add expiry policy to S3FIFO

Author: scalalang2

s3fifo/bucket_table.go

@@ -0,0 +1,54 @@
+package s3fifo
+
+import "container/list"
+
+// ghost represents the `ghost` structure in the S3FIFO algorithm.
+// At the time of writing, this implementation is not carefully designed.
+// There can be a better way to implement `ghost`.
+type ghost[K comparable] struct {
+	size  int
+	ll    *list.List
+	items map[K]*list.Element
+}
+
+func newGhost[K comparable](size int) *ghost[K] {
+	return &ghost[K]{
+		size:  size,
+		ll:    list.New(),
+		items: make(map[K]*list.Element),
+	}
+}
+
+func (b *ghost[K]) add(key K) {
+	if _, ok := b.items[key]; ok {
+		return
+	}
+
+	for b.ll.Len() >= b.size {
+		e := b.ll.Back()
+		delete(b.items, e.Value.(K))
+		b.ll.Remove(e)
+	}
+
+	e := b.ll.PushFront(key)
+	b.items[key] = e
+}
+
+func (b *ghost[K]) remove(key K) {
+	if e, ok := b.items[key]; ok {
+		b.ll.Remove(e)
+		delete(b.items, key)
+	}
+}
+
+func (b *ghost[K]) contains(key K) bool {
+	_, ok := b.items[key]
+	return ok
+}
+
+func (b *ghost[K]) clear() {
+	b.ll.Init()
+	for k := range b.items {
+		delete(b.items, k)
+	}
+}