<?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
{
}