functions.php

/**
 * Functions to handle transition from Tumblr to Wordpress
 * author: philipp fehre <philipp@fehre.co.uk
 *
 * The goal ist to make imported tumbler posts work along the normal permalinks
 * generated by tumblr
 *
 * This works for posts imported via http://tumblr2wordpress.benapps.net
 * exported file containing the tumblr post ids in the post_name field.
 */

add_action('init', 'tumblr_add_rewrite_rules');

function tumblr_add_rewrite_rules() {
  global $wp_rewrite;

  $wp_rewrite->add_rewrite_tag('%tumblrslug%', '([^/]+)', '');
  $wp_rewrite->add_permastruct('tumblrslug', '%tumblrslug%', false);
}

add_filter('post_link', 'tumblr_permalinks', 10, 3);

function tumblr_permalinks($permalink, $post, $leavename) {
  $post_name = $post->post_name;
  if ( is_numeric($post_name) ) {
    // this is a tumblr imported post, we need to attach the slug
    // so the post_name contains the tumblr post id
    $tumblr_slug = sanitize_title($post->post_title);
  } else {
    // this is not a tumblr post so just remove the slug
    $tumblr_slug = '';
  }
  $permalink = str_replace('%tumblrslug%', $tumblr_slug, $permalink);
  return $permalink;
}