// edit these defines (articles is just an example of a Page slug)
	define('RESULTS_PAGE_SLUG', 'articles');
	define('LETTER_PREFIX', '');

	function get_letter_ids() {
		$letter = get_query_var('letter');
		if($letter) {
			global $wpdb;
			// get post ID's that start with the letter from the query var
			return $wpdb->get_col("select ID from $wpdb->posts where $wpdb->posts.post_type = 'post' AND ($wpdb->posts.post_status = 'publish') AND post_title LIKE '".esc_sql(like_escape($letter))."%' ");
		}
		return;
	}

	function get_alphabet_links() {
		$alphabet_array = range('a','z');
		$current_letter = get_query_var('letter');
		$results_page = get_page_by_title( (string) RESULTS_PAGE_SLUG );
		$available_letters = get_available_post_letters();

		$html = '';
		if($results_page && !empty($available_letters)) {
			$page_permalink = get_permalink( $results_page );
			$html .= '<ul>';
			foreach($alphabet_array as $letter){
				if(in_array($letter, $available_letters)) {
					if($current_letter == $letter) {
						$html .= '<li class="current_letter">' . strtoupper ( $letter ) . '</li>';
					} else {
						if(get_option( 'permalink_structure' )) {
							$permalink = esc_url( $page_permalink .sanitize_title( (string) LETTER_PREFIX.$letter));
						} else {
							$permalink = add_query_arg( 'letter', $letter, esc_url($page_permalink));
						}

						$html .= '<li><a href="' . $permalink . '">' . strtoupper ( $letter ) . '</a></li>';
					}
				} else {
					$html .= '<li class="not_available_letter">' . strtoupper ( $letter ) . '</li>';
				}
			}
			$html .= '</ul>';
		}
		return $html;
	}


	function get_available_post_letters(){

		if ( false === ( $post_letters = get_transient( 'all_post_letters' ) ) ) {

			$args = array(
			  'posts_per_page'      => -1,
			  'ignore_sticky_posts' => 1,
			  'post_status'         => 'publish',
			  'no_found_rows'       => true,
			);

			$posts = get_posts($args);
			$post_letters = array();
			if($posts) {
				foreach ($posts as $post) {
					$first_letter = strtolower(substr($post->post_title, 0, 1));
					if(preg_match('/[a-z]/', $first_letter)) {
						$post_letters[] = $first_letter;
					}
				}
			}
			$post_letters = array_unique($post_letters);
			set_transient('all_post_letters', $post_letters);

		}

		return $post_letters;
	}

	// delete transient when updating a post
	add_action( 'save_post', 'update_all_ids' );
	add_action( 'delete_post', 'update_all_ids' );

	function update_all_ids( $post_id ) {
		//verify post is not a revision
		if ( !wp_is_post_revision( $post_id ) ) {
			delete_transient( 'all_post_letters' );
		}
	}

	add_action('init', 'add_letter_rewrite_rules');

	function add_letter_rewrite_rules() {
	  $prefix = trim((string) LETTER_PREFIX);
	  $page_slug = trim((string) RESULTS_PAGE_SLUG);

		if(!is_admin() && get_page_by_title( $page_slug )) {

		  // sanitize the letter prefix before adding the rewrite rule
		  $prefix_end = '';
		  if('-' == substr($prefix, -1)) {
		    $prefix_end = '-';
		  }
		  $prefix = sanitize_title($prefix) . $prefix_end;

			$options = array(
			  'rules'      => array(
			    '(.?.+?)/' .  $prefix . '([a-z])/page/?([0-9]{1,})/?$' => 'index.php?pagename='.$page_slug.'&letter=$matches[2]&paged=$matches[3]',
			    '(.?.+?)/' .  $prefix . '([a-z])(/[0-9]+)?/?$' => 'index.php?pagename='.$page_slug.'&letter=$matches[2]&page=$matches[3]',
			  ),
			  'query_vars' => array('letter')
			);

			$add_rewrite_rules = new ARR_add_rewrite_rules($options);
		}
	}

	if(!class_exists('ARR_add_rewrite_rules')) {
		class ARR_add_rewrite_rules {

			var $query_vars = array();
			var $rules = array();

			function __construct($options = NULL){
				if(!is_null($options)){
					$this->init($options);
				}
			}

			function init($options){
				foreach($options as $key => $value){
					 $this->$key = $value;
				}

				if(get_option( 'permalink_structure' ) && !empty($this->rules)) {
						add_action('wp_head', array(&$this, 'flush_rules'));
						add_action('generate_rewrite_rules', array(&$this, 'add_rules'));
				}

				if(!empty($this->query_vars)){
					 add_filter('query_vars', array(&$this, 'add_query_vars'));
				}

			}

			function add_query_vars($query_vars){
				foreach($this->query_vars as $var){
					$query_vars[] = $var;
				}
				return $query_vars;
			}

			function add_rules(){
				global $wp_rewrite;
				$wp_rewrite->rules = $this->rules + $wp_rewrite->rules;
			}

			function rules_exist(){
				global $wp_rewrite;

				foreach($this->rules as $key => $rule){
					if(!in_array($rule, $wp_rewrite->rules) || !key_exists($key, $wp_rewrite->rules)){
						return FALSE;
						}
				}
				return TRUE;
			}

			function flush_rules(){
				global $wp_rewrite;
				// only flush rules if new rules are added
				if(!$this->rules_exist()){
					//echo 'Rules are flused (refresh the page again)';
					$wp_rewrite->flush_rules();
				}
			}

		} // class
	} // class exists