短链

需求：创建一个指定长度唯一字符串。
条件：字符串的形式小写字母和数字组合。创建的字符串长度从4位开始。

class shortUrl
{
    private $charList = 'abcdefghijklmnlopqrstuvwxyz1234567890';//26+10=36个字符
    private $baseCounter = 36 * 36 * 36 + 1;

     /**
     * generate a unique id by a integer
     * @param number, int
     * @param charlist, string
     * @return string
     */
    public static function encode($number, $charlist)
    {
        $charArray = str_split($charlist);
        $base = count($charArray);

        $chars = '';

        $rest = $number % $base;
        $chars = $charArray[$rest];

        if ($number - $rest != 0) {
            $number = ($number - $rest ) / $base;
            $chars = self::encode($number, $charlist) . $chars;
        }

        return $chars;
    }

    /**
     * decode the encode string
     * @param input, string
     * @param charlist, string
     * @return int
     */
    public static function decode($input, $charlist)
    {
        $number = 0;
        $chars = str_split(strrev($input));
        $dictionary = str_split($charlist);
        $base = count($dictionary);
        $length = strlen($input);

        for ($i = $length; $i > 0; --$i) {
            $number += array_search($chars[$i-1], $dictionary) * pow($base, ($i - 1));
        }

        return $number;
    }
}