PHP微博客应用中的URL缩短类

#!usr/local/bin/php
<?php
/**
 * 使用单例模式 4个方法 编码 解码 测试 压缩URL
 * SQL表urls只有2个字 一个自增长的ID 一个varchar的url字符串
 * 512增加为了防止传入的url太短
 * */
class UrlShortener {
	const OFFSET = 512;
	private $base;
	private $characterMap;

	public function __construct(){
		//初始化私有变量
		$this->characterMap = array(
			'1','2','3','4','5','6','7','8','9',
			'z','x','c','b','v',
			'f','g','h','j','k','L','m','n',
			'q','w','r','t','y','p','s','d'
		);
		$this->base = sizeof($this->characterMap);
	}

	public static function getInstance(){
		static $instance = null;

		if($instance === null){
			$instance = new UrlShortener();
		}
		return $instance;
	}

	public function shorten($conn,$url){
		$escapedUrl = mysql_escape_string($url);

		$res = mysql_query("select 'id' from urls where 'url' like $escapedUrl",$conn);
		$row = mysql_fetch_row($res);
		mysql_free_result($res);

		//如果有 就进行编码 然后重写进入数据库
		if($row)
			return $this->encode($row[0]+self::OFFSET);
		//如果没有 就先编码后插入
		$res = mysql_query("
			insert into 'urls'('url') values('$escapedUrl')
			",$conn);
		return $this->encode(mysql_insert_id($conn)+self::OFFSET);
	}

	public function encode($val){

		#递归对每一个URL中的字符进行解码
		#@return 解码以后的字符串Url 
		$val += 512;
		if($val < $this->base){
			return $this->characterMap[ $val ];
		}else{
			return $this->encode(floor( $val/$this->base)).
				$this->characterMap[ $val % $this->base];
		}
	}

	public function decode($val){
		$decodeMap = array_flip($this->characterMap);#翻转字典
		$parts = array_reverse(str_split( $val ));

		$index = 0;
		$i = 0;
		foreach($parts as $part){
			$index += $decodeMap[$part] * pow($this->base,$i++);	
		}
		return $index-512;
	}

	public function expand($conn,$index){
		$id = $this->decode($index)-self::OFFSET;
		$res = mysql_query('
			select "url" from "urls" where 'id' = $id
			',$conn);
		$value = (($row = mysql_fetch_row( $res)))?$row[0]:null;
		mysql_free_result($res);
		return $value;
	}
}#end class
?>