Rack Middleware to generate short URLs for a model

shrtn.rb

# drop in /lib in a rails app
# replace MODEL with the model to run against
# enable in environment.rb with:
#   config.middleware.use 'Shrtn::Middleware'
module Shrtn
  class Helper
    include Singleton
    
    def initialize
      @chars = ('A'..'Z').to_a + ('a'..'z').to_a + ('0'..'9').to_a + ['-','_']
      @chars -= %w(a e i o u A E I O U)  # remove vowels to prevent actual (rude) words
      @base = @chars.size
    end

    def generate_id id
      localCount = id
      result = ''

      while localCount != 0
        rem = localCount % @base
        localCount = (localCount - rem) / @base
        result = @chars[rem] + result
      end
      result
    end
    
    def parse_id str
      id = 0
      count = 0
      str.reverse.each_char do |n|
        id += (@base**count) * @chars.index(n)
        count += 1
      end
      id
    end
  end
  
  class Middleware
    include ActionController::UrlWriter
    def initialize app
      @app = app
    end
  
    def call env
    
      if env['PATH_INFO'] =~ /^\/s\/(.*)/
        id = Shrtn::Helper.instance.parse_id($1)
        if id.nil? || (obj = MODEL.find_by_id(id)).nil?
          [404, {'Content-type' => 'text/plain'}, 'Not Found']
        else
          [301, {'Location' => MODEL_path(obj, :only_path => true) }, '']
        end
      else
        @app.call env
      end
    
    end
  end
end