An algorithm to find all / the largest palindrome in a string.

palindromes.rb

string = '111sjhfb1234321asdjkhbfhhjjhhkik'

long_palindromes = []

(0..string.length-1).each do |i|
  # xyx
  xyx0 = i < 1 ? nil : string[i-1]
  xyx1 = string[i]
  xyx2 = string[i+1]
  # xx
  xx0 = string[i]
  xx1 = string[i+1]
  [:xyx, :xx].each do |m|
    case m
    when :xyx
      next unless xyx0 && xyx2 # if we're out of bounds
      next unless xyx0 == xyx2
      # palindrome :)
      matched = 1
      (2..1/0.0).each do |j|
        b = i-j < 0 ? nil : string[i-j]
        a = string[i+j]
        break if a.nil? || b.nil?
        break unless a == b
        matched = j
      end
      long_palindromes << string[i-matched..i+matched]
    when :xx
      next unless xx1 # if we're out of bounds
      next unless xx0 == xx1
      # palindrome :)
      matched = 1
      (2..1/0.0).each do |j|
        b = i-j < 0 ? nil : string[i-j]
        a = string[i+j-1]
        break if a.nil? || b.nil?
        break unless a == b
        matched = j
      end
      long_palindromes << string[i-matched..i+matched-1]
    end
  end
end

longest = long_palindromes.sort {|a,b| a.length <=> b.length }.last
p long_palindromes, longest