includes/HashRing.php

   1 <?php
   2 /**
   3  * Convenience class for weighted consistent hash rings.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @author Aaron Schulz
  22  */
  23
  24 /**
  25  * Convenience class for weighted consistent hash rings
  26  *
  27  * @since 1.22
  28  */
  29 class HashRing {
  30         /** @var Array (location => (start, end)) */
  31         protected $ring = array();
  32
  33         const RING_SIZE = 268435456; // 2^28
  34
  35         /**
  36          * @param array $map (location => weight)
  37          */
  38         public function __construct( array $map ) {
  39                 $map = array_filter( $map, function( $w ) { return $w > 0; } );
  40                 if ( !count( $map ) ) {
  41                         throw new MWException( "Ring is empty or all weights are zero." );
  42                 }
  43                 // Sort the locations based on the hash of their names
  44                 $hashes = array();
  45                 foreach ( $map as $location => $weight ) {
  46                         $hashes[$location] = sha1( $location );
  47                 }
  48                 uksort( $map, function ( $a, $b ) use ( $hashes ) {
  49                         return strcmp( $hashes[$a], $hashes[$b] );
  50                 } );
  51                 // Fit the map to weight-proportionate one with a space of size RING_SIZE
  52                 $sum = array_sum( $map );
  53                 $standardMap = array();
  54                 foreach ( $map as $location => $weight ) {
  55                         $standardMap[$location] = (int)floor( $weight / $sum * self::RING_SIZE );
  56                 }
  57                 // Build a ring of RING_SIZE spots, with each location at a spot in location hash order
  58                 $index = 0;
  59                 foreach ( $standardMap as $location => $weight ) {
  60                         // Location covers half-closed interval [$index,$index + $weight)
  61                         $this->ring[$location] = array( $index, $index + $weight );
  62                         $index += $weight;
  63                 }
  64                 // Make sure the last location covers what is left
  65                 end( $this->ring );
  66                 $this->ring[key( $this->ring )][1] = self::RING_SIZE;
  67         }
  68
  69         /**
  70          * Get the location of an item on the ring
  71          *
  72          * @param string $item
  73          * @return string Location
  74          */
  75         public function getLocation( $item ) {
  76                 $locations = $this->getLocations( $item, 1 );
  77                 return $locations[0];
  78         }
  79
  80         /**
  81          * Get the location of an item on the ring, as well as the next clockwise locations
  82          *
  83          * @param string $item
  84          * @param integer $limit Maximum number of locations to return
  85          * @return array List of locations
  86          */
  87         public function getLocations( $item, $limit ) {
  88                 $locations = array();
  89                 $primaryLocation = null;
  90                 $spot = hexdec( substr( sha1( $item ), 0, 7 ) ); // first 28 bits
  91                 foreach ( $this->ring as $location => $range ) {
  92                         if ( count( $locations ) >= $limit ) {
  93                                 break;
  94                         }
  95                         // The $primaryLocation is the location the item spot is in.
  96                         // After that is reached, keep appending the next locations.
  97                         if ( ( $range[0] <= $spot && $spot < $range[1] ) || $primaryLocation !== null ) {
  98                                 if ( $primaryLocation === null ) {
  99                                         $primaryLocation = $location;
 100                                 }
 101                                 $locations[] = $location;
 102                         }
 103                 }
 104                 // If more locations are requested, wrap-around and keep adding them
 105                 reset( $this->ring );
 106                 while ( count( $locations ) < $limit ) {
 107                         list( $location, ) = each( $this->ring );
 108                         if ( $location === $primaryLocation ) {
 109                                 break; // don't go in circles
 110                         }
 111                         $locations[] = $location;
 112                 }
 113                 return $locations;
 114         }
 115 }