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<?php
/**
* Convenience class for weighted consistent hash rings.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @file
* @author Aaron Schulz
*/
/**
* Convenience class for weighted consistent hash rings
*
* @since 1.22
*/
class HashRing {
/** @var Array (location => weight) */
protected $sourceMap = array();
/** @var Array (location => (start, end)) */
protected $ring = array();
const RING_SIZE = 268435456; // 2^28
/**
* @param array $map (location => weight)
*/
public function __construct( array $map ) {
$map = array_filter( $map, function( $w ) { return $w > 0; } );
if ( !count( $map ) ) {
throw new MWException( "Ring is empty or all weights are zero." );
}
$this->sourceMap = $map;
// Sort the locations based on the hash of their names
$hashes = array();
foreach ( $map as $location => $weight ) {
$hashes[$location] = sha1( $location );
}
uksort( $map, function ( $a, $b ) use ( $hashes ) {
return strcmp( $hashes[$a], $hashes[$b] );
} );
// Fit the map to weight-proportionate one with a space of size RING_SIZE
$sum = array_sum( $map );
$standardMap = array();
foreach ( $map as $location => $weight ) {
$standardMap[$location] = (int)floor( $weight / $sum * self::RING_SIZE );
}
// Build a ring of RING_SIZE spots, with each location at a spot in location hash order
$index = 0;
foreach ( $standardMap as $location => $weight ) {
// Location covers half-closed interval [$index,$index + $weight)
$this->ring[$location] = array( $index, $index + $weight );
$index += $weight;
}
// Make sure the last location covers what is left
end( $this->ring );
$this->ring[key( $this->ring )][1] = self::RING_SIZE;
}
/**
* Get the location of an item on the ring
*
* @param string $item
* @return string Location
*/
public function getLocation( $item ) {
$locations = $this->getLocations( $item, 1 );
return $locations[0];
}
/**
* Get the location of an item on the ring, as well as the next clockwise locations
*
* @param string $item
* @param integer $limit Maximum number of locations to return
* @return array List of locations
*/
public function getLocations( $item, $limit ) {
$locations = array();
$primaryLocation = null;
$spot = hexdec( substr( sha1( $item ), 0, 7 ) ); // first 28 bits
foreach ( $this->ring as $location => $range ) {
if ( count( $locations ) >= $limit ) {
break;
}
// The $primaryLocation is the location the item spot is in.
// After that is reached, keep appending the next locations.
if ( ( $range[0] <= $spot && $spot < $range[1] ) || $primaryLocation !== null ) {
if ( $primaryLocation === null ) {
$primaryLocation = $location;
}
$locations[] = $location;
}
}
// If more locations are requested, wrap-around and keep adding them
reset( $this->ring );
while ( count( $locations ) < $limit ) {
list( $location, ) = each( $this->ring );
if ( $location === $primaryLocation ) {
break; // don't go in circles
}
$locations[] = $location;
}
return $locations;
}
/**
* Get the map of locations to weight (ignores 0-weight items)
*
* @return array
*/
public function getLocationWeights() {
return $this->sourceMap;
}
/**
* Get a new hash ring with a location removed from the ring
*
* @param string $location
* @return HashRing|bool Returns false if no non-zero weighted spots are left
*/
public function newWithoutLocation( $location ) {
$map = $this->sourceMap;
unset( $map[$location] );
if ( count( $map ) ) {
return new self( $map );
}
return false;
}
}
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