<?php

/**
 * @file
 * Drupal Module: Google Analytics
 *
 * Adds the required Javascript to all your Drupal pages to allow tracking by
 * the Google Analytics statistics package.
 *
 * @author: Alexander Hass <http://drupal.org/user/85918>
 */

/**
 * Define the default file extension list that should be tracked as download.
 */
define('GOOGLEANALYTICS_TRACKFILES_EXTENSIONS', '7z|aac|arc|arj|asf|asx|avi|bin|csv|doc(x|m)?|dot(x|m)?|exe|flv|gif|gz|gzip|hqx|jar|jpe?g|js|mp(2|3|4|e?g)|mov(ie)?|msi|msp|pdf|phps|png|ppt(x|m)?|pot(x|m)?|pps(x|m)?|ppam|sld(x|m)?|thmx|qtm?|ra(m|r)?|sea|sit|tar|tgz|torrent|txt|wav|wma|wmv|wpd|xls(x|m|b)?|xlt(x|m)|xlam|xml|z|zip');

/**
 * Define default path exclusion list to remove tracking from admin pages,
 * see http://drupal.org/node/34970 for more information.
 */
define('GOOGLEANALYTICS_PAGES', "admin\nadmin/*\nbatch\nnode/add*\nnode/*/*\nuser/*/*");

/**
 * Advertise the supported google analytics api details.
 */
function googleanalytics_api() {
  return array(
    'api' => 'analytics.js',
  );
}

/**
 * Implements hook_help().
 */
function googleanalytics_help($path, $arg) {
  switch ($path) {
    case 'admin/config/system/googleanalytics':
      return t('<a href="@ga_url">Google Analytics</a> is a free (registration required) website traffic and marketing effectiveness service.', array('@ga_url' => 'http://www.google.com/analytics/'));
  }
}

/**
 * Implements hook_theme().
 */
function googleanalytics_theme() {
  return array(
    'googleanalytics_admin_custom_var_table' => array(
      'render element' => 'form',
    ),
  );
}

/**
 * Implements hook_permission().
 */
function googleanalytics_permission() {
  return array(
    'administer google analytics' => array(
      'title' => t('Administer Google Analytics'),
      'description' => t('Perform maintenance tasks for Google Analytics.'),
    ),
    'opt-in or out of tracking' => array(
      'title' => t('Opt-in or out of tracking'),
      'description' => t('Allow users to decide if tracking code will be added to pages or not.'),
    ),
    'use PHP for tracking visibility' => array(
      'title' => t('Use PHP for tracking visibility'),
      'description' => t('Enter PHP code in the field for tracking visibility settings.'),
      'restrict access' => TRUE,
    ),
  );
}

/**
 * Implements hook_menu().
 */
function googleanalytics_menu() {
  $items['admin/config/system/googleanalytics'] = array(
    'title' => 'Google Analytics',
    'description' => 'Configure tracking behavior to get insights into your website traffic and marketing effectiveness.',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('googleanalytics_admin_settings_form'),
    'access arguments' => array('administer google analytics'),
    'type' => MENU_NORMAL_ITEM,
    'file' => 'googleanalytics.admin.inc',
  );

  return $items;
}

/**
 * Implements hook_page_alter() to insert JavaScript to the appropriate scope/region of the page.
 */
function googleanalytics_page_alter(&$page) {
  global $user;

  $id = variable_get('googleanalytics_account', '');

  // Get page status code for visibility filtering.
  $status = drupal_get_http_header('Status');
  $trackable_status_codes = array(
    '403 Forbidden',
    '404 Not Found',
  );

  // 1. Check if the GA account number has a valid value.
  // 2. Track page views based on visibility value.
  // 3. Check if we should track the currently active user's role.
  // 4. Ignore pages visibility filter for 404 or 403 status codes.
  if (preg_match('/^UA-\d+-\d+$/', $id) && (_googleanalytics_visibility_pages() || in_array($status, $trackable_status_codes)) && _googleanalytics_visibility_user($user)) {

    $debug = variable_get('googleanalytics_debug', 0);
    $url_custom = '';

    // Add link tracking.
    $link_settings = array();
    if ($track_outbound = variable_get('googleanalytics_trackoutbound', 1)) {
      $link_settings['trackOutbound'] = $track_outbound;
    }
    if ($track_mailto = variable_get('googleanalytics_trackmailto', 1)) {
      $link_settings['trackMailto'] = $track_mailto;
    }
    if (($track_download = variable_get('googleanalytics_trackfiles', 1)) && ($trackfiles_extensions = variable_get('googleanalytics_trackfiles_extensions', GOOGLEANALYTICS_TRACKFILES_EXTENSIONS))) {
      $link_settings['trackDownload'] = $track_download;
      $link_settings['trackDownloadExtensions'] = $trackfiles_extensions;
    }
    if ($track_domain_mode = variable_get('googleanalytics_domain_mode', 0)) {
      $link_settings['trackDomainMode'] = $track_domain_mode;
    }
    if ($track_cross_domains = variable_get('googleanalytics_cross_domains', '')) {
      $link_settings['trackCrossDomains'] = preg_split('/(\r\n?|\n)/', $track_cross_domains);
    }
    if ($track_url_fragments = variable_get('googleanalytics_trackurlfragments', 0)) {
      $link_settings['trackUrlFragments'] = $track_url_fragments;
      $url_custom = 'location.pathname + location.search + location.hash';
    }

    if (!empty($link_settings)) {
      drupal_add_js(array('googleanalytics' => $link_settings), 'setting');

      // Add debugging code.
      if ($debug) {
        drupal_add_js(drupal_get_path('module', 'googleanalytics') . '/googleanalytics.debug.js');
        // Add the JS test in development to the page.
        //drupal_add_js(drupal_get_path('module', 'googleanalytics') . '/googleanalytics.test.js');
      }
      else {
        drupal_add_js(drupal_get_path('module', 'googleanalytics') . '/googleanalytics.js');
      }
    }

    // Add messages tracking.
    $message_events = '';
    if ($message_types = variable_get('googleanalytics_trackmessages', array())) {
      $message_types = array_values(array_filter($message_types));
      $status_heading = array(
        'status' => t('Status message'),
        'warning' => t('Warning message'),
        'error' => t('Error message'),
      );

      foreach (drupal_get_messages(NULL, FALSE) as $type => $messages) {
        // Track only the selected message types.
        if (in_array($type, $message_types)) {
          foreach ($messages as $message) {
            // @todo: Track as exceptions?
            $message_events .= 'ga("send", "event", ' . drupal_json_encode(t('Messages')) . ', ' . drupal_json_encode($status_heading[$type]) . ', ' . drupal_json_encode(strip_tags($message)) . ');';
          }
        }
      }
    }

    // Site search tracking support.
    if (module_exists('search') && variable_get('googleanalytics_site_search', FALSE) && arg(0) == 'search' && $keys = googleanalytics_search_get_keys()) {
      // hook_preprocess_search_results() is not executed if search result is
      // empty. Make sure the counter is set to 0 if there are no results.
      $url_custom = '(window.googleanalytics_search_results) ? ' . drupal_json_encode(url('search/' . arg(1), array('query' => array('search' => $keys)))) . ' : ' . drupal_json_encode(url('search/' . arg(1), array('query' => array('search' => 'no-results:' . $keys, 'cat' => 'no-results'))));
    }

    // If this node is a translation of another node, pass the original
    // node instead.
    if (module_exists('translation') && variable_get('googleanalytics_translation_set', 0)) {
      // Check we have a node object, it supports translation, and its
      // translated node ID (tnid) doesn't match its own node ID.
      $node = menu_get_object();
      if ($node && translation_supported_type($node->type) && !empty($node->tnid) && ($node->tnid != $node->nid)) {
        $source_node = node_load($node->tnid);
        $languages = language_list();
        $url_custom = drupal_json_encode(url('node/' . $source_node->nid, array('language' => $languages[$source_node->language])));
      }
    }

    // Track access denied (403) and file not found (404) pages.
    if ($status == '403 Forbidden') {
      // See http://www.google.com/support/analytics/bin/answer.py?answer=86927
      $url_custom = '"/403.html?page=" + document.location.pathname + document.location.search + "&from=" + document.referrer';
    }
    elseif ($status == '404 Not Found') {
      $url_custom = '"/404.html?page=" + document.location.pathname + document.location.search + "&from=" + document.referrer';
    }

    // Add custom dimensions and metrics.
    $custom_var = '';
    foreach (array('dimension', 'metric') as $googleanalytics_custom_type) {
      $googleanalytics_custom_vars = variable_get('googleanalytics_custom_' . $googleanalytics_custom_type, array());
      // Are there dimensions or metrics configured?
      if (!empty($googleanalytics_custom_vars)) {
        // Add all the configured variables to the content.
        foreach ($googleanalytics_custom_vars as $googleanalytics_custom_var) {
          // Replace tokens in values.
          $types = array();
          $node = menu_get_object();
          if (is_object($node)) {
            $types += array('node' => $node);
          }
          $googleanalytics_custom_var['value'] = token_replace($googleanalytics_custom_var['value'], $types, array('clear' => TRUE));

          // Suppress empty values.
          if (!drupal_strlen(trim($googleanalytics_custom_var['value']))) {
            continue;
          }

          // Per documentation the max length of a dimension is 150 bytes.
          // A metric has no length limitation. It's not documented if this
          // limit means 150 bytes after url encoding or before.
          // See https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference#customs
          if ($googleanalytics_custom_type == 'dimension' && drupal_strlen($googleanalytics_custom_var['value']) > 150) {
            $googleanalytics_custom_var['value'] = substr($googleanalytics_custom_var['value'], 0, 150);
          }

          // Cast metric values for json_encode to data type numeric.
          if ($googleanalytics_custom_type == 'metric') {
            settype($googleanalytics_custom_var['value'], 'float');
          };

          // Add variables to tracker.
          $custom_var .= 'ga("set", ' . drupal_json_encode($googleanalytics_custom_type . $googleanalytics_custom_var['index']) . ', ' . drupal_json_encode($googleanalytics_custom_var['value']) . ');';
        }
      }
    }

    // Build tracker code.
    $script = '(function(i,s,o,g,r,a,m){';
    $script .= 'i["GoogleAnalyticsObject"]=r;i[r]=i[r]||function(){';
    $script .= '(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),';
    $script .= 'm=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)';
    $script .= '})(window,document,"script",';

    // Which version of the tracking library should be used?
    $library_tracker_url = '//www.google-analytics.com/' . ($debug ? 'analytics_debug.js' : 'analytics.js');
    $library_cache_url = 'http:' . $library_tracker_url;

    // Should a local cached copy of analytics.js be used?
    if (variable_get('googleanalytics_cache', 0) && $url = _googleanalytics_cache($library_cache_url)) {
      // A dummy query-string is added to filenames, to gain control over
      // browser-caching. The string changes on every update or full cache
      // flush, forcing browsers to load a new copy of the files, as the
      // URL changed.
      $query_string = '?' . variable_get('css_js_query_string', '0');

      $script .= '"' . $url . $query_string . '"';
    }
    else {
      $script .= '"' . $library_tracker_url . '"';
    }
    $script .= ',"ga");';

    // Add any custom code snippets if specified.
    $codesnippet_create = variable_get('googleanalytics_codesnippet_create', array());
    $codesnippet_before = variable_get('googleanalytics_codesnippet_before', '');
    $codesnippet_after = variable_get('googleanalytics_codesnippet_after', '');

    // Build the create only fields list.
    $create_only_fields = array('cookieDomain' => 'auto');
    $create_only_fields = array_merge($create_only_fields, $codesnippet_create);

    // Domain tracking type.
    global $cookie_domain;
    $domain_mode = variable_get('googleanalytics_domain_mode', 0);
    $googleanalytics_adsense_script = '';

    // Per RFC 2109, cookie domains must contain at least one dot other than the
    // first. For hosts such as 'localhost' or IP Addresses we don't set a cookie domain.
    if ($domain_mode == 1 && count(explode('.', $cookie_domain)) > 2 && !is_numeric(str_replace('.', '', $cookie_domain))) {
      $create_only_fields = array_merge($create_only_fields, array('cookieDomain' => $cookie_domain));
      $googleanalytics_adsense_script .= 'window.google_analytics_domain_name = ' . drupal_json_encode($cookie_domain) . ';';
    }
    elseif ($domain_mode == 2) {
      // Cross Domain tracking. 'autoLinker' need to be enabled in 'create'.
      $create_only_fields = array_merge($create_only_fields, array('allowLinker' => TRUE));
      $googleanalytics_adsense_script .= 'window.google_analytics_domain_name = "none";';
    }

    // Track logged in users across all devices.
    if (variable_get('googleanalytics_trackuserid', 0) && user_is_logged_in()) {
      // The USER_ID value should be a unique, persistent, and non-personally
      // identifiable string identifier that represents a user or signed-in
      // account across devices.
      $create_only_fields['userId'] = drupal_hmac_base64($user->uid, drupal_get_private_key() . drupal_get_hash_salt());
    }

    // Create a tracker.
    $script .= 'ga("create", ' . drupal_json_encode($id) . ', ' . drupal_json_encode($create_only_fields) .');';

    // Prepare Adsense tracking.
    $googleanalytics_adsense_script .= 'window.google_analytics_uacct = ' . drupal_json_encode($id) . ';';

    // Add enhanced link attribution after 'create', but before 'pageview' send.
    // @see https://support.google.com/analytics/answer/2558867
    if (variable_get('googleanalytics_tracklinkid', 0)) {
      $script .= 'ga("require", "linkid", "linkid.js");';
    }

    // Add display features after 'create', but before 'pageview' send.
    // @see https://support.google.com/analytics/answer/2444872
    if (variable_get('googleanalytics_trackdoubleclick', 0)) {
      $script .= 'ga("require", "displayfeatures");';
    }

    // Domain tracking type.
    if ($domain_mode == 2) {
      // Cross Domain tracking
      // https://developers.google.com/analytics/devguides/collection/upgrade/reference/gajs-analyticsjs#cross-domain
      $script .= 'ga("require", "linker");';
      $script .= 'ga("linker:autoLink", ' . drupal_json_encode($link_settings['trackCrossDomains']) . ');';
    }

    if (variable_get('googleanalytics_tracker_anonymizeip', 1)) {
      $script .= 'ga("set", "anonymizeIp", true);';
    }

    if (!empty($custom_var)) {
      $script .= $custom_var;
    }
    if (!empty($codesnippet_before)) {
      $script .= $codesnippet_before;
    }
    if (!empty($url_custom)) {
      $script .= 'ga("set", "page", ' . $url_custom . ');';
    }
    $script .= 'ga("send", "pageview");';

    if (!empty($message_events)) {
      $script .= $message_events;
    }
    if (!empty($codesnippet_after)) {
      $script .= $codesnippet_after;
    }

    if (variable_get('googleanalytics_trackadsense', FALSE)) {
      // Custom tracking. Prepend before all other JavaScript.
      // @TODO: https://support.google.com/adsense/answer/98142
      // sounds like it could be appended to $script.
      drupal_add_js($googleanalytics_adsense_script, array('type' => 'inline', 'group' => JS_LIBRARY-1));
    }

    drupal_add_js($script, array('scope' => 'header', 'type' => 'inline'));
  }
}

/**
 * Implements hook_field_extra_fields().
 */
function googleanalytics_field_extra_fields() {
  $extra['user']['user']['form']['googleanalytics'] = array(
    'label' => t('Google Analytics configuration'),
    'description' => t('Google Analytics module form element.'),
    'weight' => 3,
  );

  return $extra;
}

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Allow users to decide if tracking code will be added to pages or not.
 */
function googleanalytics_form_user_profile_form_alter(&$form, &$form_state) {
  $account = $form['#user'];
  $category = $form['#user_category'];

  if ($category == 'account' && user_access('opt-in or out of tracking') && ($custom = variable_get('googleanalytics_custom', 0)) != 0 && _googleanalytics_visibility_roles($account)) {
    $form['googleanalytics'] = array(
      '#type' => 'fieldset',
      '#title' => t('Google Analytics configuration'),
      '#weight' => 3,
      '#collapsible' => TRUE,
      '#tree' => TRUE
    );

    switch ($custom) {
      case 1:
        $description = t('Users are tracked by default, but you are able to opt out.');
        break;

      case 2:
        $description = t('Users are <em>not</em> tracked by default, but you are able to opt in.');
        break;
    }

    // Disable tracking for visitors who have opted out from tracking via DNT (Do-Not-Track) header.
    $disabled = FALSE;
    if (variable_get('googleanalytics_privacy_donottrack', 1) && !empty($_SERVER['HTTP_DNT'])) {
      $disabled = TRUE;

      // Override settings value.
      $account->data['googleanalytics']['custom'] = FALSE;

      $description .= '<span class="admin-disabled">';
      $description .= ' ' . t('You have opted out from tracking via browser privacy settings.');
      $description .= '</span>';
    }

    $form['googleanalytics']['custom'] = array(
      '#type' => 'checkbox',
      '#title' => t('Enable user tracking'),
      '#description' => $description,
      '#default_value' => isset($account->data['googleanalytics']['custom']) ? $account->data['googleanalytics']['custom'] : ($custom == 1),
      '#disabled' => $disabled,
    );

    return $form;
  }
}

/**
 * Implements hook_user_presave().
 */
function googleanalytics_user_presave(&$edit, $account, $category) {
  if (isset($edit['googleanalytics']['custom'])) {
    $edit['data']['googleanalytics']['custom'] = $edit['googleanalytics']['custom'];
  }
}

/**
 * Implements hook_cron().
 */
function googleanalytics_cron() {
  // Regenerate the tracking code file every day.
  if (REQUEST_TIME - variable_get('googleanalytics_last_cache', 0) >= 86400 && variable_get('googleanalytics_cache', 0)) {
    _googleanalytics_cache('http://www.google-analytics.com/analytics.js', TRUE);
    variable_set('googleanalytics_last_cache', REQUEST_TIME);
  }
}

/**
 * Implements hook_preprocess_search_results().
 *
 * Collects and adds the number of search results to the head.
 */
function googleanalytics_preprocess_search_results(&$variables) {
  if (variable_get('googleanalytics_site_search', FALSE)) {
    // There is no search result $variable available that hold the number of items
    // found. But the pager item mumber can tell the number of search results.
    global $pager_total_items;

    drupal_add_js('window.googleanalytics_search_results = ' . intval($pager_total_items[0]) . ';', array('type' => 'inline', 'group' => JS_LIBRARY-1));
  }
}

/**
 * Helper function for grabbing search keys. Function is missing in D7.
 *
 * http://api.drupal.org/api/function/search_get_keys/6
 */
function googleanalytics_search_get_keys() {
  static $return;
  if (!isset($return)) {
    // Extract keys as remainder of path
    // Note: support old GET format of searches for existing links.
    $path = explode('/', $_GET['q'], 3);
    $keys = empty($_REQUEST['keys']) ? '' : $_REQUEST['keys'];
    $return = count($path) == 3 ? $path[2] : $keys;
  }
  return $return;
}

/**
 * Download/Synchronize/Cache tracking code file locally.
 *
 * @param $location
 *   The full URL to the external javascript file.
 * @param $synchronize
 *   Synchronize to local cache if remote file has changed.
 * @return mixed
 *   The path to the local javascript file on success, boolean FALSE on failure.
 */
function _googleanalytics_cache($location, $synchronize = FALSE) {
  $path = 'public://googleanalytics';
  $file_destination = $path . '/' . basename($location);

  if (!file_exists($file_destination) || $synchronize) {
    // Download the latest tracking code.
    $result = drupal_http_request($location);

    if ($result->code == 200) {
      if (file_exists($file_destination)) {
        // Synchronize tracking code and and replace local file if outdated.
        $data_hash_local = drupal_hash_base64(file_get_contents($file_destination));
        $data_hash_remote = drupal_hash_base64($result->data);
        // Check that the files directory is writable.
        if ($data_hash_local != $data_hash_remote && file_prepare_directory($path)) {
          // Save updated tracking code file to disk.
          file_unmanaged_save_data($result->data, $file_destination, FILE_EXISTS_REPLACE);
          watchdog('googleanalytics', 'Locally cached tracking code file has been updated.', array(), WATCHDOG_INFO);

          // Change query-strings on css/js files to enforce reload for all users.
          _drupal_flush_css_js();
        }
      }
      else {
        // Check that the files directory is writable.
        if (file_prepare_directory($path, FILE_CREATE_DIRECTORY)) {
          // There is no need to flush JS here as core refreshes JS caches
          // automatically, if new files are added.
          file_unmanaged_save_data($result->data, $file_destination, FILE_EXISTS_REPLACE);
          watchdog('googleanalytics', 'Locally cached tracking code file has been saved.', array(), WATCHDOG_INFO);

          // Return the local JS file path.
          return file_create_url($file_destination);
        }
      }
    }
  }
  else {
    // Return the local JS file path.
    return file_create_url($file_destination);
  }
}

/**
 * Delete cached files and directory.
 */
function googleanalytics_clear_js_cache() {
  $path = 'public://googleanalytics';
  if (file_prepare_directory($path)) {
    file_scan_directory($path, '/.*/', array('callback' => 'file_unmanaged_delete'));
    drupal_rmdir($path);

    // Change query-strings on css/js files to enforce reload for all users.
    _drupal_flush_css_js();

    watchdog('googleanalytics', 'Local cache has been purged.', array(), WATCHDOG_INFO);
  }
}

/**
 * Tracking visibility check for an user object.
 *
 * @param $account
 *   A user object containing an array of roles to check.
 * @return boolean
 *   A decision on if the current user is being tracked by Google Analytics.
 */
function _googleanalytics_visibility_user($account) {

  $enabled = FALSE;

  // Is current user a member of a role that should be tracked?
  if (_googleanalytics_visibility_header($account) && _googleanalytics_visibility_roles($account)) {

    // Use the user's block visibility setting, if necessary.
    if (($custom = variable_get('googleanalytics_custom', 0)) != 0) {
      if ($account->uid && isset($account->data['googleanalytics']['custom'])) {
        $enabled = $account->data['googleanalytics']['custom'];
      }
      else {
        $enabled = ($custom == 1);
      }
    }
    else {
      $enabled = TRUE;
    }

  }

  return $enabled;
}

/**
 * Based on visibility setting this function returns TRUE if GA code should
 * be added for the current role and otherwise FALSE.
 */
function _googleanalytics_visibility_roles($account) {

  $visibility = variable_get('googleanalytics_visibility_roles', 0);
  $enabled = $visibility;
  $roles = variable_get('googleanalytics_roles', array());

  if (array_sum($roles) > 0) {
    // One or more roles are selected.
    foreach (array_keys($account->roles) as $rid) {
      // Is the current user a member of one of these roles?
      if (isset($roles[$rid]) && $rid == $roles[$rid]) {
        // Current user is a member of a role that should be tracked/excluded from tracking.
        $enabled = !$visibility;
        break;
      }
    }
  }
  else {
    // No role is selected for tracking, therefore all roles should be tracked.
    $enabled = TRUE;
  }

  return $enabled;
}

/**
 * Based on visibility setting this function returns TRUE if GA code should
 * be added to the current page and otherwise FALSE.
 */
function _googleanalytics_visibility_pages() {
  static $page_match;

  // Cache visibility result if function is called more than once.
  if (!isset($page_match)) {

    $visibility = variable_get('googleanalytics_visibility_pages', 0);
    $setting_pages = variable_get('googleanalytics_pages', GOOGLEANALYTICS_PAGES);

    // Match path if necessary.
    if (!empty($setting_pages)) {
      // Convert path to lowercase. This allows comparison of the same path
      // with different case. Ex: /Page, /page, /PAGE.
      $pages = drupal_strtolower($setting_pages);
      if ($visibility < 2) {
        // Convert the Drupal path to lowercase
        $path = drupal_strtolower(drupal_get_path_alias($_GET['q']));
        // Compare the lowercase internal and lowercase path alias (if any).
        $page_match = drupal_match_path($path, $pages);
        if ($path != $_GET['q']) {
          $page_match = $page_match || drupal_match_path($_GET['q'], $pages);
        }
        // When $visibility has a value of 0, the tracking code is displayed on
        // all pages except those listed in $pages. When set to 1, it
        // is displayed only on those pages listed in $pages.
        $page_match = !($visibility xor $page_match);
      }
      elseif (module_exists('php')) {
        $page_match = php_eval($setting_pages);
      }
      else {
        $page_match = FALSE;
      }
    }
    else {
      $page_match = TRUE;
    }

  }
  return $page_match;
}

/**
 * Based on headers send by clients this function returns TRUE if GA code should
 * be added to the current page and otherwise FALSE.
 */
function _googleanalytics_visibility_header($account) {

  if (($account->uid || variable_get('cache', 0) == 0) && variable_get('googleanalytics_privacy_donottrack', 1) && !empty($_SERVER['HTTP_DNT'])) {
    // Disable tracking if caching is disabled or a visitors is logged in and
    // have opted out from tracking via DNT (Do-Not-Track) header.
    return FALSE;
  }

  return TRUE;
}