repeating_dates.module

<?php
/**
 * @file
 * Repeating dates main module code.
 */

require_once __DIR__ . '/libraries/php-rrule/src/RfcParser.php';
require_once __DIR__ . '/libraries/php-rrule/src/RRuleInterface.php';
require_once __DIR__ . '/libraries/php-rrule/src/RRuleTrait.php';
require_once __DIR__ . '/libraries/php-rrule/src/RRule.php';
require_once __DIR__ . '/libraries/php-rrule/src/RSet.php';

use RRule\RRule;
use RRule\RSet;

define('REPEATING_DATES_MAX_VALUES', 730);

/**
 * Implements hook_field_info().
 */
function repeating_dates_field_info() {
  return array(
    'repeating_date' => array(
      'label' => t('Date (repeating)'),
      'description' => t('This field stores start and end datetime and a rule to repeat.'),
      'settings' => array(),
      'instance_settings' => array(),
      'default_widget' => 'repeating_date_widget',
      'default_formatter' => 'repeating_date_list',
    ),
  );
}

/**
 * Implements hook_field_is_empty().
 */
function repeating_dates_field_is_empty($item, $field) {
  // Nothing at all.
  if (empty($item['dtstart'])) {
    return TRUE;
  }
  // Start date is empty.
  if (isset($item['dtstart']['date']) && empty($item['dtstart']['date'])) {
    return TRUE;
  }

  return FALSE;
}

/**
 * Implements hook_field_widget_info().
 */
function repeating_dates_field_widget_info() {
  return array(
    'repeating_date_widget' => array(
      'label' => t('Repeating date widget'),
      'field types' => array('repeating_date'),
      'settings' => array(
        'hide_to_date' => FALSE,
        'sync_initial' => 'no',
        'offset' => '',
        'default_repeat_freq' => 'weekly',
        'default_stop_mode' => 'count',
        'exdate_helptext' => t('Exclude dates from this series of recurrences.'),
      ),
      'behaviors' => array(
        'multiple values' => FIELD_BEHAVIOR_CUSTOM,
        'default value' => FIELD_BEHAVIOR_NONE,
      ),
    ),
  );
}

/**
 * Implements hook_field_widget_settings_form().
 */
function repeating_dates_field_widget_settings_form($field, $instance) {
  $widget = $instance['widget'];
  $settings = $widget['settings'];

  if ($widget['type'] == 'repeating_date_widget') {
    $form = array();
    $form['hide_to_date'] = array(
      '#type' => 'checkbox',
      '#title' => t('Hide to-date input'),
      '#default_value' => $settings['hide_to_date'],
      '#description' => t('When to-date is hidden, only a single datetime input is available for start date, no duration.'),
    );
    $options = array(
      'no' => t('Do not synchronize'),
      'same' => t('To-date same as start date'),
      'offset' => t('Relative offset to start date'),
    );
    $form['sync_initial'] = array(
      '#type' => 'select',
      '#title' => t('Synchronize initial to-date'),
      '#options' => $options,
      '#default_value' => $settings['sync_initial'],
      '#description' => t('Initially synchronize start and to-date. Synchronization will stop once the to-date or time is overridden.'),
      '#states' => array(
        'visible' => array(
          ':input[name="instance[widget][settings][hide_to_date]"]' => array('checked' => FALSE),
        ),
      ),
    );
    $offset_help = t('Relative to start date, for example <code>+1 hour</code>. See !strtotime for more details.', array(
      '!strtotime' => l('strtotime', 'https://www.php.net/manual/function.strtotime.php'),
    ));
    $form['offset'] = array(
      '#type' => 'textfield',
      '#title' => t('To-date relative offset'),
      '#default_value' => $settings['offset'],
      '#description' => $offset_help,
      '#states' => array(
        'visible' => array(
          ':input[name="instance[widget][settings][hide_to_date]"]' => array('checked' => FALSE),
          ':input[name="instance[widget][settings][sync_initial]"]' => array('value' => 'offset'),
        ),
        'required' => array(
          ':input[name="instance[widget][settings][hide_to_date]"]' => array('checked' => FALSE),
          ':input[name="instance[widget][settings][sync_initial]"]' => array('value' => 'offset'),
        ),
      ),
      '#element_validate' => array('_repeating_dates_field_widget_settings_offset_validate'),
    );
    $form['default_repeat_freq'] = array(
      '#type' => 'select',
      '#title' => t('Default repeat frequency'),
      '#options' => _repeating_dates_frequency_options(),
      '#default_value' => $settings['default_repeat_freq'],
    );
    $form['default_stop_mode'] = array(
      '#type' => 'radios',
      '#title' => t('By default stop repeating by'),
      '#options' => array(
        'count' => t('Count'),
        'date' => t('Date'),
      ),
      '#default_value' => isset($settings['default_stop_mode']) ? $settings['default_stop_mode'] : 'count',
      );
    $form['exdate_helptext'] = array(
      '#type' => 'textfield',
      '#title' => t('Help text for excluded dates'),
      '#default_value' => $settings['exdate_helptext'],
    );

    return $form;
  }
}

/**
 * Custom element validation callback.
 *
 * @param array $element
 *   FormAPI element (textfield) to validate.
 * @param array $form_state
 *   FormAPI form_state array.
 * @param array $form
 *   Complete field widget form.
 */
function _repeating_dates_field_widget_settings_offset_validate(array $element, array $form_state, array $form) {
  $settings = $form_state['values']['instance']['widget']['settings'];
  if ($settings['sync_initial'] == 'offset') {
    // Make sure an offset pattern is set.
    if (empty(trim($element['#value']))) {
      form_error($element, t('If using a relative offset, the offset setting can not be empty.'));
    }
    $timestamp = strtotime($element['#value']);
    // Make sure that strtotime() can handle the pattern.
    if (!$timestamp) {
      form_error($element, t('The offset pattern appears to be incorrect.'));
    }
    // Make sure to-date is after start date.
    if ($timestamp < REQUEST_TIME) {
      form_error($element, t('The offset pattern results in a to-date <em>before</em> start date.'));
    }
  }
}

/**
 * Implements hook_field_widget_form().
 */
function repeating_dates_field_widget_form(&$form, &$form_state, $field, $instance, $langcode, $items, $delta, $element) {
  switch ($instance['widget']['type']) {
    case 'repeating_date_widget':
      // The first item contains all the info we need (dtstart and rrule).
      $values = !empty($items[0]) ? $items[0] : array();
      require_once __DIR__ . '/repeating_dates.widget_form.inc';

      $element += _repeating_dates_widget_form($form_state, $instance, $values, $field['cardinality']);
      if (!empty($element['#description'])) {
        $element['rrule']['#description'] = $element['#description'];
      }

      $module_path = backdrop_get_path('module', 'repeating_dates');
      $element['#attached']['css'][] = $module_path . '/css/repeating-dates.css';
      $element['#attached']['js'][] = $module_path . '/js/repeating-dates-flush.js';
      $element['#attached']['icons'] = array('backspace-fill');

      // If we have a to-date input, add some convenience for date input, if
      // configured.
      $widget_settings = $instance['widget']['settings'];
      if (!$widget_settings['hide_to_date'] && $widget_settings['sync_initial'] != 'no') {
        $offset = 0;
        if ($widget_settings['sync_initial'] == 'offset' && !empty($widget_settings['offset'])) {
          $timestamp = strtotime($widget_settings['offset']);
          if ($timestamp) {
            $offset = $timestamp - REQUEST_TIME;
          }
        }
        $element['#attached']['js'][] = $module_path . '/js/repeating-dates-sync.js';
        $element['rrule']['#attributes']['data-sync-offset'] = $offset;
        $element['rrule']['#attributes']['class'][] = 'repeating-dates-sync-widget';
      }

      break;
  }

  return $element;
}

/**
 * Custom validation callback for the widget form.
 */
function _repeating_dates_widget_validate($element, &$form_state, $form) {
  $field_name = $element['#field_name'];
  $rrule = $form_state['values'][$field_name][LANGUAGE_NONE]['rrule'];
  $parents = implode('][', $element['#parents']);

  if (!$element['#required'] && empty($rrule['dtstart']['date'])) {
    // Nothing to do.
    return;
  }

  if ($element['#required'] == TRUE && empty($rrule['dtstart']['date'])) {
    $selector = $parents . '][rrule][dtstart';
    form_set_error($selector, t('Start date is required.'));
  }

  $tz = date_default_timezone();
  $dtstart = _repeating_dates_array_to_timestamp($rrule['dtstart'], $tz);
  $type = $form[$field_name][LANGUAGE_NONE]['rrule']['dtend']['#type'];
  if ($type != 'value' && !empty($rrule['dtend']['date'])) {
    $dtend = _repeating_dates_array_to_timestamp($rrule['dtend'], $tz);
  }
  else {
    // Either dtend is empty, or the form item is hidden, in both cases we set
    // the value automatically.
    $form_state['values'][$field_name][LANGUAGE_NONE]['rrule']['dtend'] = $rrule['dtstart'];
    $dtend = $dtstart;
  }
  if ($dtstart > $dtend) {
    $selector = $parents . '][rrule][dtstart';
    form_set_error($selector, t('Start can not be after end.'));
  }

  if ($rrule['freq'] == 'weekly') {
    $weekdays = array_filter($rrule['byday_week']);
    if (empty($weekdays)) {
      $selector = $parents . '][rrule][byday_week';
      form_set_error($selector, t('Weekly frequencies need at least one weekday selected.'));
    }
  }
  if ($rrule['stopby'] == 'date' && empty($rrule['until'])) {
    $selector = $parents . '][rrule][until';
    form_set_error($selector, t('If you stop based on date, the date may not be empty.'));
  }
  if ($rrule['stopby'] == 'count' && empty($rrule['count'])) {
    $selector = $parents . '][rrule][count';
    form_set_error($selector, t('If you stop based on number, the number may not be empty.'));
  }
}

/**
 * Implements hook_field_formatter_info().
 */
function repeating_dates_field_formatter_info() {
  return array(
    'repeating_date_list' => array(
      'label' => t('Repeating date list'),
      'field types' => array('repeating_date'),
      'settings' => array(
        'hide_description' => FALSE,
        'datetime_format' => 'short',
        'dateonly_format' => 'html_date',
        'timeonly_format' => 'html_time',
      ),
    ),
  );
}

/**
 * Implements hook_field_formatter_settings_form().
 */
function repeating_dates_field_formatter_settings_form($field, $instance, $view_mode, $form, &$form_state) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];
  $element = array();

  if ($display['type'] == 'repeating_date_list') {
    $element['hide_description'] = array(
      '#type' => 'checkbox',
      '#title' => t('Hide the rule summary (description)'),
      '#default_value' => $settings['hide_description'],
    );

    $date_formats = system_get_date_formats();
    $options = array();
    foreach ($date_formats as $name => $values) {
      $options[$name] = $values['label'];
    }
    $element['datetime_format'] = array(
      '#type' => 'select',
      '#title' => t('Full date and time format'),
      '#options' => $options,
      '#default_value' => $settings['datetime_format'],
    );
    $element['dateonly_format'] = array(
      '#type' => 'select',
      '#title' => t('Date only format'),
      '#options' => $options,
      '#default_value' => $settings['dateonly_format'],
    );
    $element['timeonly_format'] = array(
      '#type' => 'select',
      '#title' => t('Time only format'),
      '#options' => $options,
      '#default_value' => $settings['timeonly_format'],
      '#description' => t('You might want to create a custom format for date only or time only.'),
    );
  }

  return $element;
}

/**
 * Implements hook_field_formatter_settings_summary().
 */
function repeating_dates_field_formatter_settings_summary($field, $instance, $view_mode) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];
  $summary = '';

  if ($display['type'] == 'repeating_date_list') {
    $list = array();
    if ($settings['hide_description']) {
      $list[] = t('The repeat rule summary is hidden.');
    }
    else {
      $list[] = t('The repeat rule summary is shown.');
    }
    $formats = system_get_date_formats();
    $list[] = t('Full datetime format is %format', array(
      '%format' => $formats[$settings['datetime_format']]['label'],
    ));
    $list[] = t('Date only format is %format', array(
      '%format' => $formats[$settings['dateonly_format']]['label'],
    ));
    $list[] = t('Time only format is %format', array(
      '%format' => $formats[$settings['timeonly_format']]['label'],
    ));

    $summary = theme('item_list', array('items' => $list));
  }

  return $summary;
}

/**
 * Implements hook_field_formatter_view().
 */
function repeating_dates_field_formatter_view($object_type, $object, $field, $instance, $langcode, $items, $display) {
  $element = array();
  switch ($display['type']) {
    case 'repeating_date_list':
      if (empty($items)) {
        return $element;
      }
      // @todo is there a way to fiddle a preview and is it worth it?
      $args = arg();
      if (count($args) == 4 && $args[1] == 'preview') {
        $output = t('Field %label does not support dynamic preview.', array(
          '%label' => $instance['label'],
        ));
        $element[0] = array('#markup' => $output);
        return $element;
      }

      $settings = $display['settings'];
      $prefix = '';
      if (!$settings['hide_description']) {
        $prefix = '<div class="rrule-description">' . _repeating_dates_rule_description($items[0], $settings) . '</div>';
      }
      $item_list = array();
      $dtstart_date = format_date($items[0]['dtstart'], 'custom', 'Y-m-d');
      $dtend_date = format_date($items[0]['dtend'], 'custom', 'Y-m-d');

      foreach ($items as $delta => $item) {
        $item_list[$delta] = format_date($item['dtstart'], $settings['datetime_format']);
        // Skip "To" datetime, if it's the same as start.
        if ($item['dtend'] != $item['dtstart']) {
          // Skip redundant date info, if both dates are the same.
          if ($dtstart_date == $dtend_date) {
            $item_list[$delta] .= ' – ' . format_date($item['dtend'], $settings['timeonly_format']);
          }
          else {
            $item_list[$delta] .= ' – ' . format_date($item['dtend'], $settings['datetime_format']);
          }
        }
      }
      $element[0] = array(
        '#type' => 'markup',
        '#markup' => $prefix . theme('item_list', array('items' => $item_list)),
      );
      break;
  }

  return $element;
}

/**
 * Implements hook_field_presave().
 */
function repeating_dates_field_presave($entity_type, $entity, $field, $instance, $langcode, &$items) {
  if (empty($items)) {
    return;
  }
  // Already processed, like in VBO.
  if (!is_array($items['0']['dtstart'])) {
    return;
  }

  // Remove wrapping delta.
  $rrule = $items['0'];
  $items_processed = array();

  $tz = date_default_timezone();
  $dtstart_str = trim(implode(' ', $rrule['dtstart']));
  $dtstart_obj = new DateTime($dtstart_str, new DateTimeZone($tz));
  $dtstart_stamp = $dtstart_obj->format('U');
  $duration = 0;
  $ical_rrule = _repeating_dates_build_rrule($rrule, $dtstart_obj);
  if (is_array($rrule['dtend']) && !empty($rrule['dtend']['date'])) {
    $dtend_stamp = _repeating_dates_array_to_timestamp($rrule['dtend'], $tz);
    $duration = $dtend_stamp - $dtstart_stamp;
  }

  if (!empty($ical_rrule)) {
    $max = $field['cardinality'];
    $exceeded = FALSE;
    if ($max == FIELD_CARDINALITY_UNLIMITED) {
      $max = REPEATING_DATES_MAX_VALUES;
    }
    try {
      $string = "DTSTART;TZID=$tz:" . $dtstart_obj->format('Ymd\THis') . "\r\n" . $ical_rrule;
      $rrule_obj = RRule::createFromRfcString($string);

      foreach ($rrule_obj as $delta => $recurrence) {
        if ($delta > $max) {
          $exceeded = TRUE;
          break;
        }
        $items_processed[$delta]['dtstart'] = $recurrence->format('U');
        $items_processed[$delta]['dtend'] = $recurrence->format('U') + $duration;
        $items_processed[$delta]['rrule'] = $ical_rrule;
      }
    }
    catch (Exception $e) {
      watchdog_exception('repeating_dates', $e);
      $items = array();
      return;
    }
    if ($exceeded) {
      $message = t('The repeat setup created more items than field %field_label allows. Truncated to @max recurrences.', array(
        '%field_label' => $instance['label'],
        '@max' => $max,
      ));
      backdrop_set_message($message, 'warning');
    }
  }
  else {
    // Single datetime pair, no repeat rule.
    $items_processed[0]['dtstart'] = $dtstart_stamp;
    $items_processed[0]['dtend'] = $dtstart_stamp + $duration;
    $items_processed[0]['rrule'] = '';
  }
  // Replace rrule with processed items.
  $items = $items_processed;
}

/**
 * Implements hook_field_views_data_views_data_alter().
 */
function repeating_dates_field_views_data_views_data_alter(&$data, $field) {
  if ($field['type'] == 'repeating_date') {
    $field_name = $field['field_name'];
    $data_table_name = 'field_data_' . $field_name;
    $revision_table_name = 'field_revision_' . $field_name;

    // Inform views that our field columns are dates (timestamps).
    $data[$data_table_name][$field_name . '_dtstart']['field']['handler'] = 'views_handler_field_date';
    $data[$data_table_name][$field_name . '_dtend']['field']['handler'] = 'views_handler_field_date';
    $data[$data_table_name][$field_name . '_dtstart']['filter']['handler'] = 'views_handler_filter_date';
    $data[$data_table_name][$field_name . '_dtend']['filter']['handler'] = 'views_handler_filter_date';
    $data[$data_table_name][$field_name . '_dtstart']['sort']['handler'] = 'views_handler_sort_date';
    $data[$data_table_name][$field_name . '_dtend']['sort']['handler'] = 'views_handler_sort_date';
  }
}

/**
 * Custom callback to add another date item to exdates.
 */
function repeating_dates_add_exdate($form, &$form_state) {
  $button = $form_state['triggering_element'];
  // Go one level up in the form, to the exdates element.
  array_pop($button['#array_parents']);
  $element = backdrop_array_get_nested_value($form, $button['#array_parents']);
  $defaults = $element['dates']['#default_value'];
  $field_name = $element['#array_parents'][0];// @todo sure?

  $exdates = array();
  if (!empty($form_state['input'][$field_name])) {
    if (!empty($form_state['input'][$field_name]['und']['rrule']['exdates']['dates'])) {
      $datelist = $form_state['input'][$field_name]['und']['rrule']['exdates']['dates'];
      foreach ($datelist as $input) {
        // Filter out invalid input.
        if (!is_string($input)) {
          continue;
        }
        // Filter out strings that are no dates as we expect them.
        $pattern = '/[0-9]{4}-[0-9]{2}-[0-9]{2}/';
        if (preg_replace($pattern, '', $input) !== '') {
          continue;
        }
        $exdates[] = $input;
      }
    }
  }
  $element['dates']['#default_value'] = $exdates;
  $element['dates']['#default_value'][] = '';

  $weight = 0;
  foreach ($element['dates']['#default_value'] as $index => $date) {
    $weight += 0.001;
    $element['dates'] += array($index => array());
    $parents_for_id = array_merge($element['dates']['#parents'], array($index));
    $element['dates'][$index] += array(
      '#type' => 'html_date',
      '#title' => t('Date !num', array('!num' => ($index + 1))),
      '#title_display' => 'invisible',
      '#default_value' => $element['dates']['#default_value'][$index],
      '#value' => $element['dates']['#default_value'][$index],
      '#attributes' => $element['dates']['#attributes'],
      '#parents' => array_merge($element['dates']['#parents'], array($index)),
      '#id' => backdrop_html_id('edit-' . implode('-', $parents_for_id)),
      '#name' => $field_name . '[und][rrule][exdates][dates][' . $index . ']',
      '#weight' => $weight,
    );
  }

  return $element['dates'];
}

/**
 * Custom ajax callback.
 */
function repeating_dates_add_exdate_callback($form, &$form_state) {
  return repeating_dates_add_exdate($form, $form_state);
}

/**
 * Implements hook_element_info().
 */
function repeating_dates_element_info() {
  $types['repeating_dates_datelist'] = array(
    '#input' => TRUE,
    '#process' => array('repeating_dates_process_datelist'),
  );
  return $types;
}

/**
 * Process callback for custom form item "datelist".
 */
function repeating_dates_process_datelist($element) {
  if (count($element['#default_value']) > 0) {
    $weight = 0;
    $field_name = $element['#parents'][0];
    // Back from preview the value got messed.
    if (isset($element['#default_value']['add_exception'])) {
      $element['#default_value'] = $element['#default_value']['dates'];
    }
    foreach ($element['#default_value'] as $index => $date) {
      $weight += 0.001;
      $element += array($index => array());
      $parents_for_id = array_merge($element['#parents'], array($index));
      $element[$index] += array(
        '#type' => 'html_date',
        '#title' => t('Date !num', array('!num' => ($index + 1))),
        '#title_display' => 'invisible',
        '#default_value' => $element['#default_value'][$index],
        '#value' => $element['#default_value'][$index],
        '#attributes' => $element['#attributes'],
        '#parents' => array_merge($element['#parents'], array($index)),
        '#id' => backdrop_html_id('edit-' . implode('-', $parents_for_id)),
        '#name' => $field_name . '[und][rrule][exdates][dates][' . $index . ']',
        '#weight' => $weight,
      );
    }
  }
  return $element;
}

/**
 * Helper function.
 *
 * @param array $values
 *   Keyed array of date and time.
 * @param string $timezone
 *   Timezone name to use for conversion.
 *
 * @return string
 *   Unix timestamp.
 */
function _repeating_dates_array_to_timestamp($values, $timezone = 'UTC') {
  if (is_array($values)) {
    $string = trim(implode(' ', $values));
    $datetime = new DateTime($string, new DateTimeZone($timezone));

    if (is_object($datetime)) {
      return $datetime->format('U');
    }
  }
  return '0';
}

/**
 * Helper function to build an ical rule from form submissions.
 *
 * @param array $rrule
 *   Array of rrule components as provided by the widget form.
 * @param object $start_date
 *   Datetime object of the start date.
 */
function _repeating_dates_build_rrule($rrule, $start_date) {
  $components = array();
  switch ($rrule['freq']) {
    case 'daily':
      $components[] = 'FREQ=DAILY';
      $components[] = 'INTERVAL=' . $rrule['interval_day'];
      break;

    case 'weekly':
      $dow = array_filter($rrule['byday_week']);
      $days_string = implode(',', array_keys($dow));
      $components[] = 'FREQ=WEEKLY';
      $components[] = 'BYDAY=' . $days_string;
      $components[] = 'INTERVAL=' . $rrule['interval_week'];
      break;

    case 'monthly':
      $components[] = 'FREQ=MONTHLY';
      if ($rrule['monthday_setpos'] === '0') {
        $components[] = 'BYMONTHDAY=' . $rrule['bymonthday_month'];
      }
      else {
        if ($rrule['byday_month'] != 'any') {
          $components[] = 'BYSETPOS=' . $rrule['bysetpos_month'];
          $components[] = 'BYDAY=' . $rrule['byday_month'];
        }
        else {
          // @todo is there an easier way to provide "last day of month"?
          $components[] = 'BYMONTHDAY=' . $rrule['bysetpos_month'];
        }
      }
      $components[] = 'INTERVAL=' . $rrule['interval_month'];
      break;

    case 'yearly':
      $components[] = 'FREQ=YEARLY';
      if ($rrule['yearly_setpos'] === '0') {
        $components[] = 'BYMONTHDAY=' . $rrule['bymonthday_year'];
      }
      else {
        $components[] = 'BYSETPOS=' . $rrule['bysetpos_year'];
        $components[] = 'BYDAY=' . $rrule['byday_year'];
      }
      $components[] = 'BYMONTH=' . $rrule['bymonth_year'];
      break;

    default:
      return '';
  }

  if ($rrule['stopby'] == 'date') {
    // Apply start date time offset to date only 'until' string.
    $full_until_string = $rrule['until'] . ' ' . $start_date->format('H:i:s');
    $until = new DateTime($full_until_string);
    // Convert 'until' to UTC.
    $until->setTimezone(new DateTimeZone('UTC'));
    $components[] = 'UNTIL=' . $until->format('Ymd\THis\Z');
  }
  else {
    $components[] = 'COUNT=' . max($rrule['count'], 1);
  }
  $first_day = config_get('system.date', 'first_day');
  $keys = _repeating_dates_get_ical_daykeys();
  $components[] = 'WKST=' . $keys[$first_day];

  $rrule_string = 'RRULE:' . implode(';', $components);

  if (!empty($rrule['exdates']['dates'])) {
    $exdates = array_filter($rrule['exdates']['dates']);
    $exdate_list = array();
    foreach ($exdates as $date_string) {
      $date = new DateTime($date_string . ' ' . $start_date->format('H:i:s'));
      $exdate_list[] = $date->format('Ymd\THis');
    }
    if (!empty($exdate_list)) {
      $timezone = $start_date->getTimeZone()->getName();
      $rrule_string .= "\r\nEXDATE;TZID=$timezone:" . implode(',', $exdate_list);
    }
  }

  return $rrule_string;
}

/**
 * Helper function to return a list of weekdays as used by ical.
 */
function _repeating_dates_get_ical_daykeys() {
  return array('SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA');
}

/**
 * Helper function to return the frequency options.
 */
function _repeating_dates_frequency_options() {
  return array(
    'daily' => t('Daily'),
    'weekly' => t('Weekly'),
    'monthly' => t('Monthly'),
    'yearly' => t('Yearly'),
    '' => t('Do not repeat'),
  );
}

/**
 * Helper function to return a descriptive repesentation of the rrule.
 *
 * @param array $values
 *   Keyed array with field values from the database.
 * @param array $settings
 *   Formatter settings for this field.
 */
function _repeating_dates_rule_description($values, $settings) {
  $tz = date_default_timezone();
  $dtstart = new DateTime('@' . $values['dtstart']);
  $dtstart->setTimezone(new DateTimeZone($tz));

  if (!empty($values['rrule'])) {
    try {
      global $language;
      $description = '';
      // @todo outcome is at least as odd as with date_repeat. :(
      // @todo and it's relatively slow.
      $rule_parts = explode("\r\n", $values['rrule']);
      $rrule_obj = new RRule("DTSTART;TZID=$tz:" . $dtstart->format('Ymd\THis') . "\r\n" . $rule_parts[0]);
      $description = $rrule_obj->humanReadable(array('locale' => $language->langcode));

      if (!empty($rule_parts[1])) {
        $dates = array();
        $datelist_parts = explode(':', $rule_parts[1]);
        // The dates' timezone might be different from the current one.
        $timezone = str_replace('EXDATE;TZID=', '', $datelist_parts[0]);
        $tz_obj = new DateTimeZone($timezone);
        $date_strings = explode(',', $datelist_parts[1]);
        foreach ($date_strings as $string) {
          $date = new DateTime($string, $tz_obj);
          $dates[] = format_date($date->format('U'), $settings['dateonly_format']);
        }
        $description .= '<br>' . t('Except for: !dates', array(
          '!dates' => implode(', ', $dates),
        ));
      }
      return $description;
    }
    catch (Exception $e) {
      // This is for node display. No dblog to prevent flooding.
      return;
    }
  }
}