part: Minimize empty Set and Map space usage

Drop the tree when the map or set becomes empty.

Signed-off-by: Jussi Maki <[email protected]>

Author: joamaki

part/map.go

@@ -32,6 +32,10 @@ type mapKVPair[K, V any] struct {
 // This is not implemented as a method on Map[K,V] as hash maps require the
 // comparable constraint and we do not need to limit Map[K, V] to that.
 func FromMap[K comparable, V any](m Map[K, V], hm map[K]V) Map[K, V] {
+	if len(hm) == 0 {
+		return Map[K, V]{}
+	}
+
 	m.ensureTree()
 	txn := m.tree.Txn()
 	for k, v := range hm {
@@ -79,6 +83,9 @@ func (m Map[K, V]) Delete(key K) Map[K, V] {
 		// Map is a struct passed by value, so we can modify
 		// it without changing the caller's view of it.
 		m.tree = txn.CommitOnly()
+		if m.tree.Len() == 0 {
+			m.tree = nil
+		}
 	}
 	return m
 }
@@ -256,6 +263,9 @@ func (m *Map[K, V]) UnmarshalYAML(value *yaml.Node) error {
 	if value.Kind != yaml.SequenceNode {
 		return fmt.Errorf("%T.UnmarshalYAML: expected sequence", m)
 	}
+	if len(value.Content) == 0 {
+		return nil
+	}
 	m.ensureTree()
 	txn := m.tree.Txn()
 	for _, e := range value.Content {

part/set.go

@@ -27,23 +27,29 @@ type Set[T any] struct {
 // NewSet creates a new set of T.
 // The value type T must be registered with RegisterKeyType.
 func NewSet[T any](values ...T) Set[T] {
-	s := Set[T]{tree: New[T](RootOnlyWatch, NoCache)}
-	s.toBytes = lookupKeyType[T]()
-	if len(values) > 0 {
-		txn := s.tree.Txn()
-		for _, v := range values {
-			txn.Insert(s.toBytes(v), v)
-		}
-		s.tree = txn.CommitOnly()
+	if len(values) == 0 {
+		return Set[T]{}
+	}
+	s := Set[T]{}
+	s.ensureTree()
+	txn := s.tree.Txn()
+	for _, v := range values {
+		txn.Insert(s.toBytes(v), v)
 	}
+	s.tree = txn.CommitOnly()
 	return s
 }
 
-// Set a value. Returns a new set. Original is unchanged.
-func (s Set[T]) Set(v T) Set[T] {
+func (s *Set[T]) ensureTree() {
 	if s.tree == nil {
-		return NewSet(v)
+		s.tree = New[T](RootOnlyWatch, NoCache)
 	}
+	s.toBytes = lookupKeyType[T]()
+}
+
+// Set a value. Returns a new set. Original is unchanged.
+func (s Set[T]) Set(v T) Set[T] {
+	s.ensureTree()
 	txn := s.tree.Txn()
 	txn.Insert(s.toBytes(v), v)
 	s.tree = txn.CommitOnly() // As Set is passed by value we can just modify it.
@@ -59,6 +65,9 @@ func (s Set[T]) Delete(v T) Set[T] {
 	txn := s.tree.Txn()
 	txn.Delete(s.toBytes(v))
 	s.tree = txn.CommitOnly()
+	if s.tree.Len() == 0 {
+		s.tree = nil
+	}
 	return s
 }
 
@@ -186,9 +195,8 @@ func (s *Set[T]) UnmarshalJSON(data []byte) error {
 		return fmt.Errorf("%T.UnmarshalJSON: expected '[' got %v", s, t)
 	}
 
-	if s.tree == nil {
-		*s = NewSet[T]()
-	}
+	*s = Set[T]{}
+	s.ensureTree()
 	txn := s.tree.Txn()
 
 	for dec.More() {
@@ -200,6 +208,9 @@ func (s *Set[T]) UnmarshalJSON(data []byte) error {
 		txn.Insert(s.toBytes(x), x)
 	}
 	s.tree = txn.CommitOnly()
+	if s.tree.Len() == 0 {
+		s.tree = nil
+	}
 
 	t, err = dec.Token()
 	if err != nil {
@@ -221,9 +232,8 @@ func (s *Set[T]) UnmarshalYAML(value *yaml.Node) error {
 		return fmt.Errorf("%T.UnmarshalYAML: expected sequence", s)
 	}
 
-	if s.tree == nil {
-		*s = NewSet[T]()
-	}
+	*s = Set[T]{}
+	s.ensureTree()
 	txn := s.tree.Txn()
 
 	for _, e := range value.Content {