lru.go

/*
Copyright 2015 To gocql authors
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.
//
// This cache has been forked from github.com/golang/groupcache/lru, but
// specialized with string keys to avoid the allocations caused by wrapping them
// in interface{}.
type Cache struct {
	// MaxEntries is the maximum number of cache entries before
	// an item is evicted. Zero means no limit.
	MaxEntries int

	// OnEvicted optionally specificies a callback function to be
	// executed when an entry is purged from the cache.
	OnEvicted func(key string, value interface{})

	ll    *list.List
	cache map[string]*list.Element
}

type entry struct {
	key   string
	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[string]*list.Element),
	}
}

// Add adds a value to the cache.
func (c *Cache) Add(key string, value interface{}) {
	if c.cache == nil {
		c.cache = make(map[string]*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 string) (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 string) bool {
	if c.cache == nil {
		return false
	}

	if ele, hit := c.cache[key]; hit {
		c.removeElement(ele)
		return true
	}

	return false
}

// 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()
}