func New(conf Config) (*EasyCache, error) {

	if !utils.IsPowerOfTwo(conf.Shards) {
		returnnil, errors.New("shards number must be power of two")
	}

	if conf.Cap <= 0 {
		conf.Cap = defaultCap
	}
	// init cache object
	cache := &EasyCache{
		shards:    make([]*cacheShard, conf.Shards),
		conf:      conf,
		hash:      conf.Hasher,
		shardMask: uint64(conf.Shards - 1), // mask
		close:     make(chanstruct{}),
	}

	var onRemove OnRemoveCallback
	if conf.OnRemoveWithReason != nil {
		onRemove = conf.OnRemoveWithReason
	} else {
		onRemove = cache.notProvidedOnRemove
	}

	// init shard
	for i := 0; i < conf.Shards; i++ {
		cache.shards[i] = newCacheShard(conf, i, onRemove, cache.close)
	}
	return cache, nil
}

func newCacheShard(conf Config, id int, onRemove OnRemoveCallback, close chan struct{}) *cacheShard {
	shard := &cacheShard{
		items:           make(map[string]*list.Element),
		expireItems:     make(map[string]*list.Element),
		cap:             conf.Cap,
		list:            list.New(),
		logger:          newLogger(conf.Logger),
		cleanupInterval: defaultInternal,
		cleanupTicker:   time.NewTicker(defaultInternal),
		addChan:         make(chanstring),
		isVerbose:       conf.Verbose,
		id:              id,
		onRemove:        onRemove,
		close:           close,
	}
	// goroutine clean expired key
	go shard.expireCleanup()
	return shard
}

/*
1.當定時器到期，執(zhí)行過期清理
2.當新增的key有過期時間，通過addChan觸發(fā)執(zhí)行
*/
func (cs *cacheShard) expireCleanup() {
	for {
		select {
		case <-cs.cleanupTicker.C:
		case <-cs.addChan: // 立即觸發(fā)
		case <-cs.close: // stop goroutine
			if cs.isVerbose {
				cs.logger.Printf("[shard %d] flush..", cs.id)
			}
			cs.flush() // free
			return
		}
		cs.cleanupTicker.Stop()
		// 記錄下一次定時器的最小間隔（目的：key過期了，盡快刪除）
		smallestInternal := 0 * time.Second
		now := time.Now()
		cs.lock.Lock()
		for key, ele := range cs.expireItems { // 遍歷過期key
			item := ele.Value.(*cacheItem)
			if item.LifeSpan() == 0 { // 沒有過期時間
				cs.logger.Printf("warning wrong data\n")
				continue
			}
			if now.Sub(item.CreatedOn()) >= item.LifeSpan() { // 過期
				// del
				delete(cs.items, key)
				delete(cs.expireItems, key)
				cs.list.Remove(ele)
				cs.onRemove(key, item.Value(), Expired)
				if cs.isVerbose {
					cs.logger.Printf("[shard %d]: expire del key <%s>  createdOn:%v,  lifeSpan:%d ms \n", cs.id, key, item.CreatedOn(), item.LifeSpan().Milliseconds())
				}
			} else {
				d := item.LifeSpan() - now.Sub(item.CreatedOn())
				if smallestInternal == 0 || d < smallestInternal {
					smallestInternal = d
				}
			}
		}
		if smallestInternal == 0 {
			smallestInternal = defaultInternal
		}
		cs.cleanupInterval = smallestInternal
		cs.cleanupTicker.Reset(cs.cleanupInterval)
		cs.lock.Unlock()
	}
}

cache.Set(key, 0, 5*time.Second) // expire 5s
cache.Set(key, 0, 0*time.Second) // expire 0s

func (cs *cacheShard) set(key string, value interface{}, lifeSpan time.Duration) error {

	cs.lock.Lock()
	defer cs.lock.Unlock()

	oldEle, ok := cs.items[key]
	if ok { // old item
		oldItem := oldEle.Value.(*cacheItem)
		oldLifeSpan := oldItem.LifeSpan()

		// modify
		oldEle.Value = newCacheItem(key, value, lifeSpan)
		cs.list.MoveToFront(oldEle)

		if oldLifeSpan > 0 && lifeSpan == 0 { // 原來的有過期時間，新的沒有過期時間
			delete(cs.expireItems, key)
		}

		if oldLifeSpan == 0 && lifeSpan > 0 { // 原有的無過期時間，當前有過期時間
			cs.expireItems[key] = oldEle
			if lifeSpan < cs.cleanupInterval {
				gofunc() {
					cs.addChan <- key
				}()
			}
		}

	} else { // new item

		iflen(cs.items) >= int(cs.cap) { // lru: No space
			delVal := cs.list.Remove(cs.list.Back())
			item := delVal.(*cacheItem)
			delete(cs.items, item.Key())
			if item.LifeSpan() > 0 {
				delete(cs.expireItems, item.Key())
			}
			cs.onRemove(key, item.Value(), NoSpace)

			if cs.isVerbose {
				cs.logger.Printf("[shard %d] no space del key <%s>\n", cs.id, item.Key())
			}
		}
		// add
		ele := cs.list.PushFront(newCacheItem(key, value, lifeSpan))
		cs.items[key] = ele
		if lifeSpan > 0 {
			cs.expireItems[key] = ele
			if lifeSpan < cs.cleanupInterval {
				gofunc() {
					cs.addChan <- key
				}()
			}
		}
	}

	if cs.isVerbose {
		if lifeSpan == 0 {
			cs.logger.Printf("[shard %d]: set persist key <%s>\n", cs.id, key)
		} else {
			cs.logger.Printf("[shard %d]: set expired key <%s>", cs.id, key)
		}
	}
	returnnil
}