func (b *LRUCache) leastUsedEntry() *entry {
	return b.lruList.Back().Value.(*entry)
}
Give me the least used entry.
package lrucache
import (
	"container/heap"
	"sync"
	"time"
)
type entry struct {
	element element      
	key     string       
	value   interface{}  
	expire  time.Time    
	index   int          
}
type LRUCache struct {
	lock          sync.Mutex
	table         map[string]*entry  
	priorityQueue priorityQueue      
	lruList       list               
	freeList      list               
	ExpireGracePeriod time.Duration  
}
func (b *LRUCache) Init(capacity uint) {
	b.table = make(map[string]*entry, capacity)
	b.priorityQueue = make([]*entry, 0, capacity)
	b.lruList.Init()
	b.freeList.Init()
	heap.Init(&b.priorityQueue)
	arrayOfEntries := make([]entry, capacity)
	for i := uint(0); i < capacity; i++ {
		e := &arrayOfEntries[i]
		e.element.Value = e
		e.index = -1
		b.freeList.PushElementBack(&e.element)
	}
}
func NewLRUCache(capacity uint) *LRUCache {
	b := &LRUCache{}
	b.Init(capacity)
	return b
}
func (b *LRUCache) expiredEntry(now time.Time) *entry {
	if len(b.priorityQueue) == 0 {
		return nil
	}
	if now.IsZero() {
		now = time.Now()
	}
	if e := b.priorityQueue[0]; e.expire.Before(now) {
		return e
	}
	return nil
}