WordPress / Preview Link - add login link

gistfile1.php

<?php
// Show login link if a user visits a preview link and is logged out

remove_action( 'init', '_show_post_preview' );
add_action( 'init', 'NAME_show_post_preview' );
function NAME_show_post_preview($result='', $id = false) {
	// if ?preview=true is set
	if ( isset($_GET['preview'])) {
		$id = (int) $_GET['preview_id'];
		
		 if ( false == wp_verify_nonce( $_GET['preview_nonce'], 'post_preview_' . $id ) ) { 
			if ( is_user_logged_in() ) {
				wp_die( __('You do not have permission to preview drafts.') );
			} else {
				wp_die('<a href="'. wp_login_url() .'">Log in</a> to see this preview.');
			}
		}
		add_filter('the_preview', '_set_preview');
	} 
}