Add ShouldUpdate() function in config (#427)

Add ShouldUpdate() function which allows users to check
whether the value should be updated in the cache on Set.

cache.go

@@ -148,6 +148,16 @@ type Config[K Key, V any] struct {
 	// as well as on rejection of the value.
 	OnExit func(val V)
 
+	// ShouldUpdate is called when a value already exists in cache and is being updated.
+	// If ShouldUpdate returns true, the cache continues with the update (Set). If the
+	// function returns false, no changes are made in the cache. If the value doesn't
+	// already exist, the cache continue with setting that value for the given key.
+	//
+	// In this function, you can check whether the new value is valid. For example, if
+	// your value has timestamp assosicated with it, you could check whether the new
+	// value has the latest timestamp, preventing you from setting an older value.
+	ShouldUpdate func(cur, prev V) bool
+
 	// KeyToHash function is used to customize the key hashing algorithm.
 	// Each key will be hashed using the provided function. If keyToHash value
 	// is not set, the default keyToHash function is used.
@@ -233,6 +243,7 @@ func NewCache[K Key, V any](config *Config[K, V]) (*Cache[K, V], error) {
 		ignoreInternalCost: config.IgnoreInternalCost,
 		cleanupTicker:      time.NewTicker(time.Duration(config.TtlTickerDurationInSec) * time.Second / 2),
 	}
+	cache.storedItems.SetShouldUpdateFn(config.ShouldUpdate)
 	cache.onExit = func(val V) {
 		if config.OnExit != nil {
 			config.OnExit(val)

store.go

@@ -21,6 +21,8 @@ import (
 	"time"
 )
 
+type updateFn[V any] func(cur, prev V) bool
+
 // TODO: Do we need this to be a separate struct from Item?
 type storeItem[V any] struct {
 	key        uint64
@@ -53,6 +55,7 @@ type store[V any] interface {
 	Cleanup(policy *defaultPolicy[V], onEvict func(item *Item[V]))
 	// Clear clears all contents of the store.
 	Clear(onEvict func(item *Item[V]))
+	SetShouldUpdateFn(f updateFn[V])
 }
 
 // newStore returns the default store implementation.
@@ -78,6 +81,12 @@ func newShardedMap[V any]() *shardedMap[V] {
 	return sm
 }
 
+func (m *shardedMap[V]) SetShouldUpdateFn(f updateFn[V]) {
+	for i := range m.shards {
+		m.shards[i].setShouldUpdateFn(f)
+	}
+}
+
 func (sm *shardedMap[V]) Get(key, conflict uint64) (V, bool) {
 	return sm.shards[key%numShards].get(key, conflict)
 }
@@ -116,17 +125,25 @@ func (sm *shardedMap[V]) Clear(onEvict func(item *Item[V])) {
 
 type lockedMap[V any] struct {
 	sync.RWMutex
-	data map[uint64]storeItem[V]
-	em   *expirationMap[V]
+	data         map[uint64]storeItem[V]
+	em           *expirationMap[V]
+	shouldUpdate updateFn[V]
 }
 
 func newLockedMap[V any](em *expirationMap[V]) *lockedMap[V] {
 	return &lockedMap[V]{
 		data: make(map[uint64]storeItem[V]),
 		em:   em,
+		shouldUpdate: func(cur, prev V) bool {
+			return true
+		},
 	}
 }
 
+func (m *lockedMap[V]) setShouldUpdateFn(f updateFn[V]) {
+	m.shouldUpdate = f
+}
+
 func (m *lockedMap[V]) get(key, conflict uint64) (V, bool) {
 	m.RLock()
 	item, ok := m.data[key]
@@ -167,6 +184,9 @@ func (m *lockedMap[V]) Set(i *Item[V]) {
 		if i.Conflict != 0 && (i.Conflict != item.conflict) {
 			return
 		}
+		if m.shouldUpdate != nil && !m.shouldUpdate(i.Value, item.value) {
+			return
+		}
 		m.em.update(i.Key, i.Conflict, item.expiration, i.Expiration)
 	} else {
 		// The value is not in the map already. There's no need to return anything.
@@ -211,6 +231,9 @@ func (m *lockedMap[V]) Update(newItem *Item[V]) (V, bool) {
 	if newItem.Conflict != 0 && (newItem.Conflict != item.conflict) {
 		return zeroValue[V](), false
 	}
+	if m.shouldUpdate != nil && !m.shouldUpdate(newItem.Value, item.value) {
+		return item.value, false
+	}
 
 	m.em.update(newItem.Key, newItem.Conflict, item.expiration, newItem.Expiration)
 	m.data[newItem.Key] = storeItem[V]{

store_test.go

@@ -76,6 +76,29 @@ func TestStoreClear(t *testing.T) {
 	}
 }
 
+func TestShouldUpdate(t *testing.T) {
+	// Create a should update function where the value only increases.
+	s := newStore[int]()
+	s.SetShouldUpdateFn(func(cur, prev int) bool {
+		return cur > prev
+	})
+
+	key, conflict := z.KeyToHash(1)
+	i := Item[int]{
+		Key:      key,
+		Conflict: conflict,
+		Value:    2,
+	}
+	s.Set(&i)
+	i.Value = 1
+	_, ok := s.Update(&i)
+	require.False(t, ok)
+
+	i.Value = 3
+	_, ok = s.Update(&i)
+	require.True(t, ok)
+}
+
 func TestStoreUpdate(t *testing.T) {
 	s := newStore[int]()
 	key, conflict := z.KeyToHash(1)