1 files changed, 113 insertions, 0 deletions
diff --git a/lib/containers/lrucache.go b/lib/containers/lrucache.go
new file mode 100644
index 0000000..e7a8d62
--- /dev/null
+++ b/lib/containers/lrucache.go
@@ -0,0 +1,113 @@
+// 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)
+}