一致性哈希

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发现了段优秀的代码，读了一下，顺手写了一写注释，防止以后再看的时候忘记。
主要实现了一致性哈希的算法，php界相当优秀的代码。读完心旷神怡。

<?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
 * @comment by MyZ (http://blog.csdn.net/mayongzhan)
 */

/**
 * 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
{
}