Enumerable#longest_common_prefix

longest_common_prefix.rb

module Enumerable
  def longest_common_prefix
    common = first
    self[1..-1].each do |item|
      length_to_test = [item.length, common.length].min
      while length_to_test > 0
        if item[0..(length_to_test-1)] === common[0..(length_to_test-1)]
          common = common[0..(length_to_test-1)]
          break
        end
        length_to_test -= 1
        return nil if length_to_test == 0
      end
    end
    common
  end
end