Canonical Url Improvements To Wordpress

function redirect_canonical($requested_url=NULL, $do_redirect=true) {
   global $wp_rewrite, $posts, $is_IIS;

   if ( is_feed() || is_trackback() || is_search() || is_comments_popup() || is_admin() || $is_IIS || ( isset($_POST) && count($_POST) ) )
      return;

   if ( !$requested_url ) {
      // build the URL in the address bar
      $requested_url  = ( isset($_SERVER['HTTPS'] ) && strtolower($_SERVER['HTTPS']) == 'on' ) ? 'https://' : 'http://';
      $requested_url .= $_SERVER['HTTP_HOST'];
      $requested_url .= $_SERVER['REQUEST_URI'];
   }

   $original = @parse_url($requested_url);
   if ( false === $original )
      return;

   // Some PHP setups turn requests for / into /index.php in REQUEST_URI
   $original['path'] = preg_replace('|/index\.php$|', '/', $original['path']);

   $redirect = $original;
   $redirect_url = false;

   // These tests give us a WP-generated permalink
   if ( is_404() ) {
      $redirect_url = redirect_guess_404_permalink();
   } elseif ( is_object($wp_rewrite) && $wp_rewrite->using_permalinks() ) {
      // rewriting of old ?p=X, ?m=2004, ?m=200401, ?m=20040101
      if ( is_single() && isset($_GET['p']) ) {
         if ( $redirect_url = get_permalink(get_query_var('p')) )
            $redirect['query'] = remove_query_arg('p', $redirect['query']);
      } elseif ( is_page() && isset($_GET['page_id']) ) {
         if ( $redirect_url = get_permalink(get_query_var('page_id')) )
            $redirect['query'] = remove_query_arg('page_id', $redirect['query']);
      } elseif ( isset($_GET['m']) && ( is_year() || is_month() || is_day() ) ) {
         $m = get_query_var('m');
         switch ( strlen($m) ) {
            case 4: // Yearly
               $redirect_url = get_year_link($m);
               break;
            case 6: // Monthly
               $redirect_url = get_month_link( substr($m, 0, 4), substr($m, 4, 2) );
               break;
            case 8: // Daily
               $redirect_url = get_day_link(substr($m, 0, 4), substr($m, 4, 2), substr($m, 6, 2));
               break;
         }
         if ( $redirect_url )
            $redirect['query'] = remove_query_arg('m', $redirect['query']);
      // now moving on to non ?m=X year/month/day links
      } elseif ( is_day() && get_query_var('year') && get_query_var('monthnum') && isset($_GET['day']) ) {
         if ( $redirect_url = get_day_link(get_query_var('year'), get_query_var('monthnum'), get_query_var('day')) )
            $redirect['query'] = remove_query_arg(array('year', 'monthnum', 'day'), $redirect['query']);
      } elseif ( is_month() && get_query_var('year') && isset($_GET['monthnum']) ) {
         if ( $redirect_url = get_month_link(get_query_var('year'), get_query_var('monthnum')) )
            $redirect['query'] = remove_query_arg(array('year', 'monthnum'), $redirect['query']);
      } elseif ( is_year() && isset($_GET['year']) ) {
         if ( $redirect_url = get_year_link(get_query_var('year')) )
            $redirect['query'] = remove_query_arg('year', $redirect['query']);
      } elseif ( is_category() && isset($_GET['cat']) ) {
         if ( $redirect_url = get_category_link(get_query_var('cat')) )
            $redirect['query'] = remove_query_arg('cat', $redirect['query']);
      } elseif ( is_author() && isset($_GET['author']) ) {
         $author = get_userdata(get_query_var('author'));
         if ( false !== $author && $redirect_url = get_author_link(false, $author->ID, $author->user_nicename) )
            $redirect['query'] = remove_query_arg('author', $redirect['author']);
      }

   // paging
      if ( $paged = get_query_var('paged') ) {
         if ( $paged > 0 ) {
            if ( !$redirect_url )
               $redirect_url = $requested_url;
            $paged_redirect = @parse_url($redirect_url);
            $paged_redirect['path'] = preg_replace('|/page/[0-9]+?(/+)?$|', '/', $paged_redirect['path']); // strip off any existing paging
            $paged_redirect['path'] = preg_replace('|/index.php/?$|', '/', $paged_redirect['path']); // strip off trailing /index.php/
            if ( $paged > 1 && !is_single() ) {
               $paged_redirect['path'] = trailingslashit($paged_redirect['path']);
               if ( $wp_rewrite->using_index_permalinks() && strpos($paged_redirect['path'], '/index.php/') === false )
                  $paged_redirect['path'] .= 'index.php/';
               $paged_redirect['path'] .= user_trailingslashit("page/$paged", 'paged');
            } elseif ( !is_home() && !is_single() ){
               $paged_redirect['path'] = user_trailingslashit($paged_redirect['path'], 'paged');
            }
            $redirect_url = $paged_redirect['scheme'] . '://' . $paged_redirect['host'] . $paged_redirect['path'];
            $redirect['path'] = $paged_redirect['path'];
         }
         $redirect['query'] = remove_query_arg('paged', $redirect['query']);
      }
   }

   // tack on any additional query vars
   if ( $redirect_url && $redirect['query'] ) {
      if ( strpos($redirect_url, '?') !== false )
         $redirect_url .= '&';
      else
         $redirect_url .= '?';
      $redirect_url .= $redirect['query'];
   }

   if ( $redirect_url )
      $redirect = @parse_url($redirect_url);

   // www.example.com vs example.com
   $user_home = @parse_url(get_option('home'));
   $redirect['host'] = $user_home['host'];

   // Handle ports
   if ( isset($user_home['port']) )
      $redirect['port'] = $user_home['port'];
   else
      unset($redirect['port']);

   // trailing /index.php/
   $redirect['path'] = preg_replace('|/index.php/$|', '/', $redirect['path']);

   // strip /index.php/ when we're not using PATHINFO permalinks
   if ( !$wp_rewrite->using_index_permalinks() )
      $redirect['path'] = str_replace('/index.php/', '/', $redirect['path']);

   // trailing slashes
   if ( is_object($wp_rewrite) && $wp_rewrite->using_permalinks() && !is_404() && (!is_home() || ( is_home() && (get_query_var('paged') > 1) ) ) ) {
      $user_ts_type = '';
      if ( get_query_var('paged') > 0 ) {
         $user_ts_type = 'paged';
      } else {
         foreach ( array('single', 'category', 'page', 'day', 'month', 'year') as $type ) {
            $func = 'is_' . $type;
            if ( call_user_func($func) )
               $user_ts_type = $type;
               break;
            }
         }
      $redirect['path'] = user_trailingslashit($redirect['path'], $user_ts_type);
   } elseif ( is_home() ) {
      $redirect['path'] = trailingslashit($redirect['path']);
   }

   // Always trailing slash the 'home' URL
   if ( $redirect['path'] == $user_home['path'] )
      $redirect['path'] = trailingslashit($redirect['path']);

   // Ignore differences in host capitalization, as this can lead to infinite redirects
   if ( strtolower($original['host']) == strtolower($redirect['host']) )
      $redirect['host'] = $original['host'];

   if ( array($original['host'], $original['port'], $original['path'], $original['query']) !== array($redirect['host'], $redirect['port'], $redirect['path'], $redirect['query']) ) {
      $redirect_url = $redirect['scheme'] . '://' . $redirect['host'];
      if ( isset($redirect['port']) )
          $redirect_url .= ':' . $redirect['port'];
      $redirect_url .= $redirect['path'];
      if ( $redirect['query'] )
         $redirect_url .= '?' . $redirect['query'];
   }

   if ( $redirect_url && $redirect_url != $requested_url ) {
      // var_dump($redirect_url); die();
      $redirect_url = apply_filters('redirect_canonical', $redirect_url, $requested_url);
      if ( $do_redirect) {
         // protect against chained redirects
         if ( !redirect_canonical($redirect_url, false) ) {
            wp_redirect($redirect_url, 301);
            exit();
         } else {
            return false;
         }
      } else {
         return $redirect_url;
      }
   } else {
      return false;
   }
}

function redirect_guess_404_permalink() {
   global $wp_query, $wpdb;
   if ( !get_query_var('name') )
      return false;

   $where = "post_name LIKE '" . $wpdb->escape(get_query_var('name')) . "%'";

   // if any of year, monthnum, or day are set, use them to refine the query
   if ( get_query_var('year') )
      $where .= " AND YEAR(post_date) = '" . $wpdb->escape(get_query_var('year')) . "'";
   if ( get_query_var('monthnum') )
      $where .= " AND MONTH(post_date) = '" . $wpdb->escape(get_query_var('monthnum')) . "'";
   if ( get_query_var('day') )
      $where .= " AND DAYOFMONTH(post_date) = '" . $wpdb->escape(get_query_var('day')) . "'";

   $post_id = $wpdb->get_var("SELECT ID FROM $wpdb->posts WHERE $where AND post_status = 'publish'");
   if ( !$post_id )
      return false;
   return get_permalink($post_id);
}

add_action('template_redirect', 'redirect_canonical');