序

base64大家肯定是很熟悉了，那base62是什么东东，它常被用来做短url的映射。

ascii编码的62个字母数字

Value Encoding  Value Encoding  Value Encoding  Value Encoding0 a            17 r            34 I            51 Z1 b            18 s            35 J            52 02 c            19 t            36 K            53 13 d            20 u            37 L            54 24 e            21 v            38 M            55 35 f            22 w            39 N            56 46 g            23 x            40 O            57 57 h            24 y            41 P            58 68 i            25 z            42 Q            59 79 j            26 A            43 R            60 810 k            27 B            44 S            61 911 l            28 C            45 T12 m            29 D            46 U13 n            30 E            47 V14 o            31 F            48 W15 p            32 G            49 X16 q            33 H            50 Y

26个小写字母+26个大写字母+10个数字=62

    public static final String BASE_62_CHAR = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";public static final int BASE = BASE_62_CHAR.length();

62进制与十进制的映射

62进制转10进制

还记得二进制转十进制的算法么，从右到左用二进制的每个数去乘以2的相应次方，次方要从0开始。62进制转10进制也类似，从右往左每个数*62的N次方，N从0开始。

    public static long toBase10(String str) {//从右边开始return toBase10(new StringBuilder(str).reverse().toString().toCharArray());}private static long toBase10(char[] chars) {long n = 0;int pow = 0;for(char item: chars){n += toBase10(BASE_62_CHAR.indexOf(item),pow);pow++;}return n;}private static long toBase10(int n, int pow) {return n * (long) Math.pow(BASE, pow);}

十进制转62进制

还记得十进制转二进制的算法么，除二取余，然后倒序排列，高位补零。转62进制也类似，不断除以62取余数，然后倒序。

    public static String fromBase10(long i) {StringBuilder sb = new StringBuilder("");if (i == 0) {return "a";}while (i > 0) {i = fromBase10(i, sb);}return sb.reverse().toString();}private static long fromBase10(long i, final StringBuilder sb) {int rem = (int)(i % BASE);sb.append(BASE_62_CHAR.charAt(rem));return i / BASE;}

短url的转换

主要思路，维护一个全局自增的id，每来一个长url，将其与一个自增id绑定，然后利用base62将该自增id转换为base62字符串，即完成转换。

public class Base62UrlShorter {private long autoIncrId = 10000;Map<Long, String> longUrlIdMap = new HashMap<Long, String>();public long incr(){return autoIncrId ++ ;}public String shorten(String longUrl){long id = incr();//add to mappinglongUrlIdMap.put(id,longUrl);return Base62.fromBase10(id);}public String lookup(String shortUrl){long id = Base62.toBase10(shortUrl);return longUrlIdMap.get(id);}
}

测试

    @Testpublic void testLongUrl2Short(){Base62UrlShorter shorter= new Base62UrlShorter();String longUrl = "https://movie.douban.com/subject/26363254/";String shortUrl = shorter.shorten(longUrl);System.out.println("short url:"+shortUrl);System.out.println(shorter.lookup(shortUrl));}

关于容量

自增id为long型，最大2^64 -1

doc

• 534. Design TinyURL

• 如何设计短网址系统(TinyURL)