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<?php

/**
 * @file
 * Framework for handling the filtering of content.
 */

use Drupal\Core\Cache\CacheBackendInterface;
use Drupal\Core\Template\Attribute;
use Drupal\filter\Plugin\Core\Entity\FilterFormat;

/**
 * Non-HTML markup language filters that generate HTML.
 */
const FILTER_TYPE_MARKUP_LANGUAGE = 0;

/**
 * HTML tag and attribute restricting filters.
 */
const FILTER_TYPE_HTML_RESTRICTOR = 1;

/**
 * Reversible transformation filters.
 */
const FILTER_TYPE_TRANSFORM_REVERSIBLE = 2;

/**
 * Irreversible transformation filters.
 */
const FILTER_TYPE_TRANSFORM_IRREVERSIBLE = 3;

/**
 * Implements hook_cache_flush().
 */
function filter_cache_flush() {
  return array('filter');
}

/**
 * Implements hook_help().
 */
function filter_help($path, $arg) {
  switch ($path) {
    case 'admin/help#filter':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Filter module allows administrators to configure text formats. A text format defines the HTML tags, codes, and other input allowed in content and comments, and is a key feature in guarding against potentially damaging input from malicious users. For more information, see the online handbook entry for <a href="@filter">Filter module</a>.', array('@filter' => 'http://drupal.org/documentation/modules/filter/')) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Configuring text formats') . '</dt>';
      $output .= '<dd>' . t('Configure text formats on the <a href="@formats">Text formats page</a>. <strong>Improper text format configuration is a security risk</strong>. To ensure security, untrusted users should only have access to text formats that restrict them to either plain text or a safe set of HTML tags, since certain HTML tags can allow embedding malicious links or scripts in text. More trusted registered users may be granted permission to use less restrictive text formats in order to create rich content.', array('@formats' => url('admin/config/content/formats'))) . '</dd>';
      $output .= '<dt>' . t('Applying filters to text') . '</dt>';
      $output .= '<dd>' . t('Each text format uses filters to manipulate text, and most formats apply several different filters to text in a specific order. Each filter is designed for a specific purpose, and generally either adds, removes, or transforms elements within user-entered text before it is displayed. A filter does not change the actual content, but instead, modifies it temporarily before it is displayed. One filter may remove unapproved HTML tags, while another automatically adds HTML to make URLs display as clickable links.') . '</dd>';
      $output .= '<dt>' . t('Defining text formats') . '</dt>';
      $output .= '<dd>' . t('One format is included by default: <em>Plain text</em> (which removes all HTML tags). Additional formats may be created by your installation profile when you install Drupal, and more can be created by an administrator on the <a href="@text-formats">Text formats page</a>.', array('@text-formats' => url('admin/config/content/formats'))) . '</dd>';
      $output .= '<dt>' . t('Choosing a text format') . '</dt>';
      $output .= '<dd>' . t('Users with access to more than one text format can use the <em>Text format</em> widget to choose between available text formats when creating or editing multi-line content. Administrators can define the text formats available to each user role, and control the order of formats listed in the <em>Text format</em> widget on the <a href="@text-formats">Text formats page</a>.', array('@text-formats' => url('admin/config/content/formats'))) . '</dd>';
      $output .= '</dl>';
      return $output;

    case 'admin/config/content/formats':
      $output = '<p>' . t('Text formats define the HTML tags, code, and other formatting that can be used when entering text. <strong>Improper text format configuration is a security risk</strong>. Learn more on the <a href="@filterhelp">Filter module help page</a>.', array('@filterhelp' => url('admin/help/filter'))) . '</p>';
      $output .= '<p>' . t('Text formats are presented on content editing pages in the order defined on this page. The first format available to a user will be selected by default.') . '</p>';
      return $output;

    case 'admin/config/content/formats/%':
      $output = '<p>' . t('A text format contains filters that change the user input, for example stripping out malicious HTML or making URLs clickable. Filters are executed from top to bottom and the order is important, since one filter may prevent another filter from doing its job. For example, when URLs are converted into links before disallowed HTML tags are removed, all links may be removed. When this happens, the order of filters may need to be re-arranged.') . '</p>';
      return $output;
  }
}

/**
 * Implements hook_theme().
 */
function filter_theme() {
  return array(
    'filter_admin_format_filter_order' => array(
      'render element' => 'element',
      'file' => 'filter.admin.inc',
    ),
    'filter_tips' => array(
      'variables' => array('tips' => NULL, 'long' => FALSE),
      'file' => 'filter.pages.inc',
    ),
    'text_format_wrapper' => array(
      'render element' => 'element',
    ),
    'filter_tips_more_info' => array(
      'variables' => array(),
    ),
    'filter_guidelines' => array(
      'variables' => array('format' => NULL),
    ),
    'filter_html_image_secure_image' => array(
      'variables' => array('image' => NULL),
    ),
  );
}

/**
 * Implements hook_element_info().
 *
 * @see filter_process_format()
 * @see text_format_wrapper()
 */
function filter_element_info() {
  $type['text_format'] = array(
    '#process' => array('filter_process_format'),
    '#base_type' => 'textarea',
    '#theme_wrappers' => array('text_format_wrapper'),
  );
  return $type;
}

/**
 * Implements hook_menu().
 */
function filter_menu() {
  $items['filter/tips'] = array(
    'title' => 'Compose tips',
    'page callback' => 'filter_tips_long',
    'access callback' => TRUE,
    'type' => MENU_SUGGESTED_ITEM,
    'file' => 'filter.pages.inc',
  );
  $items['filter/tips/%filter_format'] = array(
    'title' => 'Compose tips',
    'page callback' => 'filter_tips_long',
    'page arguments' => array(2),
    'access callback' => 'filter_access',
    'access arguments' => array(2),
    'file' => 'filter.pages.inc',
  );
  $items['admin/config/content/formats'] = array(
    'title' => 'Text formats',
    'description' => 'Configure how content input by users is filtered, including allowed HTML tags. Also allows enabling of module-provided filters.',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('filter_admin_overview'),
    'access arguments' => array('administer filters'),
    'file' => 'filter.admin.inc',
  );
  $items['admin/config/content/formats/list'] = array(
    'title' => 'List',
    'type' => MENU_DEFAULT_LOCAL_TASK,
  );
  $items['admin/config/content/formats/add'] = array(
    'title' => 'Add text format',
    'page callback' => 'filter_admin_format_page',
    'access arguments' => array('administer filters'),
    'type' => MENU_LOCAL_ACTION,
    'weight' => 1,
    'file' => 'filter.admin.inc',
  );
  $items['admin/config/content/formats/%filter_format'] = array(
    'title callback' => 'filter_admin_format_title',
    'title arguments' => array(4),
    'page callback' => 'filter_admin_format_page',
    'page arguments' => array(4),
    'access arguments' => array('administer filters'),
    'file' => 'filter.admin.inc',
  );
  $items['admin/config/content/formats/%filter_format/disable'] = array(
    'title' => 'Disable text format',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('filter_admin_disable', 4),
    'access callback' => '_filter_disable_format_access',
    'access arguments' => array(4),
    'file' => 'filter.admin.inc',
  );
  return $items;
}

/**
 * Access callback: Checks access for disabling text formats.
 *
 * @param $format
 *   A text format object.
 *
 * @return
 *   TRUE if the text format can be disabled by the current user, FALSE
 *   otherwise.
 *
 * @see filter_menu()
 */
function _filter_disable_format_access($format) {
  // The fallback format can never be disabled.
  return user_access('administer filters') && ($format->format != filter_fallback_format());
}

/**
 * Loads a text format object from the database.
 *
 * @param $format_id
 *   The format ID.
 *
 * @return
 *   A fully-populated text format object, if the requested format exists and
 *   is enabled. If the format does not exist, or exists in the database but
 *   has been marked as disabled, FALSE is returned.
 *
 * @see filter_format_exists()
 */
function filter_format_load($format_id) {
  $formats = filter_formats();
  return isset($formats[$format_id]) ? $formats[$format_id] : FALSE;
}

/**
 * Disables a text format.
 *
 * There is no core facility to re-enable a disabled format. It is not deleted
 * to keep information for contrib and to make sure the format ID is never
 * reused. As there might be content using the disabled format, this would lead
 * to data corruption.
 *
 * @param $format
 *   The text format object to be disabled.
 */
function filter_format_disable($format) {
  $format->status = 0;
  $format->save();

  // Allow modules to react on text format deletion.
  module_invoke_all('filter_format_disable', $format);

  // Clear the filter cache whenever a text format is disabled.
  filter_formats_reset();
  cache('filter')->deleteTags(array('filter_format' => $format->format));
}

/**
 * Determines if a text format exists.
 *
 * @param $format_id
 *   The ID of the text format to check.
 *
 * @return
 *   TRUE if the text format exists, FALSE otherwise. Note that for disabled
 *   formats filter_format_exists() will return TRUE while filter_format_load()
 *   will return FALSE.
 *
 * @see filter_format_load()
 */
function filter_format_exists($format_id) {
  return entity_load('filter_format', $format_id);
}

/**
 * Displays a text format form title.
 *
 * @param object $format_id
 *   A format object.
 *
 * @return string
 *   The name of the format.
 *
 * @see filter_menu()
 */
function filter_admin_format_title($format) {
  return $format->name;
}

/**
 * Implements hook_permission().
 */
function filter_permission() {
  $perms['administer filters'] = array(
    'title' => t('Administer text formats and filters'),
    'description' => t('Define how text is handled by combining filters into <a href="@url">text formats</a>.', array(
      '@url' => url('admin/config/content/formats'),
    )),
    'restrict access' => TRUE,
  );

  // Generate permissions for each text format. Warn the administrator that any
  // of them are potentially unsafe.
  foreach (filter_formats() as $format) {
    $permission = filter_permission_name($format);
    if (!empty($permission)) {
      // Only link to the text format configuration page if the user who is
      // viewing this will have access to that page.
      $format_name_replacement = user_access('administer filters') ? l($format->name, 'admin/config/content/formats/' . $format->format) : drupal_placeholder($format->name);
      $perms[$permission] = array(
        'title' => t("Use the !text_format text format", array('!text_format' => $format_name_replacement,)),
        'description' => drupal_placeholder(t('Warning: This permission may have security implications depending on how the text format is configured.')),
      );
    }
  }
  return $perms;
}

/**
 * Returns the machine-readable permission name for a provided text format.
 *
 * @param $format
 *   An object representing a text format.
 *
 * @return
 *   The machine-readable permission name, or FALSE if the provided text format
 *   is malformed or is the fallback format (which is available to all users).
 */
function filter_permission_name($format) {
  if (isset($format->format) && $format->format != filter_fallback_format()) {
    return 'use text format ' . $format->format;
  }
  return FALSE;
}

/**
 * Implements hook_modules_enabled().
 */
function filter_modules_enabled($modules) {
  // Reset the static cache of module-provided filters, in case any of the
  // newly enabled modules defines a new filter or alters existing ones.
  drupal_static_reset('filter_get_filters');
}

/**
 * Implements hook_modules_disabled().
 */
function filter_modules_disabled($modules) {
  // Reset the static cache of module-provided filters, in case any of the
  // newly disabled modules defined or altered any filters.
  drupal_static_reset('filter_get_filters');
}

/**
 * Retrieves a list of text formats, ordered by weight.
 *
 * @param $account
 *   (optional) If provided, only those formats that are allowed for this user
 *   account will be returned. All formats will be returned otherwise. Defaults
 *   to NULL.
 *
 * @return
 *   An array of text format objects, keyed by the format ID and ordered by
 *   weight.
 *
 * @see filter_formats_reset()
 */
function filter_formats($account = NULL) {
  $language_interface = language(LANGUAGE_TYPE_INTERFACE);
  $formats = &drupal_static(__FUNCTION__, array());

  // All available formats are cached for performance.
  if (!isset($formats['all'])) {
    if ($cache = cache()->get("filter_formats:{$language_interface->langcode}")) {
      $formats['all'] = $cache->data;
    }
    else {
      $filter_formats = entity_load_multiple('filter_format');
      $formats['all'] = array();
      foreach ($filter_formats as $format_name => $filter_format) {
        if (!empty($filter_format->status)) {
          $formats['all'][$format_name] = $filter_format;
        }
      }
      uasort($formats['all'], 'Drupal\Core\Config\Entity\ConfigEntityBase::sort');

      cache()->set("filter_formats:{$language_interface->langcode}", $formats['all'], CacheBackendInterface::CACHE_PERMANENT, array('filter_formats' => TRUE));
    }
  }

  // Build a list of user-specific formats.
  if (isset($account) && !isset($formats['user'][$account->uid])) {
    $formats['user'][$account->uid] = array();
    foreach ($formats['all'] as $format) {
      if (filter_access($format, $account)) {
        $formats['user'][$account->uid][$format->format] = $format;
      }
    }
  }

  return isset($account) ? $formats['user'][$account->uid] : $formats['all'];
}

/**
 * Resets the text format caches.
 *
 * @see filter_formats()
 */
function filter_formats_reset() {
  cache()->deleteTags(array('filter_formats' => TRUE));
  cache()->delete('filter_list_format');
  drupal_static_reset('filter_list_format');
  drupal_static_reset('filter_formats');
}

/**
 * Retrieves a list of roles that are allowed to use a given text format.
 *
 * @param $format
 *   An object representing the text format.
 *
 * @return
 *   An array of role names, keyed by role ID.
 */
function filter_get_roles_by_format($format) {
  // Handle the fallback format upfront (all roles have access to this format).
  if ($format->format == filter_fallback_format()) {
    return user_role_names();
  }
  // Do not list any roles if the permission does not exist.
  $permission = filter_permission_name($format);
  return !empty($permission) ? user_role_names(FALSE, $permission) : array();
}

/**
 * Retrieves a list of text formats that are allowed for a given role.
 *
 * @param $rid
 *   The user role ID to retrieve text formats for.
 *
 * @return
 *   An array of text format objects that are allowed for the role, keyed by
 *   the text format ID and ordered by weight.
 */
function filter_get_formats_by_role($rid) {
  $formats = array();
  foreach (filter_formats() as $format) {
    $roles = filter_get_roles_by_format($format);
    if (isset($roles[$rid])) {
      $formats[$format->format] = $format;
    }
  }
  return $formats;
}

/**
 * Returns the ID of the default text format for a particular user.
 *
 * The default text format is the first available format that the user is
 * allowed to access, when the formats are ordered by weight. It should
 * generally be used as a default choice when presenting the user with a list
 * of possible text formats (for example, in a node creation form).
 *
 * Conversely, when existing content that does not have an assigned text format
 * needs to be filtered for display, the default text format is the wrong
 * choice, because it is not guaranteed to be consistent from user to user, and
 * some trusted users may have an unsafe text format set by default, which
 * should not be used on text of unknown origin. Instead, the fallback format
 * returned by filter_fallback_format() should be used, since that is intended
 * to be a safe, consistent format that is always available to all users.
 *
 * @param $account
 *   (optional) The user account to check. Defaults to the currently logged-in
 *   user. Defaults to NULL.
 *
 * @return
 *   The ID of the user's default text format.
 *
 * @see filter_fallback_format()
 */
function filter_default_format($account = NULL) {
  global $user;
  if (!isset($account)) {
    $account = $user;
  }
  // Get a list of formats for this user, ordered by weight. The first one
  // available is the user's default format.
  $formats = filter_formats($account);
  $format = reset($formats);
  return $format->format;
}

/**
 * Retrieves all filter types that are used in a given text format.
 *
 * @param string $format_id
 *   A text format ID.
 *
 * @return array
 *   All filter types used by filters of a given text format.
 *
 * @throws Exception
 */
function filter_get_filter_types_by_format($format_id) {
  $filter_types = array();

  $filters = filter_list_format($format_id);

  // Ignore filters that are disabled.
  $filters = array_filter($filters, function($filter) {
    return $filter->status;
  });

  $filters_info = filter_get_filters();
  foreach ($filters as $filter) {
    if (!isset($filters_info[$filter->name]['type'])) {
      throw new Exception(t('Filter %filter has no type specified.', array ('%filter' => $filter->name)));
    }

    $filter_types[] = $filters_info[$filter->name]['type'];
  }

  return array_unique($filter_types);
}

/**
 * Returns the ID of the fallback text format that all users have access to.
 *
 * The fallback text format is a regular text format in every respect, except
 * it does not participate in the filter permission system and cannot be
 * disabled. It needs to exist because any user who has permission to create
 * formatted content must always have at least one text format they can use.
 *
 * Because the fallback format is available to all users, it should always be
 * configured securely. For example, when the Filter module is installed, this
 * format is initialized to output plain text. Installation profiles and site
 * administrators have the freedom to configure it further.
 *
 * Note that the fallback format is completely distinct from the default format,
 * which differs per user and is simply the first format which that user has
 * access to. The default and fallback formats are only guaranteed to be the
 * same for users who do not have access to any other format; otherwise, the
 * fallback format's weight determines its placement with respect to the user's
 * other formats.
 *
 * Any modules implementing a format deletion functionality must not delete this
 * format.
 *
 * @return
 *   The ID of the fallback text format.
 *
 * @see hook_filter_format_disable()
 * @see filter_default_format()
 */
function filter_fallback_format() {
  // This variable is automatically set in the database for all installations
  // of Drupal. In the event that it gets disabled or deleted somehow, there
  // is no safe default to return, since we do not want to risk making an
  // existing (and potentially unsafe) text format on the site automatically
  // available to all users. Returning NULL at least guarantees that this
  // cannot happen.
  return config('filter.settings')->get('fallback_format');
}

/**
 * Returns the title of the fallback text format.
 *
 * @return string
 *   The title of the fallback text format.
 */
function filter_fallback_format_title() {
  $fallback_format = filter_format_load(filter_fallback_format());
  return filter_admin_format_title($fallback_format);
}

/**
 * Returns a list of all filters provided by modules.
 *
 * @return array
 *   An array of filter formats.
 */
function filter_get_filters() {
  $filters = &drupal_static(__FUNCTION__, array());

  if (empty($filters)) {
    foreach (module_implements('filter_info') as $module) {
      $info = module_invoke($module, 'filter_info');
      if (isset($info) && is_array($info)) {
        // Assign the name of the module implementing the filters and ensure
        // default values.
        foreach (array_keys($info) as $name) {
          $info[$name]['module'] = $module;
          $info[$name] += array(
            'description' => '',
            'weight' => 0,
            'default settings' => array(),
          );
        }
        $filters = array_merge($filters, $info);
      }
    }
    // Allow modules to alter filter definitions.
    drupal_alter('filter_info', $filters);

    uasort($filters, '_filter_list_cmp');
  }

  return $filters;
}

/**
 * Sorts an array of filters by filter name.
 *
 * Callback for uasort() within filter_get_filters().
 */
function _filter_list_cmp($a, $b) {
  return strcmp($a['title'], $b['title']);
}

/**
 * Checks if the text in a certain text format is allowed to be cached.
 *
 * This function can be used to check whether the result of the filtering
 * process can be cached. A text format may allow caching depending on the
 * filters enabled.
 *
 * @param $format_id
 *   The text format ID to check.
 *
 * @return
 *   TRUE if the given text format allows caching, FALSE otherwise.
 */
function filter_format_allowcache($format_id) {
  $format = filter_format_load($format_id);
  return !empty($format->cache);
}

/**
 * Determines whether the output of a given text format can be cached.
 *
 * The output of a given text format can be cached when all enabled filters in
 * the text format allow caching.
 *
 * @param $format
 *   The text format object to check.
 *
 * @return
 *   TRUE if all the filters enabled in the given text format allow caching,
 *   FALSE otherwise.
 *
 * @see Drupal\filter\Plugin\Core\Entity\FilterFormat::save()
 */
function _filter_format_is_cacheable($format) {
  if (empty($format->filters)) {
    return TRUE;
  }
  $filter_info = filter_get_filters();
  foreach ($format->filters as $name => $filter) {
    // By default, 'cache' is TRUE for all filters unless specified otherwise.
    if (!empty($filter['status']) && isset($filter_info[$name]['cache']) && !$filter_info[$name]['cache']) {
      return FALSE;
    }
  }
  return TRUE;
}

/**
 * Retrieves a list of filters for a given text format.
 *
 * Note that this function returns all associated filters regardless of whether
 * they are enabled or disabled. All functions working with the filter
 * information outside of filter administration should test for $filter->status
 * before performing actions with the filter.
 *
 * @param $format_id
 *   The format ID to retrieve filters for.
 *
 * @return
 *   An array of filter objects associated to the given text format, keyed by
 *   filter name.
 */
function filter_list_format($format_id) {
  $filters = &drupal_static(__FUNCTION__, array());
  $filter_info = filter_get_filters();

  if (!isset($filters['all'])) {
    if ($cache = cache()->get('filter_list_format')) {
      $filters['all'] = $cache->data;
    }
    else {
      $filter_formats = filter_formats();
      foreach ($filter_formats as $filter_format) {
        foreach ($filter_format->filters as $filter_name => $filter) {
          $filter['name'] = $filter_name;
          $filters['all'][$filter_format->format][$filter_name] = $filter;
        }
        uasort($filters['all'][$filter_format->format], 'Drupal\filter\Plugin\Core\Entity\FilterFormat::sortFilters');
        // Convert filters into objects.
        // @todo Retain filters as arrays or convert to plugins.
        foreach ($filters['all'][$filter_format->format] as $filter_name => $filter) {
          $filters['all'][$filter_format->format][$filter_name] = (object) $filter;
        }
      }
      cache()->set('filter_list_format', $filters['all']);
    }
  }

  if (!isset($filters[$format_id])) {
    $format_filters = array();
    $filter_map = isset($filters['all'][$format_id]) ? $filters['all'][$format_id] : array();
    foreach ($filter_map as $name => $filter) {
      if (isset($filter_info[$name])) {
        $filter->title = $filter_info[$name]['title'];

        $filter->settings = isset($filter->settings) ? $filter->settings : array();
        // Merge in default settings.
        $filter->settings += $filter_info[$name]['default settings'];

        $format_filters[$name] = $filter;
      }
    }
    $filters[$format_id] = $format_filters;
  }

  return isset($filters[$format_id]) ? $filters[$format_id] : array();
}

/**
 * Runs all the enabled filters on a piece of text.
 *
 * Note: Because filters can inject JavaScript or execute PHP code, security is
 * vital here. When a user supplies a text format, you should validate it using
 * filter_access() before accepting/using it. This is normally done in the
 * validation stage of the Form API. You should for example never make a
 * preview of content in a disallowed format.
 *
 * @param $text
 *   The text to be filtered.
 * @param $format_id
 *   (optional) The format ID of the text to be filtered. If no format is
 *   assigned, the fallback format will be used. Defaults to NULL.
 * @param $langcode
 *   (optional) The language code of the text to be filtered, e.g. 'en' for
 *   English. This allows filters to be language aware so language specific
 *   text replacement can be implemented. Defaults to an empty string.
 * @param $cache
 *   (optional) A Boolean indicating whether to cache the filtered output in the
 *   {cache_filter} table. The caller may set this to FALSE when the output is
 *   already cached elsewhere to avoid duplicate cache lookups and storage.
 *   Defaults to FALSE.
 * @param array $filter_types_to_skip
 *   (optional) An array of filter types to skip, or an empty array (default)
 *   to skip no filter types. All of the format's filters will be applied,
 *   except for filters of the types that are marked to be skipped.
 *   FILTER_TYPE_HTML_RESTRICTOR is the only type that cannot be skipped.
 *
 * @return
 *   The filtered text.
 *
 * @ingroup sanitization
 */
function check_markup($text, $format_id = NULL, $langcode = '', $cache = FALSE, $filter_types_to_skip = array()) {
  if (!isset($format_id)) {
    $format_id = filter_fallback_format();
  }
  // If the requested text format does not exist, the text cannot be filtered.
  if (!$format = filter_format_load($format_id)) {
    watchdog('filter', 'Missing text format: %format.', array('%format' => $format_id), WATCHDOG_ALERT);
    return '';
  }

  // Prevent FILTER_TYPE_HTML_RESTRICTOR from being skipped.
  if (in_array(FILTER_TYPE_HTML_RESTRICTOR, $filter_types_to_skip)) {
    $filter_types_to_skip = array_diff($filter_types_to_skip, array(FILTER_TYPE_HTML_RESTRICTOR));
  }

  // When certain filters should be skipped, don't perform caching.
  if ($filter_types_to_skip) {
    $cache = FALSE;
  }

  // Check for a cached version of this piece of text.
  $cache = $cache && !empty($format->cache);
  $cache_id = '';
  if ($cache) {
    $cache_id = $format->format . ':' . $langcode . ':' . hash('sha256', $text);
    if ($cached = cache('filter')->get($cache_id)) {
      return $cached->data;
    }
  }

  // Convert all Windows and Mac newlines to a single newline, so filters only
  // need to deal with one possibility.
  $text = str_replace(array("\r\n", "\r"), "\n", $text);

  // Get a complete list of filters, ordered properly.
  $filters = filter_list_format($format->format);
  $filter_info = filter_get_filters();

  // Give filters the chance to escape HTML-like data such as code or formulas.
  foreach ($filters as $name => $filter) {
    // If necessary, skip filters of a certain type.
    if (in_array($filter_info[$name]['type'], $filter_types_to_skip)) {
      continue;
    }
    if ($filter->status && isset($filter_info[$name]['prepare callback'])) {
      $function = $filter_info[$name]['prepare callback'];
      $text = $function($text, $filter, $format, $langcode, $cache, $cache_id);
    }
  }

  // Perform filtering.
  foreach ($filters as $name => $filter) {
    // If necessary, skip filters of a certain type.
    if (in_array($filter_info[$name]['type'], $filter_types_to_skip)) {
      continue;
    }
    if ($filter->status && isset($filter_info[$name]['process callback'])) {
      $function = $filter_info[$name]['process callback'];
      $text = $function($text, $filter, $format, $langcode, $cache, $cache_id);
    }
  }

  // Cache the filtered text. This cache is infinitely valid. It becomes
  // obsolete when $text changes (which leads to a new $cache_id). It is
  // automatically flushed when the text format is updated.
  // @see Drupal\filter\Plugin\Core\Entity\FilterFormat::save()
  if ($cache) {
    cache('filter')->set($cache_id, $text, CacheBackendInterface::CACHE_PERMANENT, array('filter_format' => $format->format));
  }

  return $text;
}

/**
 * Expands an element into a base element with text format selector attached.
 *
 * The form element will be expanded into two separate form elements, one
 * holding the original element, and the other holding the text format
 * selector:
 * - value: Holds the original element, having its #type changed to the value
 *   of #base_type or 'textarea' by default.
 * - format: Holds the text format details and the text format selection,
 *   using the text format ID specified in #format or the user's default format
 *   by default, if NULL.
 *
 * The resulting value for the element will be an array holding the value and
 * the format. For example, the value for the body element will be:
 * @code
 *   $form_state['values']['body']['value'] = 'foo';
 *   $form_state['values']['body']['format'] = 'foo';
 * @endcode
 *
 * @param $element
 *   The form element to process. Properties used:
 *   - #base_type: The form element #type to use for the 'value' element.
 *     'textarea' by default.
 *   - #format: (optional) The text format ID to preselect. If NULL or not set,
 *     the default format for the current user will be used.
 *
 * @return
 *   The expanded element.
 */
function filter_process_format($element) {
  global $user;

  // Ensure that children appear as subkeys of this element.
  $element['#tree'] = TRUE;
  $blacklist = array(
    // Make form_builder() regenerate child properties.
    '#parents',
    '#id',
    '#name',
    // Do not copy this #process function to prevent form_builder() from
    // recursing infinitely.
    '#process',
    // Description is handled by theme_text_format_wrapper().
    '#description',
    // Ensure proper ordering of children.
    '#weight',
    // Properties already processed for the parent element.
    '#prefix',
    '#suffix',
    '#attached',
    '#processed',
    '#theme_wrappers',
  );
  // Move this element into sub-element 'value'.
  unset($element['value']);
  foreach (element_properties($element) as $key) {
    if (!in_array($key, $blacklist)) {
      $element['value'][$key] = $element[$key];
    }
  }

  $element['value']['#type'] = $element['#base_type'];
  $element['value'] += element_info($element['#base_type']);

  // Turn original element into a text format wrapper.
  $path = drupal_get_path('module', 'filter');
  $element['#attached']['library'][] = array('filter', 'drupal.filter');

  // Setup child container for the text format widget.
  $element['format'] = array(
    '#type' => 'container',
    '#attributes' => array('class' => array('filter-wrapper')),
  );

  // Prepare text format guidelines.
  $element['format']['guidelines'] = array(
    '#type' => 'container',
    '#attributes' => array('class' => array('filter-guidelines')),
    '#weight' => 20,
  );
  // Get a list of formats that the current user has access to.
  $formats = filter_formats($user);
  foreach ($formats as $format) {
    $options[$format->format] = $format->name;
    $element['format']['guidelines'][$format->format] = array(
      '#theme' => 'filter_guidelines',
      '#format' => $format,
    );
  }

  // Use the default format for this user if none was selected.
  if (!isset($element['#format'])) {
    $element['#format'] = filter_default_format($user);
  }

  $element['format']['format'] = array(
    '#type' => 'select',
    '#title' => t('Text format'),
    '#options' => $options,
    '#default_value' => $element['#format'],
    '#access' => count($formats) > 1,
    '#weight' => 10,
    '#attributes' => array('class' => array('filter-list')),
    '#parents' => array_merge($element['#parents'], array('format')),
  );

  $element['format']['help'] = array(
    '#type' => 'container',
    '#theme' => 'filter_tips_more_info',
    '#attributes' => array('class' => array('filter-help')),
    '#weight' => 0,
  );

  $all_formats = filter_formats();
  $format_exists = isset($all_formats[$element['#format']]);
  $user_has_access = isset($formats[$element['#format']]);
  $user_is_admin = user_access('administer filters');

  // If the stored format does not exist, administrators have to assign a new
  // format.
  if (!$format_exists && $user_is_admin) {
    $element['format']['format']['#required'] = TRUE;
    $element['format']['format']['#default_value'] = NULL;
    // Force access to the format selector (it may have been denied above if
    // the user only has access to a single format).
    $element['format']['format']['#access'] = TRUE;
  }
  // Disable this widget, if the user is not allowed to use the stored format,
  // or if the stored format does not exist. The 'administer filters' permission
  // only grants access to the filter administration, not to all formats.
  elseif (!$user_has_access || !$format_exists) {
    // Overload default values into #value to make them unalterable.
    $element['value']['#value'] = $element['value']['#default_value'];
    $element['format']['format']['#value'] = $element['format']['format']['#default_value'];

    // Prepend #pre_render callback to replace field value with user notice
    // prior to rendering.
    $element['value'] += array('#pre_render' => array());
    array_unshift($element['value']['#pre_render'], 'filter_form_access_denied');

    // Cosmetic adjustments.
    if (isset($element['value']['#rows'])) {
      $element['value']['#rows'] = 3;
    }
    $element['value']['#disabled'] = TRUE;
    $element['value']['#resizable'] = 'none';

    // Hide the text format selector and any other child element (such as text
    // field's summary).
    foreach (element_children($element) as $key) {
      if ($key != 'value') {
        $element[$key]['#access'] = FALSE;
      }
    }
  }

  return $element;
}

/**
 * Render API callback: Hides the field value of 'text_format' elements.
 *
 * To not break form processing and previews if a user does not have access to
 * a stored text format, the expanded form elements in filter_process_format()
 * are forced to take over the stored #default_values for 'value' and 'format'.
 * However, to prevent the unfiltered, original #value from being displayed to
 * the user, we replace it with a friendly notice here.
 *
 * @see filter_process_format()
 */
function filter_form_access_denied($element) {
  $element['#value'] = t('This field has been disabled because you do not have sufficient permissions to edit it.');
  return $element;
}

/**
 * Returns HTML for a text format-enabled form element.
 *
 * @param array $variables
 *   An associative array containing:
 *   - element: A render element containing #children and #description.
 *
 * @ingroup themeable
 */
function theme_text_format_wrapper($variables) {
  $element = $variables['element'];
  $output = '<div class="text-format-wrapper form-item">';
  $output .= $element['#children'];
  if (!empty($element['#description'])) {
    $output .= '<div class="description">' . $element['#description'] . '</div>';
  }
  $output .= "</div>\n";

  return $output;
}

/**
 * Checks if a user has access to a particular text format.
 *
 * @param $format
 *   An object representing the text format.
 * @param $account
 *   (optional) The user account to check access for; if omitted, the currently
 *   logged-in user is used. Defaults to NULL.
 *
 * @return
 *   Boolean TRUE if the user is allowed to access the given format.
 */
function filter_access($format, $account = NULL) {
  global $user;
  if (!isset($account)) {
    $account = $user;
  }
  // Handle special cases up front. All users have access to the fallback
  // format.
  if ($format->format == filter_fallback_format()) {
    return TRUE;
  }
  // Check the permission if one exists; otherwise, we have a non-existent
  // format so we return FALSE.
  $permission = filter_permission_name($format);
  return !empty($permission) && user_access($permission, $account);
}

/**
 * Retrieves the filter tips.
 *
 * @param $format_id
 *   The ID of the text format for which to retrieve tips, or -1 to return tips
 *   for all formats accessible to the current user.
 * @param $long
 *   (optional) Boolean indicating whether the long form of tips should be
 *   returned. Defaults to FALSE.
 *
 * @return
 *   An associative array of filtering tips, keyed by filter name. Each
 *   filtering tip is an associative array with elements:
 *   - tip: Tip text.
 *   - id: Filter ID.
 */
function _filter_tips($format_id, $long = FALSE) {
  global $user;

  $formats = filter_formats($user);
  $filter_info = filter_get_filters();

  $tips = array();

  // If only listing one format, extract it from the $formats array.
  if ($format_id != -1) {
    $formats = array($formats[$format_id]);
  }

  foreach ($formats as $format) {
    $filters = filter_list_format($format->format);
    $tips[$format->name] = array();
    foreach ($filters as $name => $filter) {
      if ($filter->status && isset($filter_info[$name]['tips callback'])) {
        $tip = $filter_info[$name]['tips callback']($filter, $format, $long);
        if (isset($tip)) {
          $tips[$format->name][$name] = array('tip' => $tip, 'id' => $name);
        }
      }
    }
  }

  return $tips;
}

/**
 * Parses an HTML snippet and returns it as a DOM object.
 *
 * This function loads the body part of a partial (X)HTML document and returns
 * a full DOMDocument object that represents this document. You can use
 * filter_dom_serialize() to serialize this DOMDocument back to a XHTML
 * snippet.
 *
 * @param $text
 *   The partial (X)HTML snippet to load. Invalid markup will be corrected on
 *   import.
 *
 * @return
 *   A DOMDocument that represents the loaded (X)HTML snippet.
 */
function filter_dom_load($text) {
  $dom_document = new DOMDocument();
  // Ignore warnings during HTML soup loading.
  @$dom_document->loadHTML('<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head><body>' . $text . '</body></html>');

  return $dom_document;
}

/**
 * Converts a DOM object back to an HTML snippet.
 *
 * The function serializes the body part of a DOMDocument back to an XHTML
 * snippet. The resulting XHTML snippet will be properly formatted to be
 * compatible with HTML user agents.
 *
 * @param $dom_document
 *   A DOMDocument object to serialize, only the tags below
 *   the first <body> node will be converted.
 *
 * @return
 *   A valid (X)HTML snippet, as a string.
 */
function filter_dom_serialize($dom_document) {
  $body_node = $dom_document->getElementsByTagName('body')->item(0);
  $body_content = '';

  foreach ($body_node->getElementsByTagName('script') as $node) {
    filter_dom_serialize_escape_cdata_element($dom_document, $node);
  }

  foreach ($body_node->getElementsByTagName('style') as $node) {
    filter_dom_serialize_escape_cdata_element($dom_document, $node, '/*', '*/');
  }

  foreach ($body_node->childNodes as $child_node) {
    $body_content .= $dom_document->saveXML($child_node);
  }
  return $body_content;
}

/**
 * Adds comments around the <!CDATA section in a dom element.
 *
 * DOMDocument::loadHTML in filter_dom_load() makes CDATA sections from the
 * contents of inline script and style tags.  This can cause HTML 4 browsers to
 * throw exceptions.
 *
 * This function attempts to solve the problem by creating a DocumentFragment
 * and imitating the behavior in drupal_get_js(), commenting the CDATA tag.
 *
 * @param $dom_document
 *   The DOMDocument containing the $dom_element.
 * @param $dom_element
 *   The element potentially containing a CDATA node.
 * @param $comment_start
 *   (optional) A string to use as a comment start marker to escape the CDATA
 *   declaration. Defaults to '//'.
 * @param $comment_end
 *   (optional) A string to use as a comment end marker to escape the CDATA
 *   declaration. Defaults to an empty string.
 */
function filter_dom_serialize_escape_cdata_element($dom_document, $dom_element, $comment_start = '//', $comment_end = '') {
  foreach ($dom_element->childNodes as $node) {
    if (get_class($node) == 'DOMCdataSection') {
      // See drupal_get_js().  This code is more or less duplicated there.
      $embed_prefix = "\n<!--{$comment_start}--><![CDATA[{$comment_start} ><!--{$comment_end}\n";
      $embed_suffix = "\n{$comment_start}--><!]]>{$comment_end}\n";

      // Prevent invalid cdata escaping as this would throw a DOM error.
      // This is the same behavior as found in libxml2.
      // Related W3C standard: http://www.w3.org/TR/REC-xml/#dt-cdsection
      // Fix explanation: http://en.wikipedia.org/wiki/CDATA#Nesting
      $data = str_replace(']]>', ']]]]><![CDATA[>', $node->data);

      $fragment = $dom_document->createDocumentFragment();
      $fragment->appendXML($embed_prefix . $data . $embed_suffix);
      $dom_element->appendChild($fragment);
      $dom_element->removeChild($node);
    }
  }
}

/**
 * Returns HTML for a link to the more extensive filter tips.
 *
 * @ingroup themeable
 */
function theme_filter_tips_more_info() {
  return '<p>' . l(t('More information about text formats'), 'filter/tips', array('attributes' => array('target' => '_blank'))) . '</p>';
}

/**
 * Returns HTML for guidelines for a text format.
 *
 * @param array $variables
 *   An associative array containing:
 *   - format: An object representing a text format.
 *
 * @ingroup themeable
 */
function theme_filter_guidelines($variables) {
  $format = $variables['format'];
  $attributes['class'][] = 'filter-guidelines-item';
  $attributes['class'][] = 'filter-guidelines-' . $format->format;
  $output = '<div' . new Attribute($attributes) . '>';
  $output .= '<h4 class="label">' . check_plain($format->name) . '</h4>';
  $output .= theme('filter_tips', array('tips' => _filter_tips($format->format, FALSE)));
  $output .= '</div>';
  return $output;
}

/**
 * @defgroup standard_filters Standard filters
 * @{
 * Filters implemented by the Filter module.
 */

/**
 * Implements hook_filter_info().
 */
function filter_filter_info() {
  $filters['filter_html'] = array(
    'title' => t('Limit allowed HTML tags'),
    'type' => FILTER_TYPE_HTML_RESTRICTOR,
    'process callback' => '_filter_html',
    'settings callback' => '_filter_html_settings',
    'default settings' => array(
      'allowed_html' => '<a> <em> <strong> <cite> <blockquote> <code> <ul> <ol> <li> <dl> <dt> <dd> <h4> <h5> <h6>',
      'filter_html_help' => 1,
      'filter_html_nofollow' => 0,
    ),
    'tips callback' => '_filter_html_tips',
    'weight' => -10,
  );
  $filters['filter_autop'] = array(
    'title' => t('Convert line breaks into HTML (i.e. <code>&lt;br&gt;</code> and <code>&lt;p&gt;</code>)'),
    'type' => FILTER_TYPE_MARKUP_LANGUAGE,
    'process callback' => '_filter_autop',
    'tips callback' => '_filter_autop_tips',
  );
  $filters['filter_url'] = array(
    'title' => t('Convert URLs into links'),
    'type' => FILTER_TYPE_MARKUP_LANGUAGE,
    'process callback' => '_filter_url',
    'settings callback' => '_filter_url_settings',
    'default settings' => array(
      'filter_url_length' => 72,
    ),
    'tips callback' => '_filter_url_tips',
  );
  $filters['filter_html_image_secure'] = array(
    'title' => t('Restrict images to this site'),
    'type' => FILTER_TYPE_HTML_RESTRICTOR,
    'description' => t('Disallows usage of &lt;img&gt; tag sources that are not hosted on this site by replacing them with a placeholder image.'),
    'process callback' => '_filter_html_image_secure_process',
    'tips callback' => '_filter_html_image_secure_tips',
    // Supposed to run after other filters and before HTML corrector by default.
    'weight' => 9,
  );
  $filters['filter_htmlcorrector'] = array(
    'title' =>  t('Correct faulty and chopped off HTML'),
    'type' => FILTER_TYPE_HTML_RESTRICTOR,
    'process callback' => '_filter_htmlcorrector',
    'weight' => 10,
  );
  $filters['filter_html_escape'] = array(
    'title' => t('Display any HTML as plain text'),
    'type' => FILTER_TYPE_HTML_RESTRICTOR,
    'process callback' => '_filter_html_escape',
    'tips callback' => '_filter_html_escape_tips',
    'weight' => -10,
  );
  return $filters;
}

/**
 * Filter settings callback for the HTML content filter.
 *
 * See hook_filter_FILTER_settings() for documentation of parameters and return
 * value.
 */
function _filter_html_settings($form, &$form_state, $filter, $format, $defaults) {
  $filter->settings += $defaults;

  $settings['allowed_html'] = array(
    '#type' => 'textfield',
    '#title' => t('Allowed HTML tags'),
    '#default_value' => $filter->settings['allowed_html'],
    '#maxlength' => 1024,
    '#description' => t('A list of HTML tags that can be used. JavaScript event attributes, JavaScript URLs, and CSS are always stripped.'),
  );
  $settings['filter_html_help'] = array(
    '#type' => 'checkbox',
    '#title' => t('Display basic HTML help in long filter tips'),
    '#default_value' => $filter->settings['filter_html_help'],
  );
  $settings['filter_html_nofollow'] = array(
    '#type' => 'checkbox',
    '#title' => t('Add rel="nofollow" to all links'),
    '#default_value' => $filter->settings['filter_html_nofollow'],
  );
  return $settings;
}

/**
 * Provides filtering of input into accepted HTML.
 */
function _filter_html($text, $filter) {
  $allowed_tags = preg_split('/\s+|<|>/', $filter->settings['allowed_html'], -1, PREG_SPLIT_NO_EMPTY);
  $text = filter_xss($text, $allowed_tags);

  if ($filter->settings['filter_html_nofollow']) {
    $html_dom = filter_dom_load($text);
    $links = $html_dom->getElementsByTagName('a');
    foreach ($links as $link) {
      $link->setAttribute('rel', 'nofollow');
    }
    $text = filter_dom_serialize($html_dom);
  }

  return trim($text);
}

/**
 * Filter tips callback: Provides help for the HTML filter.
 *
 * @see filter_filter_info()
 */
function _filter_html_tips($filter, $format, $long = FALSE) {
  global $base_url;

  if (!($allowed_html = $filter->settings['allowed_html'])) {
    return;
  }
  $output = t('Allowed HTML tags: @tags', array('@tags' => $allowed_html));
  if (!$long) {
    return $output;
  }

  $output = '<p>' . $output . '</p>';
  if (!$filter->settings['filter_html_help']) {
    return $output;
  }

  $output .= '<p>' . t('This site allows HTML content. While learning all of HTML may feel intimidating, learning how to use a very small number of the most basic HTML "tags" is very easy. This table provides examples for each tag that is enabled on this site.') . '</p>';
  $output .= '<p>' . t('For more information see W3C\'s <a href="@html-specifications">HTML Specifications</a> or use your favorite search engine to find other sites that explain HTML.', array('@html-specifications' => 'http://www.w3.org/TR/html/')) . '</p>';
  $tips = array(
    'a' => array(t('Anchors are used to make links to other pages.'), '<a href="' . $base_url . '">' . check_plain(config('system.site')->get('name')) . '</a>'),
    'br' => array(t('By default line break tags are automatically added, so use this tag to add additional ones. Use of this tag is different because it is not used with an open/close pair like all the others. Use the extra " /" inside the tag to maintain XHTML 1.0 compatibility'), t('Text with <br />line break')),
    'p' => array(t('By default paragraph tags are automatically added, so use this tag to add additional ones.'), '<p>' . t('Paragraph one.') . '</p> <p>' . t('Paragraph two.') . '</p>'),
    'strong' => array(t('Strong', array(), array('context' => 'Font weight')), '<strong>' . t('Strong', array(), array('context' => 'Font weight')) . '</strong>'),
    'em' => array(t('Emphasized'), '<em>' . t('Emphasized') . '</em>'),
    'cite' => array(t('Cited'), '<cite>' . t('Cited') . '</cite>'),
    'code' => array(t('Coded text used to show programming source code'), '<code>' . t('Coded') . '</code>'),
    'b' => array(t('Bolded'), '<b>' . t('Bolded') . '</b>'),
    'u' => array(t('Underlined'), '<u>' . t('Underlined') . '</u>'),
    'i' => array(t('Italicized'), '<i>' . t('Italicized') . '</i>'),
    'sup' => array(t('Superscripted'), t('<sup>Super</sup>scripted')),
    'sub' => array(t('Subscripted'), t('<sub>Sub</sub>scripted')),
    'pre' => array(t('Preformatted'), '<pre>' . t('Preformatted') . '</pre>'),
    'abbr' => array(t('Abbreviation'), t('<abbr title="Abbreviation">Abbrev.</abbr>')),
    'acronym' => array(t('Acronym'), t('<acronym title="Three-Letter Acronym">TLA</acronym>')),
    'blockquote' => array(t('Block quoted'), '<blockquote>' . t('Block quoted') . '</blockquote>'),
    'q' => array(t('Quoted inline'), '<q>' . t('Quoted inline') . '</q>'),
    // Assumes and describes tr, td, th.
    'table' => array(t('Table'), '<table> <tr><th>' . t('Table header') . '</th></tr> <tr><td>' . t('Table cell') . '</td></tr> </table>'),
    'tr' => NULL, 'td' => NULL, 'th' => NULL,
    'del' => array(t('Deleted'), '<del>' . t('Deleted') . '</del>'),
    'ins' => array(t('Inserted'), '<ins>' . t('Inserted') . '</ins>'),
     // Assumes and describes li.
    'ol' => array(t('Ordered list - use the &lt;li&gt; to begin each list item'), '<ol> <li>' . t('First item') . '</li> <li>' . t('Second item') . '</li> </ol>'),
    'ul' => array(t('Unordered list - use the &lt;li&gt; to begin each list item'), '<ul> <li>' . t('First item') . '</li> <li>' . t('Second item') . '</li> </ul>'),
    'li' => NULL,
    // Assumes and describes dt and dd.
    'dl' => array(t('Definition lists are similar to other HTML lists. &lt;dl&gt; begins the definition list, &lt;dt&gt; begins the definition term and &lt;dd&gt; begins the definition description.'), '<dl> <dt>' . t('First term') . '</dt> <dd>' . t('First definition') . '</dd> <dt>' . t('Second term') . '</dt> <dd>' . t('Second definition') . '</dd> </dl>'),
    'dt' => NULL, 'dd' => NULL,
    'h1' => array(t('Heading'), '<h1>' . t('Title') . '</h1>'),
    'h2' => array(t('Heading'), '<h2>' . t('Subtitle') . '</h2>'),
    'h3' => array(t('Heading'), '<h3>' . t('Subtitle three') . '</h3>'),
    'h4' => array(t('Heading'), '<h4>' . t('Subtitle four') . '</h4>'),
    'h5' => array(t('Heading'), '<h5>' . t('Subtitle five') . '</h5>'),
    'h6' => array(t('Heading'), '<h6>' . t('Subtitle six') . '</h6>')
  );
  $header = array(t('Tag Description'), t('You Type'), t('You Get'));
  preg_match_all('/<([a-z0-9]+)[^a-z0-9]/i', $allowed_html, $out);
  foreach ($out[1] as $tag) {
    if (!empty($tips[$tag])) {
      $rows[] = array(
        array('data' => $tips[$tag][0], 'class' => array('description')),
        array('data' => '<code>' . check_plain($tips[$tag][1]) . '</code>', 'class' => array('type')),
        array('data' => $tips[$tag][1], 'class' => array('get'))
      );
    }
    else {
      $rows[] = array(
        array('data' => t('No help provided for tag %tag.', array('%tag' => $tag)), 'class' => array('description'), 'colspan' => 3),
      );
    }
  }
  $output .= theme('table', array('header' => $header, 'rows' => $rows));

  $output .= '<p>' . t('Most unusual characters can be directly entered without any problems.') . '</p>';
  $output .= '<p>' . t('If you do encounter problems, try using HTML character entities. A common example looks like &amp;amp; for an ampersand &amp; character. For a full list of entities see HTML\'s <a href="@html-entities">entities</a> page. Some of the available characters include:', array('@html-entities' => 'http://www.w3.org/TR/html4/sgml/entities.html')) . '</p>';

  $entities = array(
    array(t('Ampersand'), '&amp;'),
    array(t('Greater than'), '&gt;'),
    array(t('Less than'), '&lt;'),
    array(t('Quotation mark'), '&quot;'),
  );
  $header = array(t('Character Description'), t('You Type'), t('You Get'));
  unset($rows);
  foreach ($entities as $entity) {
    $rows[] = array(
      array('data' => $entity[0], 'class' => array('description')),
      array('data' => '<code>' . check_plain($entity[1]) . '</code>', 'class' => array('type')),
      array('data' => $entity[1], 'class' => array('get'))
    );
  }
  $output .= theme('table', array('header' => $header, 'rows' => $rows));
  return $output;
}

/**
 * Filter URL settings callback: Provides settings for the URL filter.
 *
 * @see filter_filter_info()
 */
function _filter_url_settings($form, &$form_state, $filter, $format, $defaults) {
  $filter->settings += $defaults;

  $settings['filter_url_length'] = array(
    '#type' => 'number',
    '#title' => t('Maximum link text length'),
    '#default_value' => $filter->settings['filter_url_length'],
    '#min' => 1,
    '#field_suffix' => t('characters'),
    '#description' => t('URLs longer than this number of characters will be truncated to prevent long strings that break formatting. The link itself will be retained; just the text portion of the link will be truncated.'),
  );
  return $settings;
}

/**
 * Converts text into hyperlinks automatically.
 *
 * This filter identifies and makes clickable three types of "links".
 * - URLs like http://example.com.
 * - E-mail addresses like name@example.com.
 * - Web addresses without the "http://" protocol defined, like
 *   www.example.com.
 * Each type must be processed separately, as there is no one regular
 * expression that could possibly match all of the cases in one pass.
 */
function _filter_url($text, $filter) {
  // Tags to skip and not recurse into.
  $ignore_tags = 'a|script|style|code|pre';

  // Pass length to regexp callback.
  _filter_url_trim(NULL, $filter->settings['filter_url_length']);

  // Create an array which contains the regexps for each type of link.
  // The key to the regexp is the name of a function that is used as
  // callback function to process matches of the regexp. The callback function
  // is to return the replacement for the match. The array is used and
  // matching/replacement done below inside some loops.
  $tasks = array();

  // Prepare protocols pattern for absolute URLs.
  // check_url() will replace any bad protocols with HTTP, so we need to support
  // the identical list. While '//' is technically optional for MAILTO only,
  // we cannot cleanly differ between protocols here without hard-coding MAILTO,
  // so '//' is optional for all protocols.
  // @see filter_xss_bad_protocol()
  $protocols = config('system.filter')->get('protocols');
  $protocols = implode(':(?://)?|', $protocols) . ':(?://)?';

  // Prepare domain name pattern.
  // The ICANN seems to be on track towards accepting more diverse top level
  // domains, so this pattern has been "future-proofed" to allow for TLDs
  // of length 2-64.
  $domain = '(?:[A-Za-z0-9._+-]+\.)?[A-Za-z]{2,64}\b';
  $ip = '(?:[0-9]{1,3}\.){3}[0-9]{1,3}';
  $auth = '[a-zA-Z0-9:%_+*~#?&=.,/;-]+@';
  $trail = '[a-zA-Z0-9:%_+*~#&\[\]=/;?!\.,-]*[a-zA-Z0-9:%_+*~#&\[\]=/;-]';

  // Prepare pattern for optional trailing punctuation.
  // Even these characters could have a valid meaning for the URL, such usage is
  // rare compared to using a URL at the end of or within a sentence, so these
  // trailing characters are optionally excluded.
  $punctuation = '[\.,?!]*?';

  // Match absolute URLs.
  $url_pattern = "(?:$auth)?(?:$domain|$ip)/?(?:$trail)?";
  $pattern = "`((?:$protocols)(?:$url_pattern))($punctuation)`";
  $tasks['_filter_url_parse_full_links'] = $pattern;

  // Match e-mail addresses.
  $url_pattern = "[A-Za-z0-9._-]{1,254}@(?:$domain)";
  $pattern = "`($url_pattern)`";
  $tasks['_filter_url_parse_email_links'] = $pattern;

  // Match www domains.
  $url_pattern = "www\.(?:$domain)/?(?:$trail)?";
  $pattern = "`($url_pattern)($punctuation)`";
  $tasks['_filter_url_parse_partial_links'] = $pattern;

  // Each type of URL needs to be processed separately. The text is joined and
  // re-split after each task, since all injected HTML tags must be correctly
  // protected before the next task.
  foreach ($tasks as $task => $pattern) {
    // HTML comments need to be handled separately, as they may contain HTML
    // markup, especially a '>'. Therefore, remove all comment contents and add
    // them back later.
    _filter_url_escape_comments('', TRUE);
    $text = preg_replace_callback('`<!--(.*?)-->`s', '_filter_url_escape_comments', $text);

    // Split at all tags; ensures that no tags or attributes are processed.
    $chunks = preg_split('/(<.+?>)/is', $text, -1, PREG_SPLIT_DELIM_CAPTURE);
    // PHP ensures that the array consists of alternating delimiters and
    // literals, and begins and ends with a literal (inserting NULL as
    // required). Therefore, the first chunk is always text:
    $chunk_type = 'text';
    // If a tag of $ignore_tags is found, it is stored in $open_tag and only
    // removed when the closing tag is found. Until the closing tag is found,
    // no replacements are made.
    $open_tag = '';

    for ($i = 0; $i < count($chunks); $i++) {
      if ($chunk_type == 'text') {
        // Only process this text if there are no unclosed $ignore_tags.
        if ($open_tag == '') {
          // If there is a match, inject a link into this chunk via the callback
          // function contained in $task.
          $chunks[$i] = preg_replace_callback($pattern, $task, $chunks[$i]);
        }
        // Text chunk is done, so next chunk must be a tag.
        $chunk_type = 'tag';
      }
      else {
        // Only process this tag if there are no unclosed $ignore_tags.
        if ($open_tag == '') {
          // Check whether this tag is contained in $ignore_tags.
          if (preg_match("`<($ignore_tags)(?:\s|>)`i", $chunks[$i], $matches)) {
            $open_tag = $matches[1];
          }
        }
        // Otherwise, check whether this is the closing tag for $open_tag.
        else {
          if (preg_match("`<\/$open_tag>`i", $chunks[$i], $matches)) {
            $open_tag = '';
          }
        }
        // Tag chunk is done, so next chunk must be text.
        $chunk_type = 'text';
      }
    }

    $text = implode($chunks);
    // Revert back to the original comment contents
    _filter_url_escape_comments('', FALSE);
    $text = preg_replace_callback('`<!--(.*?)-->`', '_filter_url_escape_comments', $text);
  }

  return $text;
}

/**
 * Makes links out of absolute URLs.
 *
 * Callback for preg_replace_callback() within _filter_url().
 */
function _filter_url_parse_full_links($match) {
  // The $i:th parenthesis in the regexp contains the URL.
  $i = 1;

  $match[$i] = decode_entities($match[$i]);
  $caption = check_plain(_filter_url_trim($match[$i]));
  $match[$i] = check_plain($match[$i]);
  return '<a href="' . $match[$i] . '">' . $caption . '</a>' . $match[$i + 1];
}

/**
 * Makes links out of e-mail addresses.
 *
 * Callback for preg_replace_callback() within _filter_url().
 */
function _filter_url_parse_email_links($match) {
  // The $i:th parenthesis in the regexp contains the URL.
  $i = 0;

  $match[$i] = decode_entities($match[$i]);
  $caption = check_plain(_filter_url_trim($match[$i]));
  $match[$i] = check_plain($match[$i]);
  return '<a href="mailto:' . $match[$i] . '">' . $caption . '</a>';
}

/**
 * Makes links out of domain names starting with "www."
 *
 * Callback for preg_replace_callback() within _filter_url().
 */
function _filter_url_parse_partial_links($match) {
  // The $i:th parenthesis in the regexp contains the URL.
  $i = 1;

  $match[$i] = decode_entities($match[$i]);
  $caption = check_plain(_filter_url_trim($match[$i]));
  $match[$i] = check_plain($match[$i]);
  return '<a href="http://' . $match[$i] . '">' . $caption . '</a>' . $match[$i + 1];
}

/**
 * Escapes the contents of HTML comments.
 *
 * Callback for preg_replace_callback() within _filter_url().
 *
 * @param $match
 *   An array containing matches to replace from preg_replace_callback(),
 *   whereas $match[1] is expected to contain the content to be filtered.
 * @param $escape
 *   (optional) A Boolean indicating whether to escape (TRUE) or unescape
 *   comments (FALSE). Defaults to NULL, indicating neither. If TRUE, statically
 *   cached $comments are reset.
 */
function _filter_url_escape_comments($match, $escape = NULL) {
  static $mode, $comments = array();

  if (isset($escape)) {
    $mode = $escape;
    if ($escape){
      $comments = array();
    }
    return;
  }

  // Replace all HTML coments with a '<!-- [hash] -->' placeholder.
  if ($mode) {
    $content = $match[1];
    $hash = md5($content);
    $comments[$hash] = $content;
    return "<!-- $hash -->";
  }
  // Or replace placeholders with actual comment contents.
  else {
    $hash = $match[1];
    $hash = trim($hash);
    $content = $comments[$hash];
    return "<!--$content-->";
  }
}

/**
 * Shortens long URLs to http://www.example.com/long/url...
 */
function _filter_url_trim($text, $length = NULL) {
  static $_length;
  if ($length !== NULL) {
    $_length = $length;
  }

  // Use +3 for '...' string length.
  if ($_length && strlen($text) > $_length + 3) {
    $text = substr($text, 0, $_length) . '...';
  }

  return $text;
}

/**
 * Filter tips callback: Provides help for the URL filter.
 *
 * @see filter_filter_info()
 */
function _filter_url_tips($filter, $format, $long = FALSE) {
  return t('Web page addresses and e-mail addresses turn into links automatically.');
}

/**
 * Scans the input and makes sure that HTML tags are properly closed.
 */
function _filter_htmlcorrector($text) {
  return filter_dom_serialize(filter_dom_load($text));
}

/**
 * Converts line breaks into <p> and <br> in an intelligent fashion.
 *
 * Based on: http://photomatt.net/scripts/autop
 */
function _filter_autop($text) {
  // All block level tags
  $block = '(?:table|thead|tfoot|caption|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|select|form|blockquote|address|p|h[1-6]|hr|article|aside|details|figcaption|figure|footer|header|hgroup|menu|nav|section|summary)';

  // Split at opening and closing PRE, SCRIPT, STYLE, OBJECT, IFRAME tags
  // and comments. We don't apply any processing to the contents of these tags
  // to avoid messing up code. We look for matched pairs and allow basic
  // nesting. For example:
  // "processed <pre> ignored <script> ignored </script> ignored </pre> processed"
  $chunks = preg_split('@(<!--.*?-->|</?(?:pre|script|style|object|iframe|!--)[^>]*>)@i', $text, -1, PREG_SPLIT_DELIM_CAPTURE);
  // Note: PHP ensures the array consists of alternating delimiters and literals
  // and begins and ends with a literal (inserting NULL as required).
  $ignore = FALSE;
  $ignoretag = '';
  $output = '';
  foreach ($chunks as $i => $chunk) {
    if ($i % 2) {
      $comment = (substr($chunk, 0, 4) == '<!--');
      if ($comment) {
        // Nothing to do, this is a comment.
        $output .= $chunk;
        continue;
      }
      // Opening or closing tag?
      $open = ($chunk[1] != '/');
      list($tag) = preg_split('/[ >]/', substr($chunk, 2 - $open), 2);
      if (!$ignore) {
        if ($open) {
          $ignore = TRUE;
          $ignoretag = $tag;
        }
      }
      // Only allow a matching tag to close it.
      elseif (!$open && $ignoretag == $tag) {
        $ignore = FALSE;
        $ignoretag = '';
      }
    }
    elseif (!$ignore) {
      $chunk = preg_replace('|\n*$|', '', $chunk) . "\n\n"; // just to make things a little easier, pad the end
      $chunk = preg_replace('|<br />\s*<br />|', "\n\n", $chunk);
      $chunk = preg_replace('!(<' . $block . '[^>]*>)!', "\n$1", $chunk); // Space things out a little
      $chunk = preg_replace('!(</' . $block . '>)!', "$1\n\n", $chunk); // Space things out a little
      $chunk = preg_replace("/\n\n+/", "\n\n", $chunk); // take care of duplicates
      $chunk = preg_replace('/^\n|\n\s*\n$/', '', $chunk);
      $chunk = '<p>' . preg_replace('/\n\s*\n\n?(.)/', "</p>\n<p>$1", $chunk) . "</p>\n"; // make paragraphs, including one at the end
      $chunk = preg_replace("|<p>(<li.+?)</p>|", "$1", $chunk); // problem with nested lists
      $chunk = preg_replace('|<p><blockquote([^>]*)>|i', "<blockquote$1><p>", $chunk);
      $chunk = str_replace('</blockquote></p>', '</p></blockquote>', $chunk);
      $chunk = preg_replace('|<p>\s*</p>\n?|', '', $chunk); // under certain strange conditions it could create a P of entirely whitespace
      $chunk = preg_replace('!<p>\s*(</?' . $block . '[^>]*>)!', "$1", $chunk);
      $chunk = preg_replace('!(</?' . $block . '[^>]*>)\s*</p>!', "$1", $chunk);
      $chunk = preg_replace('|(?<!<br />)\s*\n|', "<br />\n", $chunk); // make line breaks
      $chunk = preg_replace('!(</?' . $block . '[^>]*>)\s*<br />!', "$1", $chunk);
      $chunk = preg_replace('!<br />(\s*</?(?:p|li|div|th|pre|td|ul|ol)>)!', '$1', $chunk);
      $chunk = preg_replace('/&([^#])(?![A-Za-z0-9]{1,8};)/', '&amp;$1', $chunk);
    }
    $output .= $chunk;
  }
  return $output;
}

/**
 * Filter tips callback: Provides help for the auto-paragraph filter.
 *
 * @see filter_filter_info()
 */
function _filter_autop_tips($filter, $format, $long = FALSE) {
  if ($long) {
    return t('Lines and paragraphs are automatically recognized. The &lt;br /&gt; line break, &lt;p&gt; paragraph and &lt;/p&gt; close paragraph tags are inserted automatically. If paragraphs are not recognized simply add a couple blank lines.');
  }
  else {
    return t('Lines and paragraphs break automatically.');
  }
}

/**
 * Escapes all HTML tags, so they will be visible instead of being effective.
 */
function _filter_html_escape($text) {
  return trim(check_plain($text));
}

/**
 * Filter tips callback: Provides help for the HTML escaping filter.
 *
 * @see filter_filter_info()
 */
function _filter_html_escape_tips($filter, $format, $long = FALSE) {
  return t('No HTML tags allowed.');
}

/**
 * Process callback for local image filter.
 */
function _filter_html_image_secure_process($text) {
  // Find the path (e.g. '/') to Drupal root.
  $base_path = base_path();
  $base_path_length = drupal_strlen($base_path);

  // Find the directory on the server where index.php resides.
  $local_dir = DRUPAL_ROOT . '/';

  $html_dom = filter_dom_load($text);
  $images = $html_dom->getElementsByTagName('img');
  foreach ($images as $image) {
    $src = $image->getAttribute('src');
    // Remove absolute URLs pointing to the local domain to prevent mixed
    // content errors.
    $image->setAttribute('src', preg_replace('|^https?://' . $_SERVER['HTTP_HOST'] . '|', '', $src));

    // Verify that $src starts with $base_path.
    // This also ensures that external images cannot be referenced.
    $src = $image->getAttribute('src');
    if (drupal_substr($src, 0, $base_path_length) === $base_path) {
      // Remove the $base_path to get the path relative to the Drupal root.
      // Ensure the path refers to an actual image by prefixing the image source
      // with the Drupal root and running getimagesize() on it.
      $local_image_path = $local_dir . drupal_substr($src, $base_path_length);
      if (@getimagesize($local_image_path)) {
        // The image has the right path. Erroneous images are dealt with below.
        continue;
      }
    }
    // Replace an invalid image with an error indicator.
    theme('filter_html_image_secure_image', array('image' => $image));
  }
  $text = filter_dom_serialize($html_dom);
  return $text;
}

/**
 * Formats an image DOM element that has an invalid source.
 *
 * @param DOMElement $image
 *   An IMG node to format, parsed from the filtered text.
 *
 * @return void
 *   Unlike other theme functions, the passed in $image is altered by reference.
 *
 * @see _filter_html_image_secure_process()
 * @ingroup themeable
 */
function theme_filter_html_image_secure_image(&$variables) {
  $image = $variables['image'];

  // Turn an invalid image into an error indicator.
  $image->setAttribute('src', base_path() . 'core/misc/message-16-error.png');
  $image->setAttribute('alt', t('Image removed.'));
  $image->setAttribute('title', t('This image has been removed. For security reasons, only images from the local domain are allowed.'));

  // Add a CSS class to aid in styling.
  $class = ($image->getAttribute('class') ? trim($image->getAttribute('class')) . ' ' : '');
  $class .= 'filter-image-invalid';
  $image->setAttribute('class', $class);
}

/**
 * Filter tips callback for secure HTML image filter.
 */
function _filter_html_image_secure_tips($filter, $format, $long = FALSE) {
  return t('Only images hosted on this site may be used in &lt;img&gt; tags.');
}

/**
 * @} End of "defgroup standard_filters".
 */

/**
 * Implements hook_library_info().
 */
function filter_library_info() {
  $libraries['drupal.filter.admin'] = array(
    'title' => 'Filter',
    'version' => VERSION,
    'js' => array(
      drupal_get_path('module', 'filter') . '/filter.admin.js' => array(),
    ),
    'css' => array(
      drupal_get_path('module', 'filter') . '/filter.admin.css'
    ),
    'dependencies' => array(
      array('system', 'jquery'),
      array('system', 'drupal'),
      array('system', 'jquery.once'),
      array('system', 'drupal.form'),
    ),
  );
  $libraries['drupal.filter'] = array(
    'title' => 'Filter',
    'version' => VERSION,
    'js' => array(
      drupal_get_path('module', 'filter') . '/filter.js' => array(),
    ),
    'css' => array(
      drupal_get_path('module', 'filter') . '/filter.admin.css'
    ),
    'dependencies' => array(
      array('system', 'jquery'),
      array('system', 'drupal'),
      array('system', 'jquery.once'),
    ),
  );

  return $libraries;
}