Gets the id of the topmost ancestor of the current page and returns the current page's id if there is no parent.

get_post_top_ancestor_id.php

<?php
/**
 * --------------------------------------------------------
 * Gets the id of the topmost ancestor of the current page.
 * Returns the current page's id if there is no parent.
 * @uses Object $post
 * @return Int
 * --------------------------------------------------------
 */
function get_post_top_ancestor_id () {
  global $post;
  
  if ( $post->post_parent ) {
    $ancestors = array_reverse( get_post_ancestors( $post->ID ) );
    return $ancestors[0];
  }
  
  return $post->ID;
}