php版一致性哈希算法（consistent hashing）

<?php
/**
* Flexihash - A simple consistent hashing implementation for PHP.
*
* The MIT License
*
* Copyright (c) 2008 Paul Annesley
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the “Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED “AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*
* @author Paul Annesley
* @link http://paul.annesley.cc/
* @copyright Paul Annesley, 2008
*/
/**
* A simple consistent hashing implementation with pluggable hash algorithms.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash
{
    /**
    * The number of positions to hash each target to.
    *
    * @var int
    * @comment 虚拟节点数,解决节点分布不均的问题
    */
    private $_replicas = 64;
    /**
    * The hash algorithm, encapsulated in a Flexihash_Hasher implementation.
    * @var object Flexihash_Hasher
    * @comment 使用的hash方法 : md5,crc32
    */
    private $_hasher;
    /**
    * Internal counter for current number of targets.
    * @var int
    * @comment 节点记数器
    */
    private $_targetCount = 0;
    /**
    * Internal map of positions (hash outputs) to targets
    * @var array { position => target, … }
    * @comment 位置对应节点,用于lookup中根据位置确定要访问的节点
    */
    private $_positionToTarget = array();
    /**
    * Internal map of targets to lists of positions that target is hashed to.
    * @var array { target => [ position, position, ... ], … }
    * @comment 节点对应位置,用于删除节点
    */
    private $_targetToPositions = array();
    /**
    * Whether the internal map of positions to targets is already sorted.
    * @var boolean
    * @comment 是否已排序
    */
    private $_positionToTargetSorted = false;
    /**
    * Constructor
    * @param object $hasher Flexihash_Hasher
    * @param int $replicas Amount of positions to hash each target to.
    * @comment 构造函数,确定要使用的hash方法和需拟节点数,虚拟节点数越多,分布越均匀,但程序的分布式运算越慢
    */
    public function __construct(Flexihash_Hasher $hasher = null, $replicas = null)
    {
        $this->_hasher = $hasher ? $hasher : new Flexihash_Crc32Hasher();
        if (!empty($replicas)) $this->_replicas = $replicas;
    }
    /**
    * Add a target.
    * @param string $target
    * @chainable
    * @comment 添加节点,根据虚拟节点数,将节点分布到多个虚拟位置上
    */
    public function addTarget($target)
    {
        if (isset($this->_targetToPositions[$target]))
        {
            throw new Flexihash_Exception("Target '$target' already exists.");
        }
        $this->_targetToPositions[$target] = array();
        // hash the target into multiple positions
        for ($i = 0; $i < $this->_replicas; $i++)
        {
            $position = $this->_hasher->hash($target . $i);
            $this->_positionToTarget[$position] = $target; // lookup
            $this->_targetToPositions[$target] []= $position; // target removal
        }
        $this->_positionToTargetSorted = false;
        $this->_targetCount++;
        return $this;
    }
    /**
    * Add a list of targets.
    * @param array $targets
    * @chainable
    */
    public function addTargets($targets)
    {
        foreach ($targets as $target)
        {
            $this->addTarget($target);
        }
        return $this;
    }
    /**
    * Remove a target.
    * @param string $target
    * @chainable
    */
    public function removeTarget($target)
    {
        if (!isset($this->_targetToPositions[$target]))
        {
            throw new Flexihash_Exception("Target '$target' does not exist.");
        }
        foreach ($this->_targetToPositions[$target] as $position)
        {
            unset($this->_positionToTarget[$position]);
        }
        unset($this->_targetToPositions[$target]);
        $this->_targetCount--;
        return $this;
    }
    /**
    * A list of all potential targets
    * @return array
    */
    public function getAllTargets()
    {
        return array_keys($this->_targetToPositions);
    }
    /**
    * Looks up the target for the given resource.
    * @param string $resource
    * @return string
    */
    public function lookup($resource)
    {
        $targets = $this->lookupList($resource, 1);
        if (empty($targets)) throw new Flexihash_Exception('No targets exist');
        return $targets[0];
    }
    /**
    * Get a list of targets for the resource, in order of precedence.
    * Up to $requestedCount targets are returned, less if there are fewer in total.
    *
    * @param string $resource
    * @param int $requestedCount The length of the list to return
    * @return array List of targets
    * @comment 查找当前的资源对应的节点,
    * 节点为空则返回空,节点只有一个则返回该节点,
    * 对当前资源进行hash,对所有的位置进行排序,在有序的位置列上寻找当前资源的位置
    * 当全部没有找到的时候,将资源的位置确定为有序位置的第一个(形成一个环)
    * 返回所找到的节点
    */
    public function lookupList($resource, $requestedCount)
    {
        if (!$requestedCount)
            throw new Flexihash_Exception('Invalid count requested');
        // handle no targets
        if (empty($this->_positionToTarget))
            return array();
        // optimize single target
        if ($this->_targetCount == 1)
            return array_unique(array_values($this->_positionToTarget));
        // hash resource to a position
        $resourcePosition = $this->_hasher->hash($resource);
        $results = array();
        $collect = false;
        $this->_sortPositionTargets();
        // search values above the resourcePosition
        foreach ($this->_positionToTarget as $key => $value)
        {
            // start collecting targets after passing resource position
            if (!$collect && $key > $resourcePosition)
            {
                $collect = true;
            }
            // only collect the first instance of any target
            if ($collect && !in_array($value, $results))
            {
                $results []= $value;
            }
            // return when enough results, or list exhausted
            if (count($results) == $requestedCount || count($results) == $this->_targetCount)
            {
                return $results;
            }
        }
        // loop to start - search values below the resourcePosition
        foreach ($this->_positionToTarget as $key => $value)
        {
            if (!in_array($value, $results))
            {
                $results []= $value;
            }
            // return when enough results, or list exhausted
            if (count($results) == $requestedCount || count($results) == $this->_targetCount)
            {
                return $results;
            }
        }
        // return results after iterating through both “parts"
        return $results;
    }
    public function __toString()
    {
        return sprintf(
        '%s{targets:[%s]}',
        get_class($this),
        implode(',', $this->getAllTargets())
        );
    }
    // —————————————-
    // private methods
    /**
    * Sorts the internal mapping (positions to targets) by position
    */
    private function _sortPositionTargets()
    {
        // sort by key (position) if not already
        if (!$this->_positionToTargetSorted)
        {
        ksort($this->_positionToTarget, SORT_REGULAR);
        $this->_positionToTargetSorted = true;
        }
    }
}
/**
* Hashes given values into a sortable fixed size address space.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
interface Flexihash_Hasher
{
    /**
    * Hashes the given string into a 32bit address space.
    *
    * Note that the output may be more than 32bits of raw data, for example
    * hexidecimal characters representing a 32bit value.
    *
    * The data must have 0xFFFFFFFF possible values, and be sortable by
    * PHP sort functions using SORT_REGULAR.
    *
    * @param string
    * @return mixed A sortable format with 0xFFFFFFFF possible values
    */
    public function hash($string);
}
/**
* Uses CRC32 to hash a value into a signed 32bit int address space.
* Under 32bit PHP this (safely) overflows into negatives ints.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash_Crc32Hasher
implements Flexihash_Hasher
{
    /* (non-phpdoc)
    * @see Flexihash_Hasher::hash()
    */
    public function hash($string)
    {
        return crc32($string);
    }
}
/**
* Uses CRC32 to hash a value into a 32bit binary string data address space.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash_Md5Hasher
implements Flexihash_Hasher
{
    /* (non-phpdoc)
    * @see Flexihash_Hasher::hash()
    */
    public function hash($string)
    {
        return substr(md5($string), 0, 8); // 8 hexits = 32bit
        // 4 bytes of binary md5 data could also be used, but
        // performance seems to be the same.
    }
}
/**
* An exception thrown by Flexihash.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash_Exception extends Exception
{
}
?>