shorturl.rb

module ShortURL
  CHARS = [*('a'..'z')] + [*('0'..'9')]
  def next_after(string)
    return CHARS[0] if string.empty?
    if nex = next_char(string[-1].chr)
      return string[0..-2] + nex
    end
    next_after(string[0..-2])+CHARS[0]
  end
  #nil if we need to increment. the next char otherwise
  def next_char(char)
    CHARS[1 + CHARS.index(char)]
  end
end