Encoding/decoding functions for a URL Shortner -- in Ruby

url_shortener.rb

module UrlShortener
  class << self
    
    CODESET = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
    BASE    = CODESET.length # 62
    
    def encode(id)
      hash = ''
      while id > 0
        hash = CODESET[id % BASE].chr + hash
        id = (id.to_f / BASE.to_f).floor
      end
      hash
    end
    
    def decode(hash)
      id = 0
      hash.chars.each_with_index do |c,i|
        n = CODESET.index(c)
        return if n < 0 # invalid codeset
        id += n * (BASE ** (hash.length-i-1))
      end
      id
    end
    
  end
end

__END__

Usage:

Normally a web resource is referenced by a unique integer as in: http://site.com/posts/300.
The methods above simply encode the integer to base 62 with the codeset defined above.
It's the same thing as converting base 10 to base 16 (hex), but in this case we're converting
base 10 to our own base 62.

UrlShortener.encode(300)
=> "4Q"

UrlShortener.decode("4Q")
=> 300

UrlShortener.encode(1000000)
=> "4c92"