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tailscale/util/lru/lru.go

111 lines
2.8 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package lru contains a typed Least-Recently-Used cache.
package lru
import (
"container/list"
)
// Cache is container type keyed by K, storing V, optionally evicting the least
// recently used items if a maximum size is exceeded.
//
// The zero value is valid to use.
//
// It is not safe for concurrent access.
//
// The current implementation is just the traditional LRU linked list; a future
// implementation may be more advanced to avoid pathological cases.
type Cache[K comparable, V any] struct {
// MaxEntries is the maximum number of cache entries before
// an item is evicted. Zero means no limit.
MaxEntries int
ll *list.List
m map[K]*list.Element // of *entry[K,V]
}
// entry is the element type for the container/list.Element.
type entry[K comparable, V any] struct {
key K
value V
}
// Set adds or replaces a value to the cache, set or updating its associated
// value.
//
// If MaxEntries is non-zero and the length of the cache is greater
// after any addition, the least recently used value is evicted.
func (c *Cache[K, V]) Set(key K, value V) {
if c.m == nil {
c.m = make(map[K]*list.Element)
c.ll = list.New()
}
if ee, ok := c.m[key]; ok {
c.ll.MoveToFront(ee)
ee.Value.(*entry[K, V]).value = value
return
}
ele := c.ll.PushFront(&entry[K, V]{key, value})
c.m[key] = ele
if c.MaxEntries != 0 && c.Len() > c.MaxEntries {
c.DeleteOldest()
}
}
// Get looks up a key's value from the cache, returning either
// the value or the zero value if it not present.
//
// If found, key is moved to the front of the LRU.
func (c *Cache[K, V]) Get(key K) V {
v, _ := c.GetOk(key)
return v
}
// Contains reports whether c contains key.
//
// If found, key is moved to the front of the LRU.
func (c *Cache[K, V]) Contains(key K) bool {
_, ok := c.GetOk(key)
return ok
}
// GetOk looks up a key's value from the cache, also reporting
// whether it was present.
//
// If found, key is moved to the front of the LRU.
func (c *Cache[K, V]) GetOk(key K) (value V, ok bool) {
if ele, hit := c.m[key]; hit {
c.ll.MoveToFront(ele)
return ele.Value.(*entry[K, V]).value, true
}
var zero V
return zero, false
}
// Delete removes the provided key from the cache if it was present.
func (c *Cache[K, V]) Delete(key K) {
if e, ok := c.m[key]; ok {
c.deleteElement(e)
}
}
// DeleteOldest removes the item from the cache that was least recently
// accessed. It is a no-op if the cache is empty.
func (c *Cache[K, V]) DeleteOldest() {
if c.ll != nil {
if e := c.ll.Back(); e != nil {
c.deleteElement(e)
}
}
}
func (c *Cache[K, V]) deleteElement(e *list.Element) {
c.ll.Remove(e)
delete(c.m, e.Value.(*entry[K, V]).key)
}
// Len returns the number of items in the cache.
func (c *Cache[K, V]) Len() int { return len(c.m) }