alpha_id.rb

# TODO: extract to gem, w/ configurable index

class AlphaId
  INDEX = %w[a b c d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z]
  BASE  = INDEX.size
  
  def self.encode(int)
    return INDEX.first if int.zero?
    
    res = ''
    
    start = (Math.log(int) / Math.log(BASE)).floor
    start.downto(0) do |n|
      idx = (int / BASE ** n) % BASE
      res << INDEX.fetch(idx)
    end
    
    res.reverse
  end
  
  def self.decode(str)
    chars = str.split(//).reverse
    length = chars.size

    res = 0
    
    chars.each_with_index do |char, i|
      res += INDEX.index(char) * (BASE ** (length - i - 1))
    end
    
    res
  end
end