Encode/decode numbers as tiny strings

tiny_id.rb

class TinyId
  # All characters that URL encode to 1 character in size, minus the
  # period which requires special Rails routing:
  #   (0..255).map(&:chr).select { |c| ERB::Util::u(c).size == 1 } - ['.']
  RAILS_URL = %w(- 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 _ 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)

  # All BSON encodable string characters
  BSON_STRING = (1..127).map(&:chr)

  def initialize(alphabet)
    @alphabet = alphabet
    @size     = @alphabet.size
  end

  # Encodes an integer as a TinyId.
  def encode(integer)
    remainder = integer.to_i
    output = ''
    loop do
      output = @alphabet[remainder % @size] + output
      remainder = remainder / @size
      break if remainder == 0
    end
    output
  end

  # Decodes a TinyId encoded string.
  def decode(string)
    value = 0
    string.reverse.split(//).each_with_index do |c, i|
      value += @alphabet.index(c) * @size ** i
    end
    value
  end
end