HashRing.php

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<?php
class HashRing {
    protected $sourceMap = array();
    protected $ring = array();

    const RING_SIZE = 268435456; // 2^28

    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;
    }

    public function getLocation( $item ) {
        $locations = $this->getLocations( $item, 1 );
        return $locations[0];
    }

    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;
    }

    public function getLocationWeights() {
        return $this->sourceMap;
    }

    public function newWithoutLocation( $location ) {
        $map = $this->sourceMap;
        unset( $map[$location] );
        if ( count( $map ) ) {
            return new self( $map );
        }
        return false;
    }
}