Detailed explanation of how PHP implements consistent hashing algorithm

How does PHP implement consistent hashing algorithm? This article mainly introduces the consistent hash algorithm implemented in PHP, and analyzes the relevant techniques of the PHP hash algorithm in the form of complete examples. It has certain reference value and friends in need can refer to it. I hope to be helpful.

<?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 &#39;$target&#39; 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 &#39;$target&#39; does not exist.");
    }
    foreach ($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(&#39;No targets exist&#39;);
    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(&#39;Invalid count requested&#39;);
    // 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 $results;
  }
  public function __toString()
  {
    return sprintf(
      &#39;%s{targets:[%s]}&#39;,
      get_class($this),
      implode(&#39;,&#39;, $this->getAllTargets())
    );
  }
  // ----------------------------------------
  // private methods
  /**
   * Sorts the internal mapping (positions to targets) by position
   */
  private function _sortPositionTargets()
  {
    // sort by key (position) if not already
    if (!$this->_positionToTargetSorted)
    {
      ksort($this->_positionToTarget, SORT_REGULAR);
      $this->_positionToTargetSorted = true;
    }
  }
}
/**
 * Hashes given values into a sortable fixed size address space.
 *
 * @author Paul Annesley
 * @package Flexihash
 */
interface Flexihash_Hasher
{
  /**
   * Hashes the given string into a 32bit address space.
   *
   * Note that the output may be more than 32bits of raw data, for example
   * hexidecimal characters representing a 32bit value.
   *
   * The data must have 0xFFFFFFFF possible values, and be sortable by
   * PHP sort functions using SORT_REGULAR.
   *
   * @param string
   * @return mixed A sortable format with 0xFFFFFFFF possible values
   */
  public function hash($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
 */
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
 */
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.
  }
}
class Flexihash_Exception extends Exception
{
}

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