LRUPageFrame.java

package com.coding.basic.homework_04.lru;

/**
 * 用双向链表实现LRU算法
 * @author liuxin
 *
 */
public class LRUPageFrame {
	
	private static class Node {
		
		Node prev;
		Node next;
		int pageNum;

		Node() {
		}
		Node(int pageNum){
			this.pageNum = pageNum;
		}
	}

	private int capacity;
	
	
	private Node first;// 链表头
	private Node last;// 链表尾

	
	public LRUPageFrame(int capacity) {
		
		this.capacity = capacity;
		
	}

	/**
	 * 获取缓存中对象
	 * 
	 * @param key
	 * @return
	 */
	public void access(int pageNum) {
		if(first == null){ //缓存第一个数据
			first = new Node(pageNum);
			first.next = first.prev = null;
			last = first;
		}else if(size() < capacity && first != null){ //缓存页未满时
			Node node = new Node(pageNum);
			if(last.prev == null){
				last.prev = node;
				node.next = last;
			}else{
				node.next = first;
				first.prev = node;
				node.prev = null;
			}
			first = node;
		}else if(size() >= capacity){ //缓存页满了
			Node node = getNode(pageNum);
			LRU(node, pageNum);
		}
	}
	
	/**
	 * lru算法
	 * @param node
	 * @param pageNum
	 */
	private void LRU(Node node, int pageNum){
		if(node != null){
			if(last.pageNum == node.pageNum){ //缓存是last
				last = node.prev;
				last.next = null;
				node.next = first;
				first.prev = node;
				node.prev = null;
				first = node;
			}else if(last.pageNum != node.pageNum && first.pageNum != node.pageNum){ 
				//缓存在first和last的中间范围
				node.prev.next = node.next;
				node.next.prev = node.prev;
				node.prev = null;
				node.next = first;
				first = node;
			}	
		}else{
			//新缓存
			last = last.prev;
			last.next = null;
			node = new Node(pageNum);
			node.next = first;
			first.prev = node;
			first = node;
		}
	}
	
	/**
	 * 根据数据在缓存中获取节点
	 * @param pageNum
	 * @return
	 */
	private Node getNode(int pageNum){
		Node node = first;
		while(node != null){
			if(node.pageNum == pageNum){
				return node;
			}
			node = node.next;
		}
		return null;
	}
	
	public int size(){
		int num = 0;
		Node node = first;
		while(node != null){
			num++;
			node = node.next;
		}
		return num;
	}

	public String toString(){
		StringBuilder buffer = new StringBuilder();
		Node node = first;
		while(node != null){
			buffer.append(node.pageNum);			
			
			node = node.next;
			if(node != null){
				buffer.append(",");
			}
		}
		return buffer.toString();
	}
	
}