Tests and code in same file

example.txt

minitest-same-file $ ruby fib.rb
minitest-same-file $ ruby -rminitest/autorun fib.rb
Run options: --seed 15401

# Running:

.

Finished in 0.000960s, 1041.6668 runs/s, 1041.6668 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

fib.rb

class Fib
  def initialize
    @cache = {}
  end

  def call(i)
    @cache[i] ||= _call(i)
  end

  def _call(i)
    if i <= 1
      i
    else
      call(i-1) + call(i-2)
    end
  end

  class FibTest < Minitest::Test
    def test_fib
      fib = Fib.new
      assert_equal 8, fib.call(6)
    end
  end if defined?(Minitest)
end