Go LRU Cache

目录

1. LRU Cache

2. container/list.go

2.1 list 数据结构

2.2 list 使用例子

3. transport.go connLRU

4. 结尾

正文

1. LRU Cache

package cache

import (
    "container/list"
    "sync"
)

// entry 存储的实体
type entry struct {
    key, val interface{}
}

// Cache 缓存结构
type Cache struct {
    // m 保证 LRU Cache 访问线程安全
    rw sync.RWMutex

    // max 标识缓存容量的最大值, = 0 标识无限缓存
    max int

    // list 是 entry 循环双向链表
    list *list.List

    // pond entry 缓存池子 key -> entry
    pond map[interface{}]*list.Element
}

// New 构建 LRU Cache 缓存结构
func New(max int) *Cache {
    return &Cache{
        max:  max,
        list: list.New(),
        pond: make(map[interface{}]*list.Element),
    }
}

func (c *Cache) delete(key interface{}) {
    element, ok := c.pond[key]
    if ok {
        delete(c.pond, key)
        c.list.Remove(element)
        return
    }
}

// Set 设置缓存
func (c *Cache) Set(key, val interface{}) {
    c.rw.Lock()
    defer c.rw.Unlock()

    // 看是否进入删除分支
    if val == nil {
        c.delete(key)
        return
    }

    element, ok := c.pond[key]
    if ok {
        // 重新设置 value 数据
        element.Value.(*entry).val = val
        // set key nil exists 进入 update 逻辑
        c.list.MoveToFront(element)
        return
    }

    // 首次添加
    c.pond[key] = c.list.PushFront(&entry{key, val})

    // 数据过多, 删除尾巴数据
    if c.list.Len() > c.max && c.max > 0 {
        delete(c.pond, c.list.Remove(c.list.Back()).(*entry).key)
    }
}

// Get 获取缓存
func (c *Cache) Get(key interface{}) (val interface{}, ok bool) {
    c.rw.RLock()
    defer c.rw.RUnlock()

    if element, ok := c.pond[key]; ok {
        // 获取指定缓存值
        val, ok = element.Value.(*entry).val, true
        // 调整缓存热点
        c.list.MoveToFront(element)
    }
    return
}

原理是 1. RWLock 做线程安全 2. list 双向链表保存时间新老关系 2. map 为了让时间复杂度到 O(1).

使用教程: 

// 创建
c := cache.New(1)

// 设置
c.Set("123", "123")
c.Set("234", "234")

// 使用
fmt.Println(c.Get("123"))
fmt.Println(c.Get("234"))

// 删除
c.Set("123", nil)

2. container/list.go

2.1 list 数据结构

上面 LRU Cache 代码中引用了 "container/list" , 简单分析下 list, 加深基础数据结构的了解. 

// Copyright 2009 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 list implements a doubly linked list.
//
// To iterate over a list (where l is a *List):
//    for e := l.Front(); e != nil; e = e.Next() {
//        // do something with e.Value
//    }
//
package list

// Element is an element of a linked list.
type Element struct {
    // Next and previous pointers in the doubly-linked list of elements.
    // To simplify the implementation, internally a list l is implemented
    // as a ring, such that &l.root is both the next element of the last
    // list element (l.Back()) and the previous element of the first list
    // element (l.Front()).
    next, prev *Element

    // The list to which this element belongs.
    list *List

    // The value stored with this element.
    Value interface{}
}

// Next returns the next list element or nil.
func (e *Element) Next() *Element {
    if p := e.next; e.list != nil && p != &e.list.root {
        return p
    }
    return nil
}

// Prev returns the previous list element or nil.
func (e *Element) Prev() *Element {
    if p := e.prev; e.list != nil && p != &e.list.root {
        return p
    }
    return nil
}

// List represents a doubly linked list.
// The zero value for List is an empty list ready to use.
type List struct {
    root Element // sentinel list element, only &root, root.prev, and root.next are used
    len  int     // current list length excluding (this) sentinel element
}

它是个特殊循环双向链表数据结构, 特殊之处在于 Element::List  指向头结点(root list).

关于业务 list.go 具体实现部分我们不表. 

2.2 list 使用例子

func Test_List_Demo(t *testing.T) {

    // Persion 普通人
    type Persion struct {
        Name string
        Age  int
    }

    pers := list.New()

    // 链表数据填充
    pers.PushBack(&Persion{Name: "wang", Age: 31})
    pers.PushFront(&Persion{Name: "zhi", Age: 31})

    fmt.Printf("List Len() = %d
", pers.Len())
    if pers.Len() != 2 {
        t.Fatal("pers.Len() != 2 data faild")
    }

    // 开始遍历数据
    for element := pers.Front(); element != nil; element = element.Next() {
        per, ok := element.Value.(*Persion)
        if !ok {
            panic(fmt.Sprint("Persion list faild", element.Value))
        }
        fmt.Println(per)
    }

    // 数据删除
    for element := pers.Front(); element != nil; {
        next := element.Next()
        pers.Remove(element)
        element = next
    }

    fmt.Printf("List Len() = %d
", pers.Len())
    if pers.Len() != 0 {
        t.Fatal("pers.Len() != 0 data faild")
    }
}

单元测试结果: 

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^Test_List_Demo$ demo/src/container/list -v -count=1

=== RUN   Test_List_Demo
List Len() = 2
&{zhi 31}
&{wang 31}
List Len() = 0
--- PASS: Test_List_Demo (0.00s)
PASS
ok      demo/src/container/list    0.002s

3. transport.go connLRU

抛一段 Go 源码中一处应用, 小学以小用 

//
// src/net/http/transport.go
//


// persistConn wraps a connection, usually a persistent one
// (but may be used for non-keep-alive requests as well)
type persistConn struct {
　　... 
　　.. 
　　.
}


type connLRU struct {
    ll *list.List // list.Element.Value type is of *persistConn
    m  map[*persistConn]*list.Element
}

// add adds pc to the head of the linked list.
func (cl *connLRU) add(pc *persistConn) {
    if cl.ll == nil {
        cl.ll = list.New()
        cl.m = make(map[*persistConn]*list.Element)
    }
    ele := cl.ll.PushFront(pc)
    if _, ok := cl.m[pc]; ok {
        panic("persistConn was already in LRU")
    }
    cl.m[pc] = ele
}

func (cl *connLRU) removeOldest() *persistConn {
    ele := cl.ll.Back()
    pc := ele.Value.(*persistConn)
    cl.ll.Remove(ele)
    delete(cl.m, pc)
    return pc
}

// remove removes pc from cl.
func (cl *connLRU) remove(pc *persistConn) {
    if ele, ok := cl.m[pc]; ok {
        cl.ll.Remove(ele)
        delete(cl.m, pc)
    }
}

// len returns the number of items in the cache.
func (cl *connLRU) len() int {
    return len(cl.m)
}

4. 结尾

很多代码, 很多事情也都平淡无奇, 但凡事种种都离不开用心, 反复琢磨 ~ 方能长久 

欢迎批评指正交流 ~ good luckly ~