CodeIQ×はてな エンジニア夏祭り2013 和田 卓人さんからのテスト駆動開発の問題（https://codeiq.jp/ace/wada_takuto/q469）

codeiq_q469.rb

class Greeter
  def initialize(time_proc = -> { Time.now.strftime("%H:%M:%S") })
    @time_proc = time_proc
  end

  def greet
    time = @time_proc.call
    if '05:00:00' <= time && time < '12:00:00'
      'おはようございます'
    elsif '12:00:00' <= time && time < '18:00:00'
      'こんにちは'
    else
      'こんばんは'
    end
  end
end

require 'minitest/spec'
require 'minitest/autorun'

class MiniTest::Spec
  def self.at(time_str, &block)
    described_class = Class.const_get(describe_stack.last.name)
    wrapped_with_stub_time_proc = ->(&spec_proc) do
      self.class.singleton_class.class_eval do
        define_method(:greeter) do
          described_class.new(-> { "#{time_str}" })
        end
      end
      yield
    end
    it("at #{time_str}", &wrapped_with_stub_time_proc)
  end
end

class Object
  def must_greet(greeting)
    MiniTest::Spec.current.assert_equal(greeting, greet)
  end
end

describe Greeter do
  at('05:00:00') { greeter.must_greet 'おはようございます' }
  at('11:59:59') { greeter.must_greet 'おはようございます' }
  at('12:00:00') { greeter.must_greet 'こんにちは' }
  at('17:59:59') { greeter.must_greet 'こんにちは' }
  at('18:00:00') { greeter.must_greet 'こんばんは' }
  at('23:59:59') { greeter.must_greet 'こんばんは' }
  at('00:00:00') { greeter.must_greet 'こんばんは' }
  at('04:59:59') { greeter.must_greet 'こんばんは' }
end