added Store.SetFunc method

tools/store/store.go

@@ -136,7 +136,7 @@ func (s *Store[K, T]) Values() []T {
 	return values
 }
 
-// Set sets (or overwrite if already exist) a new value for key.
+// Set sets (or overwrite if already exists) a new value for key.
 func (s *Store[K, T]) Set(key K, value T) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
@@ -148,6 +148,29 @@ func (s *Store[K, T]) Set(key K, value T) {
 	s.data[key] = value
 }
 
+// SetFunc sets (or overwrite if already exists) a new value resolved
+// from the function callback for the provided key.
+//
+// The function callback receives as argument the old store element value (if exists).
+// If there is no old store element, the argument will be the T zero value.
+//
+// Example:
+//
+//	s := store.New[string, int](nil)
+//	s.SetFunc("count", func(old int) int {
+//	    return old + 1
+//	})
+func (s *Store[K, T]) SetFunc(key K, fn func(old T) T) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if s.data == nil {
+		s.data = make(map[K]T)
+	}
+
+	s.data[key] = fn(s.data[key])
+}
+
 // GetOrSet retrieves a single existing value for the provided key
 // or stores a new one if it doesn't exist.
 func (s *Store[K, T]) GetOrSet(key K, setFunc func() T) T {

tools/store/store_test.go

@@ -249,6 +249,32 @@ func TestSet(t *testing.T) {
 	}
 }
 
+func TestSetFunc(t *testing.T) {
+	s := store.Store[string, int]{}
+
+	// non existing value
+	s.SetFunc("test", func(old int) int {
+		if old != 0 {
+			t.Fatalf("Expected old value %d, got %d", 0, old)
+		}
+		return old + 2
+	})
+	if v := s.Get("test"); v != 2 {
+		t.Fatalf("Expected the stored value to be %d, got %d", 2, v)
+	}
+
+	// increment existing value
+	s.SetFunc("test", func(old int) int {
+		if old != 2 {
+			t.Fatalf("Expected old value %d, got %d", 2, old)
+		}
+		return old + 1
+	})
+	if v := s.Get("test"); v != 3 {
+		t.Fatalf("Expected the stored value to be %d, got %d", 3, v)
+	}
+}
+
 func TestGetOrSet(t *testing.T) {
 	s := store.New(map[string]int{
 		"test1": 0,