calculator.rb

module Operations
  def self.list
    @list ||= []
  end

  class Base
    def self.inherited(klass)
      Operations.list << klass
    end

    def initialize(calculator)
      @calculator = calculator
    end

    def x
      @calculator.get(:x).to_f
    end

    def y
      @calculator.get(:y)
    end

    def equals
      result % 1 == 0 ? result.to_i : result
    end
  end

  class Sum < Base
    def self.key
      "s"
    end

    def self.description
      "Add values"
    end

    def result
      x + y
    end
  end

  class Quotient < Base
    def self.key
      "q"
    end

    def self.description
      "Divide values"
    end

    def result
      x / y
    end
  end

  class Product < Base
    def self.key
      "p"
    end

    def self.description
      "Multiply values"
    end

    def result
      x * y
    end
  end

  class Difference < Base
    def self.key
      "d"
    end

    def self.description
      "Subtract values"
    end

    def result
      x - y
    end
  end
end

class Calculator
  include Operations

  def initialize
    @values = {}
  end

  def run
    prompt(:x, 'First value: ')
    prompt(:y, 'Second value: ')

    say "These are your options:"

    Operations.list.each do |operation|
      say "To #{operation.to_s}, type '#{operation.key}'"
    end

    perform prompt(:operation, 'Choose an option: ')
  end

  def perform(key)
    if op = Operations.list.detect { |operation| operation.key == key }
      result = op.new(self).equals
      say "Result is #{result}"
      return result
    end
  end

  def get(key)
    @values[key].to_i
  end

  def set(key, val)
    @values[key] = val
  end

  def prompt(key, description="Enter a number: ")
    say description
    @values[key] = STDIN.gets.chomp
  end

  private

  def say(something)
    STDOUT.puts(something)
  end
end

Calculator.new.run if __FILE__ == $0

calculator_spec.rb

require 'rubygems'
require 'spec'
require 'rr'
require File.join(File.dirname(__FILE__), 'calculator')

Spec::Runner.configure do |c|
  c.mock_with(:rr)
end

describe Calculator do
  before(:each) do
    stub(STDOUT).puts(anything)
    stub(STDIN).gets.returns('1')
    @calculator = Calculator.new
  end

  it "prompts for values" do
    mock(STDIN).gets.returns('1')
    result = @calculator.prompt(:x, 'Enter first number')
    result.should == '1'
    @calculator.get(:x).to_s.should == '1'
  end

  it "returns values" do
    mock(STDIN).gets.returns('3')
    @calculator.prompt(:foo)
    @calculator.get(:foo).to_s.should == '3'
  end

  it "typecasts values" do
    mock(STDIN).gets.returns('3')
    @calculator.prompt(:y)
    @calculator.get(:y).should == 3
  end

  describe "sum" do
    it "adds x and y" do
      @calculator.set(:x, '1')
      @calculator.set(:y, '2')
      @calculator.perform('s').should == 3
    end
  end

  describe "quotient" do
    it "finds quotient" do
      @calculator.set(:x, '4')
      @calculator.set(:y, '2')
      @calculator.perform('q').should == 2
    end
  end

  describe "product" do
    it "multiplies x and y" do
      @calculator.set(:x, 3)
      @calculator.set(:y, 4)
      @calculator.perform('p').should == 12
    end
  end

  describe "difference" do
    it "subtracts y from x" do
      @calculator.set(:x, 10)
      @calculator.set(:y, 3)
      @calculator.perform('d').should == 7
    end
  end
end