String decoder in Ruby

decoder.rb

# build a hash that links number to char, so { "1" => "a", "2" => "b", ... }
CODES = (0..25).reduce Hash.new do |memo, code|
  memo[(code + 1).to_s] = (code + 'a'.ord).chr
  memo
end

# recursive method
def decode(str)

  # terminal case if the string given is empty
  return [] if str.empty?

  results = []

  # loop over each pair and see if the string begins with each char code
  CODES.each_pair do |key, value|
    if str.start_with?(key) && !str.start_with?(key + "0")

      # get a new string without the leading keycode
      new_str = str.gsub(/^#{key}/, '')

      if new_str.empty?
        # if the new string is empty, this is a valid solution
        results << value
      else
        # otherwise, decode the new string and each one of those is a correct solution
        decode(new_str).each do |possability|
          results << (value + possability)
        end
      end
    end
  end

  results
end

# tdd ftw
raise "wrong" unless decode('105') == ["je"]
raise "wrong" unless decode('2175') == ['bage', 'bqe', 'uge']
raise "wrong" unless decode('2222') == ['bbbb','bbv','bvb','vbb','vv']
raise "wrong" unless decode('0000') == []

And in python: https://gist.github.com/3379979
:-)