Recency Ranking Normalization

The code in the 6.x-1.1 release does not normalize the timestamps before applying the exponential decay function. The code in HEAD now does.

/**
 * Implementation of hook_cron().
 *
 * Store the timestamp of the oldest node for recency normalization.
 */
function search_ranking_cron() {
  $oldest_time = db_result(db_query_range("SELECT changed FROM {node} ORDER BY changed ASC", 0, 1));
  variable_set('node_rank_recent_oldest_time', $oldest_time);
}

/**
 * Implementation of hook_ranking().
 */
function search_ranking_ranking() {
  variable_get('node_rank_recent_oldest_time', 0);
  $ranking['recent'] => array(
    'title' => t('Recently posted'),
    'score' => 'POW(2.718, -5 * (1 - (n.changed - %d) / %d))',
    'arguments' => array(
      $oldest_time,
      time() - $oldest_time
    ),
  );
  return $ranking;
}

$oldest_time is the oldest changed date of a node in the system calculated during cron. We use a linear normalization function on the timestamp (output of the normalization function is range [0,1]) before inserting the value into an exponential decay function.