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Flexihash.php
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Flexihash.php
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<?php
/**
* A simple consistent hashing implementation with pluggable hash algorithms.
*
* @author Paul Annesley
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash
{
/**
* The number of positions to hash each target to.
*
* @var int
*/
private $replicas = 64;
/**
* The hash algorithm, encapsulated in a Flexihash_Hasher implementation.
* @var object Flexihash_Hasher
*/
private $hasher;
/**
* Internal counter for current number of targets.
* @var int
*/
private $targetCount = 0;
/**
* Internal map of positions (hash outputs) to targets.
* @var array { position => target, ... }
*/
private $positionToTarget = array();
/**
* Internal map of targets to lists of positions that target is hashed to.
* @var array { target => [ position, position, ... ], ... }
*/
private $targetToPositions = array();
/**
* Whether the internal map of positions to targets is already sorted.
* @var bool
*/
private $positionToTargetSorted = false;
/**
* Constructor.
* @param object $hasher Flexihash_Hasher
* @param int $replicas Amount of positions to hash each target to.
*/
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
* @param float $weight
* @chainable
*/
public function addTarget($target, $weight = 1)
{
if (isset($this->targetToPositions[$target])) {
throw new Flexihash_Exception("Target '$target' already exists.");
}
$this->targetToPositions[$target] = array();
// hash the target into multiple positions
for ($i = 0; $i < round($this->replicas * $weight); ++$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
* @param float $weight
* @chainable
*/
public function addTargets($targets, $weight = 1)
{
foreach ($targets as $target) {
$this->addTarget($target, $weight);
}
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 $requestedCount The length of the list to return
* @return array List of targets
*/
public function lookupList($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;
}
}
}