[Chore] Add pinned weight

builder.go

@@ -91,6 +91,9 @@ func (b *Builder[K, V]) InitialCapacity(initialCapacity int) *Builder[K, V] {
 // of this method requires a corresponding call to MaximumWeight prior to calling Build.
 // Weights are measured and recorded when entries are inserted into or updated in
 // the cache, and are thus effectively static during the lifetime of a cache entry.
+//
+// When the weight of an entry is zero it will not be considered for size-based eviction (though
+// it still may be evicted by other means).
 func (b *Builder[K, V]) Weigher(weigher func(key K, value V) uint32) *Builder[K, V] {
 	b.weigher = weigher
 	return b

cache.go

@@ -61,6 +61,7 @@ func (dc DeletionCause) String() string {
 
 const (
 	minWriteBufferSize uint32 = 4
+	pinnedWeight       uint32 = 0
 )
 
 var (
@@ -436,6 +437,23 @@ func (c *Cache[K, V]) evictNode(n node.Node[K, V]) {
 	}
 }
 
+func (c *Cache[K, V]) addToPolicies(n node.Node[K, V]) {
+	if !n.IsAlive() {
+		return
+	}
+
+	c.expiryPolicy.Add(n)
+	if n.Weight() != pinnedWeight {
+		c.policy.Add(n, c.clock.Offset())
+	}
+}
+
+func (c *Cache[K, V]) deleteFromPolicies(n node.Node[K, V], cause DeletionCause) {
+	c.expiryPolicy.Delete(n)
+	c.policy.Delete(n)
+	c.notifyDeletion(n.Key(), n.Value(), cause)
+}
+
 func (c *Cache[K, V]) onWrite(t task[K, V]) {
 	if t.isClear() || t.isClose() {
 		c.writeBuffer.Clear()
@@ -453,27 +471,16 @@ func (c *Cache[K, V]) onWrite(t task[K, V]) {
 	n := t.node()
 	switch {
 	case t.isAdd():
-		if n.IsAlive() {
-			c.expiryPolicy.Add(n)
-			c.policy.Add(n, c.clock.Offset())
-		}
+		c.addToPolicies(n)
 	case t.isUpdate():
-		oldNode := t.oldNode()
-		c.expiryPolicy.Delete(oldNode)
-		c.policy.Delete(oldNode)
-		if n.IsAlive() {
-			c.expiryPolicy.Add(n)
-			c.policy.Add(n, c.clock.Offset())
-		}
-		c.notifyDeletion(oldNode.Key(), oldNode.Value(), Replaced)
+		c.deleteFromPolicies(t.oldNode(), Replaced)
+		c.addToPolicies(n)
 	case t.isDelete():
-		c.expiryPolicy.Delete(n)
-		c.policy.Delete(n)
-		c.notifyDeletion(n.Key(), n.Value(), Explicit)
+		c.deleteFromPolicies(n, Explicit)
 	case t.isExpired():
-		c.expiryPolicy.Delete(n)
-		c.policy.Delete(n)
-		c.notifyDeletion(n.Key(), n.Value(), Expired)
+		c.deleteFromPolicies(n, Expired)
+	default:
+		panic("invalid task type")
 	}
 }
 

cache_test.go

@@ -87,6 +87,72 @@ func TestCache_Unbounded(t *testing.T) {
 	if m[Replaced] != replaced {
 		t.Fatalf("cache was supposed to replace %d, but replaced %d entries", replaced, m[Replaced])
 	}
+	if hitRatio := statsCounter.Snapshot().HitRatio(); hitRatio != 0.5 {
+		t.Fatalf("not valid hit ratio. expected %.2f, but got %.2f", 0.5, hitRatio)
+	}
+}
+
+func TestCache_PinnedWeight(t *testing.T) {
+	size := 10
+	pinned := 4
+	m := make(map[DeletionCause]int)
+	mutex := sync.Mutex{}
+	c, err := NewBuilder[int, int]().
+		MaximumWeight(uint64(size)).
+		Weigher(func(key int, value int) uint32 {
+			if key == pinned {
+				return pinnedWeight
+			}
+			return 1
+		}).
+		WithTTL(3 * time.Second).
+		DeletionListener(func(key int, value int, cause DeletionCause) {
+			mutex.Lock()
+			m[cause]++
+			mutex.Unlock()
+		}).
+		Build()
+	if err != nil {
+		t.Fatalf("can not create cache: %v", err)
+	}
+
+	for i := 0; i < size; i++ {
+		c.Set(i, i)
+	}
+	for i := 0; i < size; i++ {
+		if !c.Has(i) {
+			t.Fatalf("the key must exist: %d", i)
+		}
+		c.Has(i)
+	}
+	for i := size; i < 2*size; i++ {
+		c.Set(i, i)
+	}
+	time.Sleep(time.Second)
+	for i := size; i < 2*size; i++ {
+		if !c.Has(i) {
+			t.Fatalf("the key must exist: %d", i)
+		}
+		c.Has(i)
+	}
+	if !c.Has(pinned) {
+		t.Fatalf("the key must exist: %d", pinned)
+	}
+
+	time.Sleep(3 * time.Second)
+
+	if c.Has(pinned) {
+		t.Fatalf("the key must not exist: %d", pinned)
+	}
+
+	mutex.Lock()
+	defer mutex.Unlock()
+	if len(m) != 2 || m[Size] != size-1 {
+		t.Fatalf("cache was supposed to evict %d, but evicted %d entries", size-1, m[Size])
+	}
+	if m[Expired] != size+1 {
+		t.Fatalf("cache was supposed to expire %d, but expired %d entries", size+1, m[Expired])
+	}
 }
 
 func TestCache_SetWithWeight(t *testing.T) {