ConsistentHash.java

import java.util.Collection;
import java.util.SortedMap;
import java.util.TreeMap;
// From: http://www.tom-e-white.com/2007/11/consistent-hashing.html
public class ConsistentHash<T> {
 
    private final HashFunction hashFunction;
    private final int numberOfReplicas;
    private final SortedMap<Integer, T> circle =
        new TreeMap<Integer, T>();
 
    public ConsistentHash(HashFunction hashFunction,
                          int numberOfReplicas, Collection<T> nodes) {
 
        this.hashFunction = hashFunction;
        this.numberOfReplicas = numberOfReplicas;
 
        for (T node : nodes) {
            add(node);
        }
    }
 
    public void add(T node) {
        for (int i = 0; i < numberOfReplicas; i++) {
            circle.put(hashFunction.hash(node.toString() + i),
                       node);
        }
    }
 
    public void remove(T node) {
        for (int i = 0; i < numberOfReplicas; i++) {
            circle.remove(hashFunction.hash(node.toString() + i));
        }
    }
 
    public T get(Object key) {
        if (circle.isEmpty()) {
            return null;
        }
        int hash = hashFunction.hash(key);
        if (!circle.containsKey(hash)) {
            SortedMap<Integer, T> tailMap =
                circle.tailMap(hash);
            hash = tailMap.isEmpty() ?
                circle.firstKey() : tailMap.firstKey();
        }
        return circle.get(hash);
    }
 
}