lru.go

/*
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 main

import (
	"container/list"
	"time"
)

// 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.
	MaxEntries int

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

	ll    *list.List
	cache map[interface{}]*list.Element

	expires time.Duration
}

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

type entry struct {
	key           Key
	value         interface{}
	time_accessed time.Time
}

// 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 NewLRUCache(maxEntries int, expires time.Duration) *Cache {
	return &Cache{
		MaxEntries: maxEntries,
		ll:         list.New(),
		cache:      make(map[interface{}]*list.Element),
		expires:    expires,
	}
}

// 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
		ee.Value.(*entry).time_accessed = time.Now()
		return
	}
	ele := c.ll.PushFront(&entry{key, value, time.Now()})
	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 {
		duration := time.Now().Sub(ele.Value.(*entry).time_accessed)
		if duration > c.expires {
			return nil, false
		}
		c.ll.MoveToFront(ele)
		ele.Value.(*entry).time_accessed = time.Now()
		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()
}