/* The Blogger URL passes the permlink in the query string */
function labnol_blogger_query_vars_filter( $vars ) {
  $vars[] = "blogger";
  return $vars;
}

add_filter('query_vars', 'labnol_blogger_query_vars_filter');

/* We take the blogger parameter and map it to the right WordPress post */
function labnol_blogger_template_redirect() {
  global $wp_query;
  $blogger = $wp_query->query_vars['blogger'];
  if ( isset ( $blogger ) ) {
    wp_redirect( labnol_get_wordpress_url ( $blogger ) , 301 );
    exit;
  }
}

add_action( 'template_redirect', 'labnol_blogger_template_redirect' );

function labnol_get_wordpress_url($blogger) {

  /* Extract the blogger slug from the full blogspot.com url */
  if ( preg_match('@^(?:https?://)?([^/]+)(.*)@i', $blogger, $url_parts) ) {

    /* Query the WordPress database to find matching posts */
    $query = new WP_Query ( array (
      "meta_key" => "blogger_permalink", "meta_value" => $url_parts[2] ) );

    /* if a match is found, get the permalink of the WordPress post */
    if ($query->have_posts()) {
      $query->the_post();
      $url = get_permalink();
    }

    wp_reset_postdata();

  }

  /* If permalink is not found, return the blog home page url */
  return $url ? $url : home_url();

}