Add Support for PreviousTTL

cache.go

@@ -148,6 +148,10 @@ func (c *Cache[K, V]) set(key K, value V, ttl time.Duration) *Item[K, V] {
 		c.evict(EvictionReasonCapacityReached, c.items.lru.Back())
 	}
 
+	if ttl == PreviousOrDefaultTTL {
+		ttl = c.options.ttl
+	}
+
 	// create a new item
 	item := newItem(key, value, ttl, c.options.enableVersionTracking)
 	elem = c.items.lru.PushFront(item)

cache_test.go

@@ -189,6 +189,10 @@ func Test_Cache_set(t *testing.T) {
 			Key: existingKey,
 			TTL: DefaultTTL,
 		},
+		"Set with existing key and PreviousOrDefaultTTL": {
+			Key: existingKey,
+			TTL: PreviousOrDefaultTTL,
+		},
 		"Set with new key and eviction caused by small capacity": {
 			Capacity: 3,
 			Key:      newKey,
@@ -232,6 +236,14 @@ func Test_Cache_set(t *testing.T) {
 			},
 			ExpectFns: true,
 		},
+		"Set with new key and PreviousOrDefaultTTL": {
+			Key: newKey,
+			TTL: PreviousOrDefaultTTL,
+			Metrics: Metrics{
+				Insertions: 1,
+			},
+			ExpectFns: true,
+		},
 	}
 
 	for cn, c := range cc {
@@ -245,6 +257,9 @@ func Test_Cache_set(t *testing.T) {
 				evictionFnsCalls int
 			)
 
+			// calculated based on how addToCache sets ttl
+			existingKeyTTL := time.Hour + time.Minute
+
 			cache := prepCache(time.Hour, evictedKey, existingKey, "test3")
 			cache.options.capacity = c.Capacity
 			cache.options.ttl = time.Minute * 20
@@ -295,6 +310,13 @@ func Test_Cache_set(t *testing.T) {
 				assert.Equal(t, c.TTL, item.ttl)
 				assert.WithinDuration(t, time.Now(), item.expiresAt, c.TTL)
 				assert.Equal(t, c.Key, cache.items.expQueue[0].Value.(*Item[string, string]).key)
+			case c.TTL == PreviousOrDefaultTTL:
+				expectedTTL := cache.options.ttl
+				if c.Key == existingKey {
+					expectedTTL = existingKeyTTL
+				}
+				assert.Equal(t, expectedTTL, item.ttl)
+				assert.WithinDuration(t, time.Now(), item.expiresAt, expectedTTL)
 			default:
 				assert.Equal(t, c.TTL, item.ttl)
 				assert.Zero(t, item.expiresAt)

item.go

@@ -9,6 +9,10 @@ const (
 	// NoTTL indicates that an item should never expire.
 	NoTTL time.Duration = -1
 
+	// PreviousOrDefaultTTL indicates that existing TTL of item should be used
+	// default TTL will be used as fallback if item doesn't exist
+	PreviousOrDefaultTTL time.Duration = -2
+
 	// DefaultTTL indicates that the default TTL value of the cache
 	// instance should be used.
 	DefaultTTL time.Duration = 0
@@ -58,17 +62,23 @@ func (item *Item[K, V]) update(value V, ttl time.Duration) {
 	defer item.mu.Unlock()
 
 	item.value = value
+
+	// update version if enabled
+	if item.version > -1 {
+		item.version++
+	}
+
+	// no need to update ttl or expiry in this case
+	if ttl == PreviousOrDefaultTTL {
+		return
+	}
+
 	item.ttl = ttl
 
 	// reset expiration timestamp because the new TTL may be
 	// 0 or below
 	item.expiresAt = time.Time{}
 	item.touchUnsafe()
-
-	// update version if enabled
-	if item.version > -1 {
-		item.version++
-	}
 }
 
 // touch updates the item's expiration timestamp.

item_test.go

@@ -31,10 +31,16 @@ func Test_Item_update(t *testing.T) {
 	assert.Equal(t, int64(1), item.version)
 	assert.WithinDuration(t, time.Now().Add(time.Hour), item.expiresAt, time.Minute)
 
+	item.update("previous ttl", PreviousOrDefaultTTL)
+	assert.Equal(t, "previous ttl", item.value)
+	assert.Equal(t, time.Hour, item.ttl)
+	assert.Equal(t, int64(2), item.version)
+	assert.WithinDuration(t, time.Now().Add(time.Hour), item.expiresAt, time.Minute)
+
 	item.update("hi", NoTTL)
 	assert.Equal(t, "hi", item.value)
 	assert.Equal(t, NoTTL, item.ttl)
-	assert.Equal(t, int64(2), item.version)
+	assert.Equal(t, int64(3), item.version)
 	assert.Zero(t, item.expiresAt)
 }