PHP一致性hash的实现

本文是剽窃了Flexihash的实现，不是本人所为，简化了实现的细节 http://code.google.com/p/flexihash/ 
一致性hash大多用于缓存集群中，为了使在缓存中由于一台或多台服务器宕机，导致后端数据库压力过大而崩溃，他对添加和减少缓存服务器迁移的数据量最小化，相对于取模来说

对于一致性hash的原理和介绍，网上有很多，我这里就不转帖了

<?php
/**
*        Consisten Hashing
*
*/

class ConsistenHash {

private $_targetNum = 0;

//值越大，分布在环上的数据就越均匀
private $_replicas = 64;

private $_targetToPositions = array();

private $_positionsToTargets = array();

public function __construct($replicas = null){
if(!empty($replicas)) $this->_replicas = $replicas;
}

//添加单个目标
public function addTarget($target){
//如果target为空则返回false
if(empty($target)) return false;

//如果target存在则返回
if(array_key_exists($target,$this->_targetToPositions)) return $this;

for($i=0;$i<$this->_replicas;$i++){
$position = $this->hash($target.$i);
$this->_positionsToTargets[$position] = $target;
$this->_targetToPositions[$target][] = $position;
}

$this->_targetNum++;
//ksort($this->_positionsToTargets,SORT_REGULAR);

}

//批量添加目标对象，在分布式缓存中为缓存服务器，如果在分表中，则为表名列表
public function addTargets($targets){
if(is_array($targets)){
foreach($targets as $target){
$this->addTarget($target);
}
}else{
$this->addTarget($target);
}
return $this;
}

//获取目标对应的位置对象
public function getTarget($id){

if(empty($id)) return array();

if($this->_targetNum == 1)
return array_unique(array_values($this->_positionToTarget));

$position = $this->hash($id);

ksort($this->_positionsToTargets,SORT_REGULAR);

foreach($this->_positionsToTargets as $k=>$v){
if($k > $position){
return $v;
}
}
}

//获取所有位置与目标对象列表
public function getAllList($flag = null){
if(empty($flag)) {
return $this->_targetToPositions;

4000
}
ksort($this->_positionsToTargets,SORT_REGULAR);
return $this->_positionsToTargets;
}

//通过crc32计算target和key的值
public function hash($target){
$hash = sprintf("%u",crc32($target));
return $hash;
}
}
?>

以下是测试用的code：

<?php

require_once "consisten_hash.class.php";

$hash = new ConsistenHash(8);

$hash->addTargets(array('cache-001','cache-002','cache-003','cache-004'));

print_r($hash->getAllList(true));

$position = $hash->hash('abc');
$target = $hash->getTarget('abc');
echo $position."\n";
echo $target."\n";
?>