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// Copyright (C) 2023 Luke Shumaker <lukeshu@lukeshu.com>
//
// SPDX-License-Identifier: GPL-2.0-or-later
package containers
type lruEntry[K comparable, V any] struct {
key K
val V
}
// lruCache is a NON-thread-safe cache with Least Recently Used
// eviction.
//
// lruCache is non-thread-safe and unexported because it only exists
// for internal use by the more sophisticated ARCache.
//
// Similarly, it does not implement some common features of an LRU
// cache, such as specifying a maximum size (instead requiring the
// `.EvictOldest` method to be called), because it is only meant for
// use by the ARCache; which does not need such features.
type lruCache[K comparable, V any] struct {
// OnRemove is (if non-nil) called *after* removal whenever
// an entry is removed, for any reason:
//
// - evicted by .EvictOldest()
// - replaced by .Store(k, v)
// - deleted by .Delete(k)
OnRemove func(K, V)
// OnEvict is (if non-nil) called *after* removal whenever an
// entry is evicted by .EvictOldest(). If both OnEvict and
// OnRemove are set, OnRemove is called first.
OnEvict func(K, V)
byAge LinkedList[lruEntry[K, V]]
byName map[K]*LinkedListEntry[lruEntry[K, V]]
}
func (c *lruCache[K, V]) rem(entry *LinkedListEntry[lruEntry[K, V]]) {
k, v := entry.Value.key, entry.Value.val
delete(c.byName, entry.Value.key)
c.byAge.Delete(entry)
if c.OnRemove != nil {
c.OnRemove(k, v)
}
}
func (c *lruCache[K, V]) evict(entry *LinkedListEntry[lruEntry[K, V]]) {
k, v := entry.Value.key, entry.Value.val
c.rem(entry)
if c.OnEvict != nil {
c.OnEvict(k, v)
}
}
// EvictOldest deletes the oldest entry in the cache.
//
// It is a panic to call EvictOldest if the cache is empty.
func (c *lruCache[K, V]) EvictOldest() {
c.evict(c.byAge.Oldest())
}
// Store a key/value pair in to the cache.
func (c *lruCache[K, V]) Store(k K, v V) {
if c.byName == nil {
c.byName = make(map[K]*LinkedListEntry[lruEntry[K, V]])
} else if old, ok := c.byName[k]; ok {
c.rem(old)
}
c.byName[k] = c.byAge.Store(lruEntry[K, V]{key: k, val: v})
}
// Load an entry from the cache, recording a "use" for the purposes of
// "least-recently-used" eviction.
func (c *lruCache[K, V]) Load(k K) (v V, ok bool) {
entry, ok := c.byName[k]
if !ok {
var zero V
return zero, false
}
c.byAge.MoveToNewest(entry)
return entry.Value.val, true
}
// Peek is like Load, but doesn't count as a "use" for
// "least-recently-used".
func (c *lruCache[K, V]) Peek(k K) (v V, ok bool) {
entry, ok := c.byName[k]
if !ok {
var zero V
return zero, false
}
return entry.Value.val, true
}
// Has returns whether an entry is present in the cache. Calling Has
// does not count as a "use" for "least-recently-used".
func (c *lruCache[K, V]) Has(k K) bool {
_, ok := c.byName[k]
return ok
}
// Delete an entry from the cache.
func (c *lruCache[K, V]) Delete(k K) {
if entry, ok := c.byName[k]; ok {
c.rem(entry)
}
}
// Len returns the number of entries in the cache.
func (c *lruCache[K, V]) Len() int {
return len(c.byName)
}
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