Create a node's summary when the node is saved

<?php
/**
 * Implements hook_node_presave().
 */
function MODULENAME_node_presave($node) {
  if ($node->type == 'news') {
    $summary = $node->body[$node->language][0]['summary'];
    if (empty($summary)) {
      $text = $node->body[$node->language][0]['value'];
      $size = variable_get('teaser_length', 600) + 200;
      $summary = truncate_utf8($text, $size);
      $search = array(
        '#[\r\n\t]#', // Remove line breaks and tabs
        '#<img[^>]*?>#is', // Remove images
        '#<h\d>(.*?)</h\d>#is', // Change headings to strong
        '#<a name="[^"]*"></a>#is', // Remove named anchors
        '#</?p>#is' // Remove paragraph tags
      );
      $replace = array(
        '',
        '',
        '<strong>\1</strong> — ',
        '',
        ' ',
      );
      $summary = preg_replace($search, $replace, $summary);
      $node->body[$node->language][0]['summary'] = text_summary($summary, $node->body[$node->language][0]['format']);
    }
  }
}
?>