require 'rack'

class MixpanelMiddleware
  def initialize(app, mixpanel_token)
    @app = app
    @token = mixpanel_token
  end

  def call(env)
    @env = env

    @status, @headers, @response = @app.call(env)

    update_response!
    update_content_length!
    delete_event_queue!

    [@status, @headers, @response]
  end

  private

  def update_response!
    @response.each do |part|
      if is_regular_request? && is_html_response?
        part.gsub!("</head>", "#{render_mixpanel_scripts}</head>")
        part.gsub!("</head>", "#{render_event_tracking_scripts}</head>")
      elsif is_ajax_request? && is_html_response?
        part.gsub!(part, render_event_tracking_scripts + part)
      elsif is_ajax_request? && is_javascript_response?
        part.gsub!(part, render_event_tracking_scripts(false) + part)
      end
    end
  end

  def update_content_length!
    new_size = 0
    @response.each{|part| new_size += part.bytesize}
    @headers.merge!("Content-Length" => new_size.to_s)
  end

  def is_regular_request?
    !is_ajax_request?
  end

  def is_ajax_request?
    @env.has_key?("HTTP_X_REQUESTED_WITH") && @env["HTTP_X_REQUESTED_WITH"] == "XMLHttpRequest"
  end

  def is_html_response?
    @headers["Content-Type"].include?("text/html") if @headers.has_key?("Content-Type")
  end

  def is_javascript_response?
    @headers["Content-Type"].include?("text/javascript") if @headers.has_key?("Content-Type")
  end

  def render_mixpanel_scripts
    <<-EOT
  <script type='text/javascript'>
    var mp_protocol = (('https:' == document.location.protocol) ? 'https://' : 'http://');
    document.write(unescape('%3Cscript src="' + mp_protocol + 'api.mixpanel.com/site_media/js/api/mixpanel.js" type="text/javascript"%3E%3C/script%3E'));
  </script>
  <script type='text/javascript'>
    try {
      var mpmetrics = new MixpanelLib('#{@token}');
    } catch(err) {
      null_fn = function () {};
      var mpmetrics = {
        track: null_fn,  track_funnel: null_fn,  register: null_fn,  register_once: null_fn, register_funnel: null_fn
      };
    }
  </script>
    EOT
  end

  def delete_event_queue!
    @env.delete('mixpanel_events')
  end

  def queue
    return [] if !@env.has_key?('mixpanel_events') || @env['mixpanel_events'].empty?
    @env['mixpanel_events']
  end

  def render_event_tracking_scripts(include_script_tag=true)
    return "" if queue.empty?

    output = queue.map {|event| %(mpmetrics.track("#{event[:event]}", #{event[:properties].to_json});) }.join("\n")

    output = "try {#{output}} catch(err) {}"

    include_script_tag ? "<script type='text/javascript'>#{output}</script>" : output
  end
end