function CacheLRU(capacity) {
/* 利用Buffer寫的一個(gè)LRU緩存，capacity為緩存容量，為0時(shí)不限容量。
myCache = new CacheLRU(capacity); //構(gòu)造緩存
myCache.get(key); //讀取名為key的緩存值
myCache.put(key, value); //寫入名為key的緩存值
myCache.remove(key); //刪除名為key的緩存值
myCache.removeAll(); //清空緩存
myCache.info(); //返回myCache緩存信息
LRU原理：對(duì)所有緩存數(shù)據(jù)的key構(gòu)建hash鏈表，當(dāng)對(duì)某一數(shù)據(jù)進(jìn)行g(shù)et或put操作時(shí)，將其key提到鏈表前端（最新）。當(dāng)進(jìn)行put數(shù)據(jù)超出容量時(shí)，刪除鏈表尾端（最舊）的緩存數(shù)據(jù)。
hash鏈表操作可直接定位key，無需歷遍整個(gè)hash對(duì)象，故讀寫極快。緩存容量不再影響讀寫速度。
*/
  this.capacity = capacity || Number.MAX_VALUE;
  this.data = {};
  this.hash = {};
  this.linkedList = {
    length: 0,
    head: null,
    end: null
  }
  if (capacity <= 0) this.capacity = Number.MAX_VALUE;
};
CacheLRU.prototype.get = function(key) {
  key = '_' + key;
  var lruEntry = this.hash[key];
  if (!lruEntry) return;
  refresh(this.linkedList, lruEntry);
  return JSON.parse(this.data[key].toString());
};
CacheLRU.prototype.put = function(key, value) {
  key = '_' + key;
  var lruEntry = this.hash[key];
  if (value === undefined) return this;
  if (!lruEntry) {
    this.hash[key] = {key: key};
    this.linkedList.length += 1;
    lruEntry = this.hash[key];
  }
  refresh(this.linkedList, lruEntry);
  this.data[key] = new Buffer(JSON.stringify(value));
  if (this.linkedList.length > this.capacity) this.remove(this.linkedList.end.key.slice(1));
  return this;
};
CacheLRU.prototype.remove = function(key) {
  key = '_' + key;
  var lruEntry = this.hash[key];
  if (!lruEntry) return this;
  if (lruEntry === this.linkedList.head) this.linkedList.head = lruEntry.p;
  if (lruEntry === this.linkedList.end) this.linkedList.end = lruEntry.n;
  link(lruEntry.n, lruEntry.p);
  delete this.hash[key];
  delete this.data[key];
  this.linkedList.length -= 1;
  return this;
};
CacheLRU.prototype.removeAll = function() {
  this.data = {};
  this.hash = {};
  this.linkedList = {
    length: 0,
    head: null,
    end: null
  }
  return this;
};
CacheLRU.prototype.info = function() {
  var size = 0,
    data = this.linkedList.head;
  while (data){
    size += this.data[data.key].length;
    data = data.p;
  }
  return {
    capacity: this.capacity,
    length: this.linkedList.length,
    size: size
  };
};
// 更新鏈表，把get或put方法操作的key提到鏈表head，即表示最新
function refresh(linkedList, entry) {
  if (entry != linkedList.head) {
    if (!linkedList.end) {
      linkedList.end = entry;
    } else if (linkedList.end == entry) {
      linkedList.end = entry.n;
    }
    link(entry.n, entry.p);
    link(entry, linkedList.head);
    linkedList.head = entry;
    linkedList.head.n = null;
  }
}
// 對(duì)兩個(gè)鏈表對(duì)象建立鏈接，形成一條鏈
function link(nextEntry, prevEntry) {
  if (nextEntry != prevEntry) {
    if (nextEntry) nextEntry.p = prevEntry;
    if (prevEntry) prevEntry.n = nextEntry;
  }
}
module.exports = CacheLRU;
// test:
/*var user = new CacheLRU(5);
user.put('user1', {name:'admin', age: 30});
user.put('user2', {name:'user', age: 31});
user.put('user3', {name:'guest', age: 32});
user.put('user4', {name:'guest', age: 34});
user.put('user5', {name:'guest', age: 35});
console.log(user.get('user1'));
console.log(user.get('user2'));
console.log(user.get('user3'));
user.put('user6', {name:'guest', age: 36});
console.log(user.info());*/