(function ($) {

/**
 * Set the client's system time zone as default values of form fields.
 */
Backdrop.behaviors.setTimezone = {
  attach: function (context, settings) {
    var $timezone = $(context).find('.timezone-detect').once('timezone');
    if ($timezone.length) {
      // Modern approach: use JavaScript timezone capability.
      try {
        var resolvedTimezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
        if (resolvedTimezone && !resolvedTimezone.match('Etc/')) {
          // Select lists, ensure the option is available, otherwise fallback to
          // AJAX implementation.
          if ($timezone.is('select')) {
            if ($timezone.find('[value="' + resolvedTimezone + '"]').length) {
              $timezone.val(resolvedTimezone);
              return
            }
          }
          // Text or hidden elements, just set the value directly.
          else {
            $timezone.val(resolvedTimezone);
            return;
          }
        }
      }
      catch (e) {}

      // Legacy approach: Combine the date offset and an AJAX request.
      var dateString = Date();
      // In some client environments, date strings include a time zone
      // abbreviation, between 3 and 5 letters enclosed in parentheses,
      // which can be interpreted by PHP.
      var matches = dateString.match(/\(([A-Z]{3,5})\)/);
      var abbreviation = matches ? matches[1] : 0;

      // For all other client environments, the abbreviation is set to "0"
      // and the current offset from UTC and daylight saving time status are
      // used to guess the time zone.
      var dateNow = new Date();
      var offsetNow = dateNow.getTimezoneOffset() * -60;

      // Use January 1 and July 1 as test dates for determining daylight
      // saving time status by comparing their offsets.
      var dateJan = new Date(dateNow.getFullYear(), 0, 1, 12, 0, 0, 0);
      var dateJul = new Date(dateNow.getFullYear(), 6, 1, 12, 0, 0, 0);
      var offsetJan = dateJan.getTimezoneOffset() * -60;
      var offsetJul = dateJul.getTimezoneOffset() * -60;

      var isDaylightSavingTime;
      // If the offset from UTC is identical on January 1 and July 1,
      // assume daylight saving time is not used in this time zone.
      if (offsetJan == offsetJul) {
        isDaylightSavingTime = '';
      }
      // If the maximum annual offset is equivalent to the current offset,
      // assume daylight saving time is in effect.
      else if (Math.max(offsetJan, offsetJul) == offsetNow) {
        isDaylightSavingTime = 1;
      }
      // Otherwise, assume daylight saving time is not in effect.
      else {
        isDaylightSavingTime = 0;
      }

      // Submit request to the system/timezone callback and set the form field
      // to the response time zone. The client date is passed to the callback
      // for debugging purposes. Submit a synchronous request to avoid database
      // errors associated with concurrent requests during install.
      var path = 'system/timezone/' + abbreviation + '/' + offsetNow + '/' + isDaylightSavingTime;
      $.ajax({
        async: false,
        url: settings.basePath,
        data: { q: path, date: dateString },
        dataType: 'json',
        success: function (data) {
          if (data) {
            $timezone.val(data.timezone);
          }
        }
      });
    }
  }
};

})(jQuery);
