微博短链接算法php版本实现代码

思路：
1)将长网址md5生成32位签名串,分为4段, 每段8个字节;
2)对这四段循环处理, 取8个字节, 将他看成16进制串与0x3fffffff(30位1)与操作, 即超过30位的忽略处理;
3)这30位分成6段, 每5位的数字作为字母表的索引取得特定字符, 依次进行获得6位字符串;
4)总的md5串可以获得4个6位串; 取里面的任意一个就可作为这个长url的短url地址;
下面是php代码:

function shorturl($url='', $prefix='', $suffix='') {
$base = array (
'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h',
'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p',
'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
'y', 'z', '0', '1', '2', '3', '4', '5');
$hex = md5($prefix.$url.$suffix);
$hexlen = strlen($hex);
$subhexlen = $hexlen / 8;
$output = array();
for ($i = 0; $i < $subhexlen; $i++) {
$subhex = substr ($hex, $i * 8, 8);
$int = 0x3fffffff & (1 * ('0x'.$subhex));
$out = '';
for ($j = 0; $j < 6; $j++) {
$val = 0x0000001f & $int;
$out .= $base[$val];
$int = $int >> 5;
}
$output[] = $out;
}
return $output;
}
$urls = shorturl('//www.jb51.net/');
var_dump($urls);

结果

array(4) {
[0]=>
string(6) "alms1l"
[1]=>
string(6) "2ipmby"
[2]=>
string(6) "avo1hu"
[3]=>
string(6) "fdlban"
}

另外一个版本：

function shorturl($url='', $prefix='', $suffix='') {
$base = array(
"a","b","c","d","e","f","g","h",
"i","j","k","l","m","n","o","p",
"q","r","s","t","u","v","w","x",
"y","z","0","1","2","3","4","5",
"6","7","8","9","a","b","c","d",
"e","f","g","h","i","j","k","l",
"m","n","o","p","q","r","s","t",
"u","v","w","x","y","z");
$hex = md5($prefix.$url.$suffix);
$hexlen = strlen($hex);
$subhexlen = $hexlen / 8;
$output = array();
for ($i = 0; $i < $subhexlen; $i++) {
$subhex = substr ($hex, $i * 8, 8);
$int = 0x3fffffff & (1 * ('0x'.$subhex));
$out = '';
for ($j = 0; $j < 6; $j++) {
$val = 0x0000003d & $int;
$out .= $base[$val];
$int = $int >> 5;
}
$output[] = $out;
}
return $output;
}

结果：

array(4) {
[0] =>
string(6) "6jmmvj"
[1] =>
string(6) "2eniby"
[2] =>
string(6) "6vivfu"
[3] =>
string(6) "b7fb6n"
}

但是升级版本碰撞率反而更高了，不知道为什么。
测试碰撞的测试代码：

$result = array();
$repeats= array();
$loop = 20000;
for($i=0;$i<$loop;$i++){
$url = '//www.jb51.net/?id='.$i;
$shorta = shorturl($url);
$short = $shorta[0];
if(in_array($short, $result)){
$repeats[] = $short;
}
$result[] = $short;
}
$result = array();
for($i=0;$i<$loop;$i++){
$url = '//www.jb51.net/?id='.$i;
$shorta = shorturl($url);
$short = $shorta[0];
if(in_array($short, $repeats)){
$result[$short][] = $url;
}
}
var_dump($repeats);
var_dump($result);

结果：

array(8) {
[0] =>
string(6) "3eqbzq"
[1] =>
string(6) "uqfnay"
[2] =>
string(6) "qezbiv"
[3] =>
string(6) "fmneyf"
[4] =>
string(6) "fjj6fr"
[5] =>
string(6) "3eviym"
[6] =>
string(6) "j2mmuy"
[7] =>
string(6) "jyqfiv"
}
array(8) {
'jyqfiv' =>
array(2) {
[0] =>
string(26) "//www.jb51.net/?id=1640"
[1] =>
string(27) "//www.jb51.net/?id=18661"
}
'fmneyf' =>
array(2) {
[0] =>
string(26) "//www.jb51.net/?id=2072"
[1] =>
string(26) "//www.jb51.net/?id=8480"
}
'3eqbzq' =>
array(2) {
[0] =>
string(26) "//www.jb51.net/?id=4145"
[1] =>
string(26) "//www.jb51.net/?id=4273"
}
'j2mmuy' =>
array(2) {
[0] =>
string(26) "//www.jb51.net/?id=7131"
[1] =>
string(27) "//www.jb51.net/?id=17898"
}
'qezbiv' =>
array(2) {
[0] =>
string(26) "//www.jb51.net/?id=7320"
[1] =>
string(26) "//www.jb51.net/?id=8134"
}
'uqfnay' =>
array(2) {
[0] =>
string(26) "//www.jb51.net/?id=7347"
[1] =>
string(26) "//www.jb51.net/?id=7962"
}
'fjj6fr' =>
array(2) {
[0] =>
string(26) "//www.jb51.net/?id=8628"
[1] =>
string(26) "//www.jb51.net/?id=9031"
}
'3eviym' =>
array(2) {
[0] =>
string(27) "//www.jb51.net/?id=11175"
[1] =>
string(27) "//www.jb51.net/?id=14437"
}
}