<?php
/**
 * 對服務器進行一致性hash分布算法
 */
class HashRing
{
  private $servers = array();
  private $nodeList = array();
  private $nodeHashList = array();
  private $nodeTotalNum = 0;
  private $virtualNodeNum = 32;
  private $keyHash = '';
  public function __construct($servers)
  {
    $this->servers = $servers;
    foreach ($servers as $server) {
      for ($i = 0; $i < $this->virtualNodeNum; $i++) {
        $this->nodeList[sprintf("%u", crc32($server.'-'.$i))] = array($server, $i);
      }
    }
    ksort($this->nodeList);
    $this->nodeHashList = array_keys($this->nodeList);
  }
  private function getNodeIndex($key)
  {
    $this->keyHash = sprintf("%u", crc32($key));
    if ($this->keyHash > end($this->nodeHashList)) {
      $this->keyHash = $this->keyHash % end($this->nodeHashList);
    }
    if ($this->keyHash <= reset($this->nodeHashList)) {
      return 0;
    }
    $this->nodeTotalNum = count($this->nodeHashList);
    return $this->binaryChopIndex(0, $this->nodeTotalNum);
  }
  private function binaryChopIndex($l=0, $r=0)
  {
    if ($l < $r) {
      $avg = intval(($l+$r) / 2);
      if ($this->nodeHashList[$avg] == $this->keyHash) {
        return $avg;
      } elseif ($this->keyHash < $this->nodeHashList[$avg] && ($avg > 0)) {
        return $this->binaryChopIndex($l, $avg-1);
      } else {
        return $this->binaryChopIndex($avg+1, $r);
      }
    } else {
      return $l;
    }
  }
  public function getServersByKey($key, $num=1)
  {
    $index = $this->getNodeIndex($key);
    $server = $this->nodeList[$this->nodeHashList[$index]];
    if ($num == 1) {
      return $server[0];
    }
    if ($num >= count($this->servers)) {
      $num = count($this->servers);
    }
    $result = array($server[0]);
    for ($i=$index+1; true; $i++) {
      if ($i >= $this->nodeTotalNum) {
        $i = 0;
      }
      $nextServer = $this->nodeList[$this->nodeHashList[$i]];
      if (!in_array($nextServer[0], $result)) {
        $result[] = $nextServer[0];
      }
      if (count($result) == $num) {
        break;
      }
    }
    return $result;
  }
}
//示例
$servers = array(
  '127.0.0.1:11211',
  '127.0.0.1:11212',
  '127.0.0.1:11213',
  '127.0.0.1:11214',
  '127.0.0.1:11215'
);
$obj = new HashRing($servers);
$servers = $obj->getServersByKey('testkey', 2);
print_r($servers);
echo "\n";