一致性hash

在缓存场景中，一致性hash算法避免了余数法在因为节点失效导致大面积缓存失效的问题。

直接上代码,使用了Java自带的TreeMap, 参考了dubbo代码中的一致性hash负载

package org.example;

import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.*;

public class ConsistentHash<T> {

    private int VIRTUAL_NODE_NUM = 40;
    private final TreeMap<Long, T> circle = new TreeMap<>();

    public ConsistentHash(List<T> nodes, int VIRTUAL_NODE_NUM) {
        this.VIRTUAL_NODE_NUM = VIRTUAL_NODE_NUM;
        for (T node : nodes) {
            for (int i = 0; i < VIRTUAL_NODE_NUM; i++) {
                byte[] digest = md5(node.toString() + i);
                for (int h = 0; h < 4; h++) {
                    long m = hash(digest, h);
                    circle.put(m, node);
                }
            }
        }
    }

    public T get(Object key) {
        if (circle.isEmpty()) {
            return null;
        }
        byte[] digest = md5(key.toString());
        long hash = hash(digest, 0);
        Map.Entry<Long, T> entry = circle.ceilingEntry(hash);
        if (entry == null) {
            entry = circle.firstEntry();
        }
        return entry.getValue();
    }


    private long hash(byte[] digest, int number) {
        return (((long) (digest[3 + number * 4] & 0xFF) << 24)
                | ((long) (digest[2 + number * 4] & 0xFF) << 16)
                | ((long) (digest[1 + number * 4] & 0xFF) << 8)
                | (digest[number * 4] & 0xFF))
                & 0xFFFFFFFFL;
    }

    private byte[] md5(String value) {
        MessageDigest md5;
        try {
            md5 = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
        md5.reset();
        byte[] bytes = value.getBytes(StandardCharsets.UTF_8);
        md5.update(bytes);
        return md5.digest();
    }
}