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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package lru implements an LRU cache.
package lru
import (
// Cache is an LRU cache, safe for concurrent access.
type Cache struct {
maxEntries int
mu sync.Mutex
ll *list.List
cache map[interface{}]*list.Element
// *entry is the type stored in each *list.Element.
type entry struct {
key, value interface{}
// New returns a new cache with the provided maximum items.
func New(maxEntries int) *Cache {
return &Cache{
maxEntries: maxEntries,
ll: list.New(),
cache: make(map[interface{}]*list.Element),
// Add adds the provided key and value to the cache, evicting
// an old item if necessary.
func (c *Cache) Add(key, value interface{}) {
// Already in cache?
if ee, ok := c.cache[key]; ok {
ee.Value.(*entry).value = value
// Add to cache if not present
ele := c.ll.PushFront(&entry{key, value})
c.cache[key] = ele
if c.ll.Len() > c.maxEntries {
// Get fetches the key's value from the cache.
// The ok result will be true if the item was found.
func (c *Cache) Get(key interface{}) (value interface{}, ok bool) {
if ele, hit := c.cache[key]; hit {
return ele.Value.(*entry).value, true
// RemoveOldest removes the oldest item in the cache and returns its key and value.
// If the cache is empty, the empty string and nil are returned.
func (c *Cache) RemoveOldest() (key, value interface{}) {
return c.removeOldest()
// note: must hold
func (c *Cache) removeOldest() (key, value interface{}) {
ele := c.ll.Back()
if ele == nil {
ent := ele.Value.(*entry)
delete(c.cache, ent.key)
return ent.key, ent.value
// Len returns the number of items in the cache.
func (c *Cache) Len() int {
return c.ll.Len()