Minitesting

reverse_str.rb

def reverse_str(str)
  reversed = ''
  i = str.length - 1
  until i == -1 do
    reversed << str[i]
    i -= 1
  end
  reversed
end

def reverse_str2(str)
  str.reverse
end

test_reverse_str.rb

require 'minitest/autorun'
require 'minitest/benchmark'

require_relative 'reverse_str'

module SharedAsserts
  def assert_string_reverse(input,expected)
    assert_equal expected, reverse_str(input)
  end
end

class TestReverseStr < Minitest::Test
  include SharedAsserts

  def test_ab
    assert_string_reverse 'ab','ba' 
  end

  def test_bat
    assert_string_reverse 'tab', 'bat'
  end

  def test_racercar
    assert_string_reverse 'racercar', 'racrecar'
  end
end

class TestReverseStr2 < Minitest::Test
  include SharedAsserts

  def test_ab
    assert_string_reverse 'ab','ba' 
  end

  def test_bat
    assert_string_reverse 'tab', 'bat'
  end

  def test_racercar
    assert_string_reverse 'racercar', 'racrecar'
  end
end

class TestReverseStrBenchmark < Minitest::Benchmark
  bench_range { bench_exp 1, 10_000 }

  def bench_reverse_str
    assert_performance_linear do |n|
      n.times { reverse_str string_for_bench }
    end
  end

  def bench_reverse_str2
    assert_performance_linear do |n|
      n.times { reverse_str2 string_for_bench }
    end
  end

  private

    def string_for_bench
      [*'A'..'Z'].sample(10).join
    end
end