package com.github.HarryHook.coding2017.linklist;
/**
* 用双向链表实现LRU算法
*
* @author HarryHook
*
*/
public class LRUPageFrame {
private static class Node {
Node prev;
Node next;
int pageNum;
Node() {
}
}
private int capacity;
private int currentSize = 0;
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 pageNum
* @return
*/
public void access(int pageNum) {
if (currentSize == 0) {
Node node = new Node();
node.pageNum = pageNum;
node.prev = node.next = null;
first = last = node;
currentSize++;
} else {
if (first.pageNum == pageNum) {
return;
}
if (currentSize == 1) {
addToFirst(pageNum);
} else {
if (last.pageNum == pageNum) {
moveLastToFirst();
return;
}
Node iteratorNode = first;
while (iteratorNode.next != null && iteratorNode.pageNum != pageNum) {
iteratorNode = iteratorNode.next;
}
if (iteratorNode == last) {
addToFirst(pageNum);
// 若缓存已满，移除最后一个节点
if (currentSize > capacity) {
last = last.prev;
last.next = null;
}
} else {
moveToFirst(iteratorNode);
}
}
}
}
// 将节点/缓存页添加到fitrst
public void addToFirst(int pageNum) {
Node node = new Node();
node.pageNum = pageNum;
node.prev = null;
node.next = first;
first.prev = node;
first = node;
currentSize++;
}
// 将last节点移动到first
public void moveLastToFirst() {
last.next = first;
first.prev = last;
first = last;
last = last.prev;
last.next = null;
first.prev = null;
}
// 将最近使用的已有缓存页移动到first
public void moveToFirst(Node iteratorNode) {
iteratorNode.prev.next = iteratorNode.next;
iteratorNode.next.prev = iteratorNode.prev;
iteratorNode.prev = null;
iteratorNode.next = first;
first.prev = iteratorNode;
first = iteratorNode;
}
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();
}
}