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<?
function cleaner_gallery($output, $attr) {
  $base_class = 'c-content-gallery';
  if ( is_feed() ) {
    return $output;
  }
  if ( isset( $attr['orderby'] ) ) {
    $attr['orderby'] = sanitize_sql_orderby( $attr['orderby'] );
    if ( !$attr['orderby'] ) {
      unset( $attr['orderby'] );
    }
  }
  $defaults = array(
    'order'       => 'ASC',
    'orderby'     => 'menu_order ID',
    'id'          => get_the_ID(),
    'link'        => '',
    'itemtag'     => 'figure',
    'icontag'     => 'dt',
    'captiontag'  => 'figcaption',
    'columns'     => 10,
    'size'        => 'thumbnail',
    'ids'         => '',
    'include'     => '',
    'exclude'     => '',
    'numberposts' => -1,
    'offset'      => ''
  );
  $attr = shortcode_atts($defaults, $attr);
  extract($attr);
  $id = intval($id);
  $children = array(
    'post_status'      => 'inherit',
    'post_type'        => 'attachment',
    'post_mime_type'   => 'image',
    'order'            => $order,
    'orderby'          => $orderby,
    'exclude'          => $exclude,
    'include'          => $include,
    'numberposts'      => $numberposts,
    'offset'           => $offset,
    'suppress_filters' => true
  );
  if (empty($include)) {
    $attachments = get_children(array_merge(array('post_parent' => $id), $children));
  } else {
    $attachments = get_posts($children);
  }
  if (empty($attachments)) {
    return '<!-- Here be dragons but no images. -->';
  }
  $itemtag    = tag_escape($itemtag);
  $captiontag = tag_escape($captiontag);
  $size_class = sanitize_html_class($size);
  $output = '<ul id="gallery-' . $id . '" class="s-textcontent__gallery ' . $base_class . ' u-size-' . $size_class . '">';
  foreach ($attachments as $attachment) {
    $output .= '<li class="' . $base_class . '__item"><' . $itemtag . ' class="' . $base_class . '__item-inner">';
    $image = ( ( isset( $attr['link'] ) && 'file' == $attr['link'] ) ? wp_get_attachment_link( $attachment->ID, $size, false, false, false, array('class' => $base_class . '__image') ) : wp_get_attachment_link( $attachment->ID, $size, true, false, false, array('class' => $base_class . '__image') ) );
    $output .= $image;
    $title = wptexturize($attachment->post_title);
    $caption = wptexturize($attachment->post_excerpt);
    if (!empty($caption) || !empty($title)) {
      $output .= '<' . $captiontag . ' class="' . $base_class . '__caption">' . $title . (($title && $caption) ? '<br>' : '') . $caption . '</' . $captiontag . '>';
    }
    $output .= '</' . $itemtag . '></li>';
  }
  $output .= '</ul>';
  return $output;
}

add_filter( 'post_gallery', 'cleaner_gallery', 10, 2 );