Testing recursion using mocking/stubbing.

factorial.rb

class Factorial
  def execute(n, method = method(:execute))
    n.zero? ? 1 : n * method.call(n - 1, method)
  end
end

factorial_test.rb

require 'bundler/setup'
require 'minitest/unit'
require 'mocha/mini_test'
require 'minitest/autorun'

class FactorialTest < MiniTest::Unit::TestCase
  def setup
    @factorial = Factorial.new
    @factorial.stubs(:recurse)
    @recursive_method = @factorial.method(:recurse)
  end

  def test_execute_returns_one_when_number_reaches_zero
    assert_equal 1, @factorial.execute(0)
  end

  def test_execute_does_not_recurse_when_number_reaches_zero
    @factorial.expects(:recurse).never
    @factorial.execute(0, @factorial.method(:recurse))
  end

  def test_execute_does_recurse_when_number_is_greater_than_zero
    @factorial.expects(:recurse).with(0, anything).returns(1)
    @factorial.execute(1, @recursive_method)
  end

  def test_execute_passes_recursive_method_when_number_is_greater_than_zero
    @factorial.expects(:recurse).with(anything, @recursive_method).returns(1)
    @factorial.execute(1, @recursive_method)
  end

  def test_execute_returns_result_of_recursive_method_when_number_is_greater_than_zero
    @factorial.expects(:recurse).returns(1)
    assert_equal 1, @factorial.execute(1, @recursive_method)
  end
end