手撕 | FIFO缓存

底层维护了一个队列,越先进队列的,越先被淘汰

package com.darwin.redis.eliminated;

import java.util.HashMap;
import java.util.Map;

/**
 * FIFO Java代码实现
 * @author yanghang
 */
public class FIFOUtil {

    /**
     * 用来快速定位节点位置
     */
    private final Map<String, Node> map;
    /**
     * 缓存大小
     */
    private int size;
    /**
     * 虚拟头尾节点
     */
    Node head;
    Node tail;
    /**
     * 缓存大小
     */
    int capacity;

    public FIFOUtil(int capacity) {
        head = new Node("", null);
        tail = new Node("", null);
        head.next = tail;
        tail.pre = head;
        size = 0;
        this.capacity = capacity;
        map = new HashMap<>();
    }

    /**
     * 添加元素进入缓存
     *
     * @param key   键
     * @param value 值
     */
    public void put(String key, Object value) {
        Node node = new Node(key, value);
        node.pre = tail.pre;
        node.pre.next = node;
        node.next = tail;
        tail.pre = node;
        map.put(key, node);
        if (size < capacity) {
            size++;
        } else {
            // 删除队首
            Node next = head.next;
            head.next = next.next;
            head = head.next.pre;
            map.remove(next.key);
        }
    }

    /**
     * 获取缓存的值
     *
     * @param key 键
     * @return 缓存键对应的值
     */
    public Object get(String key) {
        if (map.containsKey(key)) {
            return map.get(key).val;
        }
        return null;
    }

    /**
     * 内置双向链表
     */
    static class Node {
        String key;
        Object val;

        public Node(String key, Object val) {
            this.key = key;
            this.val = val;
        }

        Node pre;
        Node next;
    }

    public static void main(String[] args) {
        FIFOUtil FIFOUtil = new FIFOUtil(10);
        for (int i = 0; i < 100; i++) {
            FIFOUtil.put(i + "", i);
        }
        for (int i = 90; i < 100; i++) {
            System.out.println(FIFOUtil.get(i + ""));
        }
    }
}