Tinyurl sinatra + datamapper

tinyurl.rb

#!/bin/env ruby
# encoding: utf-8
# Author: kimoto
require 'sinatra'
require 'erubis'
require 'dm-core'
require 'dm-migrations'
require 'dm-timestamps'

# Models
class ShortUrl
  # properties
  include DataMapper::Resource
  property :id, Serial
  property :digest, String, :length => 64, :unique_index => true
  property :url, String, :length => 1024, :unique => true
  timestamps :at

  # exceptions
  class MaxRetryError < StandardError; end

  # class methods
  MAX_RETRY = 100
  @@table = [
    ('0' .. '9'),
    ('a' .. 'z'),
    ('A' .. 'Z'),
    ["-", "_"]
  ].map(&:to_a).flatten.freeze

  # copied from http://lokka.kksg.net/bitly
  def self.conv64(i)
    digit = []
    begin
      i, c = i.divmod(62)
      digit << @@table[c+1]
    end while i > 0
    return digit.reverse.join('')
  end

  def self.gen_random_digest
    self.conv64(rand(256**4)) # 256**4 (4294967296) pattern 
  end

  def self.digest_to_url(digest)
    if r = self.first(:digest => digest)
      r.url
    else
      nil
    end
  end

  def self.url_to_digest(url)
    self.first_or_create(:url => url).digest
  end

  def digest_already_exist?
    if self.class.first(:digest => self.digest)
      return true
    else
      return false
    end
  end

  def gen_digest
    MAX_RETRY.times{ |n|
      self.digest = self.class.gen_random_digest
      if not digest_already_exist?
        return
      end
    }
    raise MaxRetryError
  end

  before :create do |short_url|
    short_url.gen_digest
  end
end

# Sinatra
configure do
  enable :inline_templates
  set :erb, :escape_html => true
  DataMapper.setup(:default, "mysql://127.0.0.1/tinyurl")
  DataMapper.finalize
  DataMapper.auto_upgrade!
end

helpers do
  def base_url
    @base_url ||= "#{request.env['rack.url_scheme']}://#{request.env['HTTP_HOST']}"
  end
end

get '/' do
  erb :index
end

get '/:digest' do
  if url = ShortUrl.digest_to_url(params[:digest])
    redirect url
  else
    not_found
  end
end

post '/new' do
  begin
    digest = ShortUrl.url_to_digest(params[:url])
    redirect URI.join(base_url, "/created/#{digest}").to_s
  rescue => RetryMaxError
    not_found
  end
end

get '/created/:digest' do
  URI.join(base_url, ShortUrl.first(:digest => params[:digest]).digest).to_s
end

not_found do
  "404 Not Found"
end

__END__
@@ index
<h1>shorturl</h1>
<form method="post" action="/new">
<input type="text" name="url" style="width: 500px;" value="" />
<input type="submit" value="create" />
</form>