Absolute path to drupal

at http://drupal.org/node/63579 but so far 0 comments have appeared.

You say that you can determine the path to the module... assuming it's in the standard module location, wouldn't the path to the drupal directory be the module path's parent? Mind sharing the progress you've made so far?

After looking everywhere, here's the function I wrote. You can chuck it anywhere in your site and it will work.
In the site I'm working on, the method of using DOCUMENT_ROOT and base_path() doesn't work for our XML feed, which does not run from index.php, but a script in a subfolder. Hence the need for this solution.

/**
* Get the true path to the root of the Drupal site.
* Better than using DOCUMENT_ROOT and base_path().
*/
function absolute_path_to_drupal() {
static $absolute_path_to_drupal = NULL;

if ($absolute_path_to_drupal === NULL) {
// Get the absolute path to this file:
$dir = rtrim(str_replace('\\', '/', dirname(__FILE__)), '/');
$parts = explode('/', $dir);

// Iterate up the directory hierarchy until we find the website root:
$done = FALSE;
do {
// Check a couple of obvious things:
$done = is_dir("$dir/sites") && is_dir("$dir/includes") && is_file("$dir/index.php");
if (!$done) {
// If there's no more path to examine, we didn't find the site root:
if (empty($parts)) {
$absolute_path_to_drupal = FALSE;
break;
}
// Go up one level and look again:
array_pop($parts);
$dir = implode('/', $parts);
}
} while (!$done);

$absolute_path_to_drupal = $dir;
}

return $absolute_path_to_drupal;
}

function dirroot() {
    if ( strstr( __FILE__, $_SERVER['DOCUMENT_ROOT']) ) {
        $dir = $_SERVER['DOCUMENT_ROOT'];
    } else {
        $dir = dirname(__FILE__);
        while ( ! file_exists($dir . '/install.php') ) {
            $dir = dirname($dir);
        }
    }
    return $dir;
}

Why not use DRUPAL_ROOT constant ?