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