iv018-redis的LRU算法简介

LRU：least recently used 最近最少使用的
LRU的算法核心是哈希链表（本质就是HashMap+DoubleLinkedList）

编码实现：

public class LRUDemo<K, V> {

    private int cacheSize;

    private Map<K, Node<K, V>> map;

    private DoubleLinkList<K, V> linkList;

    public LRUDemo(int cacheSize) {
        this.cacheSize = cacheSize;
        map = new HashMap<>();
        linkList = new DoubleLinkList<>();
    }

    public V get(K key) {
        if (!map.containsKey(key)) {
            return null;
        }
        Node<K, V> node = map.get(key);
        linkList.removeNode(node);
        linkList.addHead(node);
        return node.value;
    }

    public void put(K key, V value) {
        if (map.containsKey(key)) {
            Node<K, V> node = map.get(key);
            node.value = value;
            linkList.removeNode(node);
            linkList.addHead(node);
        } else {
            if (map.size() >= cacheSize) {
                linkList.removeNode(linkList.getLastNode());
            }
            Node<K, V> newNode = new Node<>(key, value);
            linkList.addHead(newNode);
            map.put(key, newNode);
        }
    }

    class Node<K, V> {
        K key;
        V value;
        Node<K, V> prev;
        Node<K, V> next;

        Node() {
            prev = next = null;
        }

        Node(K key, V value) {
            this.key = key;
            this.value = value;
            prev = next = null;
        }

        @Override
        public String toString() {
            return "Node{" +
                    "key=" + key +
                    ", value=" + value +
                    '}';
        }
    }

    class DoubleLinkList<K, V> {
        Node<K, V> head;
        Node<K, V> tail;

        DoubleLinkList() {
            head = new Node<>();
            tail = new Node<>();
            head.next = tail;
            tail.prev = head;
        }

        public void addHead(Node<K, V> node) {
            node.prev = head;
            node.next = head.next;
            head.next.prev = node;
            head.next = node;
        }

        public void removeNode(Node<K, V> node) {
            if (node != null && node != head && node != tail) {
                node.prev.next = node.next;
                node.next.prev = node.prev;
                node.prev = null;
                node.next = null;
            }
        }

        public Node<K, V> getLastNode() {
            return tail.prev != head ? tail.prev : null;
        }

        @Override
        public String toString() {
            StringBuilder sb = new StringBuilder("[");
            Node<K, V> node = head.next;
            while (node != null && node != tail) {
                sb.append(node.key);
                node = node.next;
                if (node != tail) {
                    sb.append(",");
                }
            }
            return sb.append("]").toString();
        }
    }

    public static void main(String[] args) {
        LRUDemo<Integer, Integer> lruDemo = new LRUDemo<>(3);
        lruDemo.put(1, 1);
        lruDemo.put(2, 2);
        lruDemo.put(3, 3);
        System.out.println(lruDemo.linkList);
        lruDemo.put(4, 4);
        System.out.println(lruDemo.linkList);
        lruDemo.put(3, 3);
        System.out.println(lruDemo.linkList);
        lruDemo.put(3, 33);
        System.out.println(lruDemo.linkList);
        lruDemo.put(3, 3);
        System.out.println(lruDemo.linkList);
        lruDemo.put(5, 5);
        System.out.println(lruDemo.linkList);
        lruDemo.get(4);
        System.out.println(lruDemo.linkList);
    }
}

运行结果：

[3,2,1]
[4,3,2]
[3,4,2]
[3,4,2]
[3,4,2]
[5,3,4]
[4,5,3]