longest_palindrome_finder.rb

class PalindromeFinder
  def initialize(string)
    @input = string
  end

  def longest
    max, out = 0, ''
    @input.chars.to_a.each_index do |i|
      index, length = max_palindrome(i)
      max, out = [length, @input[index, length]] if length > max
    end
    out
  end

  private

  init_max = {"even" => 0, "odd" => 1}
  init_b   = {"even" => 1, "odd" => 2}
  %w(even odd).each do |type|
    define_method "max_#{type}" do |a, b, max|
      max ||= init_max[type]
      b   ||= a + init_b[type]
      if @input[a] == @input[b] && a > 0 && b <= @input.size-1
        send "max_#{type}", a-1, b+1, max + 2
      elsif @input[a] == @input[b]
        [a, max + 2]
      elsif @input[a] != @input[b]
        [a+1, max]
      end
    end
  end

  def max_palindrome(i)
    [max_even(i, nil, nil), max_odd(i, nil, nil)].max{ |a,b| a.last <=> b.last }
  end
end