quick_factorial.rb

require 'test/unit'

def factorial(max)
  (1..max).inject(1) { |sum, current| sum *= current; sum }
end

class FactorialTest < Test::Unit::TestCase

  def test_factorial_0_is_1
    assert_equal 1, factorial(0)
  end

  def test_factorial_1_is_1
    assert_equal 1, factorial(1)
  end

  def test_factorial_2_is_2
    assert_equal 2, factorial(2)
  end

  def test_factorial_3_is_6
    assert_equal 6, factorial(3)
  end

  def test_factorial_4_is_24
    assert_equal 24, factorial(4)
  end

end