基于groupcache的源碼學(xué)習(xí)

/*
Copyright 2013 Google Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

// Package lru implements an LRU cache.
package lru

import "container/list"

// Cache is an LRU cache. It is not safe for concurrent access.
type Cache struct {
    // MaxEntries is the maximum number of cache entries before
    // an item is evicted. Zero means no limit.
        // 最多存放個數(shù)
    MaxEntries int

    // OnEvicted optionally specifies a callback function to be
    // executed when an entry is purged from the cache
    // 回調(diào)函數(shù)
    OnEvicted func(key Key, value interface{} 
       //鏈表存儲
    ll    *list.List
       //輔助map
    cache map[interface{}]*list.Element
}

// A Key may be any value that is comparable. See http://golang.org/ref/spec#Comparison_operators
//泛型
type Key interface{}

type entry struct {
    key   Key
    value interface{}
}

// New creates a new Cache.
// If maxEntries is zero, the cache has no limit and it's assumed
// that eviction is done by the caller.
func New(maxEntries int) *Cache {
    return &Cache{
        MaxEntries: maxEntries,
        ll:         list.New(),
        cache:      make(map[interface{}]*list.Element),
    }
}

// Add adds a value to the cache.
func (c *Cache) Add(key Key, value interface{}) {
    if c.cache == nil {
        c.cache = make(map[interface{}]*list.Element)
        c.ll = list.New()
    }
    if ee, ok := c.cache[key]; ok {
        c.ll.MoveToFront(ee)
        ee.Value.(*entry).value = value
        return
    }
    ele := c.ll.PushFront(&entry{key, value})
    c.cache[key] = ele
    if c.MaxEntries != 0 && c.ll.Len() > c.MaxEntries {
        c.RemoveOldest()
    }
}

// Get looks up a key's value from the cache.
func (c *Cache) Get(key Key) (value interface{}, ok bool) {
    if c.cache == nil {
        return
    }
    if ele, hit := c.cache[key]; hit {
        c.ll.MoveToFront(ele)
        return ele.Value.(*entry).value, true
    }
    return
}

// Remove removes the provided key from the cache.
func (c *Cache) Remove(key Key) {
    if c.cache == nil {
        return
    }
    if ele, hit := c.cache[key]; hit {
        c.removeElement(ele)
    }
}

// RemoveOldest removes the oldest item from the cache.
func (c *Cache) RemoveOldest() {
    if c.cache == nil {
        return
    }
    ele := c.ll.Back()
    if ele != nil {
        c.removeElement(ele)
    }
}

func (c *Cache) removeElement(e *list.Element) {
    c.ll.Remove(e)
    kv := e.Value.(*entry)
    delete(c.cache, kv.key)
    if c.OnEvicted != nil {
        c.OnEvicted(kv.key, kv.value)
    }
}

// Len returns the number of items in the cache.
func (c *Cache) Len() int {
    if c.cache == nil {
        return 0
    }
    return c.ll.Len()
}

// Clear purges all stored items from the cache.
func (c *Cache) Clear() {
    if c.OnEvicted != nil {
        for _, e := range c.cache {
            kv := e.Value.(*entry)
            c.OnEvicted(kv.key, kv.value)
        }
    }
    c.ll = nil
    c.cache = nil
}