require 'nokogiri'
require 'pstore'
 
 
#
#  Rack::DomainSprinkler
#
#  Modifies outgoing HTML markup so that common static assets like script source
#  files, images, and stylesheets will be served from one of a number of domains
#  rather than just a single one in order to improve parallelization of resource
#  downloads during page loads.
#
module Rack
  class DomainSprinkler
 
    #
    #  Options:
    #    A list of domains across which to spread requests
    #    The name of the file to be used for caching URL/path mappings
    #
    def initialize(app, domains=[], cache_loc="tmp/sprinklins.pstore")
      @app = app
      @@domains = domains
      @@url_cache = PStore.new(cache_loc)
    end
 
 
    def call(env)
      dup._call(env)
    end
 
 
    #
    #  Currently only looks for <link>, <script>, and <img> tags.
    #  Should be refactored to provide more generic mapping.
    #
    def _call(env)
      status, headers, response = @app.call(env)
      if headers["Content-Type"] =~ /^text\/html/
        document = Nokogiri::HTML(response.body)
        
        sprinkle(env, document, 'link', 'href')
        sprinkle(env, document, 'script', 'src')
        sprinkle(env, document, 'img', 'src')
        
        response.body = document.to_html
      end
 
      [status, headers, response]
    end
 
 
    def sprinkle(env, document, selector, property)
      document.search(selector).each do |node|
        url = node[property]
        if !url.empty? and url !~ /^\#/ and url !~ /^http\:\/\//
          node[property] = domainify(url, env)
        end
      end
    end
 
 
    def domainify(path, env)
      if path !~ /\//
        path = absolutify(path)
      end
      
      full_url = nil
      @@url_cache.transaction do
        unless @@url_cache[path]
          domain = @@domains.shift and @@domains.push(domain)
          @@url_cache[path] = "http://#{ domain }#{ path }"
        end
        full_url = @@url_cache[path]
      end
 
      full_url
    end
 
 
    def absolutify(path, env)
      path_prefix = env['REQUEST_PATH']
      unless path_prefix =~ /\/$/
        path_prefix.sub!(/\/[^\/]+$/, '')
      end
 
      "#{ tmp_path }/#{ path }"
    end
 
  end
end