bp_activity_action_permalink_router()

function bp_activity_action_permalink_router() {
	// Not viewing activity.
	if ( ! bp_is_activity_component() || ! bp_is_current_action( 'p' ) )
		return false;

	// No activity to display.
	if ( ! bp_action_variable( 0 ) || ! is_numeric( bp_action_variable( 0 ) ) )
		return false;

	// Get the activity details.
	$activity = bp_activity_get_specific( array( 'activity_ids' => bp_action_variable( 0 ), 'show_hidden' => true ) );

	// 404 if activity does not exist
	if ( empty( $activity['activities'][0] ) ) {
		bp_do_404();
		return;
	} else {
		$activity = $activity['activities'][0];
	}

	// Do not redirect at default.
	$redirect = false;

	// Redirect based on the type of activity.
	if ( bp_is_active( 'groups' ) && $activity->component == buddypress()->groups->id ) {

		// Activity is a user update.
		if ( ! empty( $activity->user_id ) ) {
			$redirect = bp_core_get_user_domain( $activity->user_id, $activity->user_nicename, $activity->user_login ) . bp_get_activity_slug() . '/' . $activity->id . '/';

		// Activity is something else.
		} else {

			// Set redirect to group activity feed.
			if ( $group = groups_get_group( $activity->item_id ) ) {
				$redirect = bp_get_group_permalink( $group ) . bp_get_activity_slug() . '/' . $activity->id . '/';
			}
		}

	// Set redirect to users' activity feed.
	} elseif ( ! empty( $activity->user_id ) ) {
		$redirect = bp_core_get_user_domain( $activity->user_id, $activity->user_nicename, $activity->user_login ) . bp_get_activity_slug() . '/' . $activity->id . '/';
	}

	// If set, add the original query string back onto the redirect URL.
	if ( ! empty( $_SERVER['QUERY_STRING'] ) ) {
		$query_frags = array();
		wp_parse_str( $_SERVER['QUERY_STRING'], $query_frags );
		$redirect = add_query_arg( urlencode_deep( $query_frags ), $redirect );
	}

	/**
	 * Filter the intended redirect url before the redirect occurs for the single activity item.
	 *
	 * @since BuddyPress 1.2.2
	 *
	 * @param array $value Array with url to redirect to and activity related to the redirect.
	 */
	if ( ! $redirect = apply_filters_ref_array( 'bp_activity_permalink_redirect_url', array( $redirect, &$activity ) ) ) {
		bp_core_redirect( bp_get_root_domain() );
	}

	// Redirect to the actual activity permalink page.
	bp_core_redirect( $redirect );
}