Longest palindrome

longest_palindrome.rb

def longest_palindrome str
  palindromes = []
  str.split('').each.with_index do |c, i|
    0.upto i do |j|
      # get all substrings up to the current letter
      chunk = str[j..i]
      reverse_chunk = chunk.reverse
      # check if they're a palindrome
      palindromes << chunk if chunk == reverse_chunk
    end
  end
  palindromes.max_by(&:length)
end

if __FILE__ == $0
  require 'test/unit'

  class TestLongestPalindrome < Test::Unit::TestCase

    def test_simple
      assert_equal('aba', longest_palindrome('aba'))
    end

    def test_two
      assert_equal('aba', longest_palindrome('abaa'))
    end

    def test_three
      assert_equal('anana', longest_palindrome('banana'))
    end

    def test_four
      assert_equal('racecar', longest_palindrome('racecar'))
    end

    def test_five
      assert_equal('aba', longest_palindrome('cabad'))
    end

    def test_six
      assert_equal('fuf', longest_palindrome('qwertyuiopfuflkjhgfdsamnbvcxz'))
    end

  end
end

I won't be winning any awards for efficiency, but it works!