Palindrom searcher class

palindrom_searcher.rb

class PalindromSearcher
  attr_accessor :string

  def initialize(string)
    @string = string
  end

  def longest
    return unless all.any?
    all.max_by(&:length)
  end

  def all
    words & reverse_words
  end

  private

  def words
    substrings(string)
  end

  def reverse_words
    substrings(string.reverse)
  end

  def substrings(string)
    return if string.nil?
    return if string.empty?
    substrings = []
    for first_char in 1..(string.length - 1)
      for last_char in (first_char + 1)..string.length
        substrings << string[(first_char - 1)..(last_char - 1)]
      end
    end
    substrings
  end
end

puts PalindromSearcher.new('xyzzy').longest # yzzy
puts PalindromSearcher.new('afbbbfjdjklgdfdhfdkjfffhhfffjkdfhdhkyejejfjkd').longest # dhfdkjfffhhfffjkdfhd
puts PalindromSearcher.new('bartarcarracecarbartar').longest # racecar
puts PalindromSearcher.new('1415926535897932384626433832795028841971').longest # 46264