new(nil)
  end
  def initialize head, next_stack = nil
    if head == nil
      @elem = head
      @next = nil
    else
      raise StandardError.new if (not next_stack.is_a?(MyStack)) && next_stack != nil
      @elem = head
      @next = next_stack
    end
  end

  def empty?
    @elem == nil
  end

  def size
    if empty? then
      0
    else
      1 + @next.size
    end
  end

  def push val
    MyStack.new(val, self)
  end

  def pop
    raise EmptyStackError.new if @elem == nil
    @next
  end

  def pop2
    {head: @elem, tail: pop}
  end

  class EmptyStackError < StandardError
  end
end

test_mystack.rb

require 'test/unit'
require_relative 'mystack'

class TestMyStack < Test::Unit::TestCase
  def setup
    @empty = MyStack.empty
    @empty.freeze
  end
  def test_new
    assert_raise(StandardError) do
      MyStack.new(1, 2)
    end
    MyStack.new(1)
  end
  def test_epmty_by_empty_mystack
    assert_equal @empty.empty?, true
  end
  def test_epmty_by_non_mystack
    assert_equal MyStack.new(3).empty?, false
  end
  def test_empty_by_nonempty_mystack
    stack = MyStack.new(3)
    assert stack.empty? == false, "non epmpty stack must return false"
  end
  def test_size_by_mystack
    assert_equal @empty.size, 0

    oneStack = @empty.push(3)
    assert_equal oneStack.size, 1

    twoStack = oneStack.push(5)
    assert_equal twoStack.size, 2
  end
  def test_pop_by_empty_mystack
    assert_raise(MyStack::EmptyStackError) do
      @empty.pop
    end
  end
  def test_pop_by_mystack
    stack = @empty.push(3).push(5)

    assert_equal stack.pop2[:head], 5
    assert_equal stack.pop2[:tail].pop2[:head], 3
  end
end