Implement efficient deep copying

protobuf/lib/src/protobuf/extension_field_set.dart

@@ -199,4 +199,8 @@ class _ExtensionFieldSet {
           'use `ExtensionRegistry.reparseMessage`.');
     }
   }
+
+  _ExtensionFieldSet deepCopy() {
+    throw 'TODO';
+  }
 }

protobuf/lib/src/protobuf/field_set.dart

@@ -78,6 +78,9 @@ class _FieldSet {
       : _values = _makeValueList(meta.byIndex.length),
         _oneofCases = meta.oneofs.isEmpty ? null : <int, int>{};
 
+  _FieldSet._(this._message, this._eventPlugin, this._values, this._extensions,
+      this._unknownFields, this._frozenState, this._oneofCases);
+
   static List _makeValueList(int length) {
     if (length == 0) return _zeroList;
     return List.filled(length, null, growable: false);
@@ -892,4 +895,31 @@ class _FieldSet {
 
     _oneofCases?.addAll(original._oneofCases!);
   }
+
+  _FieldSet deepCopy(GeneratedMessage message, {bool freeze = false}) {
+    final values = _makeValueList(_values.length);
+
+    for (var valueIdx = 0; valueIdx < _values.length; valueIdx += 1) {
+      final value = _values[valueIdx];
+      if (value is PbMap) {
+        values[valueIdx] = value.deepCopy(freeze: freeze);
+      } else if (value is PbList) {
+        values[valueIdx] = value.deepCopy(freeze: freeze);
+      } else if (value is GeneratedMessage) {
+        values[valueIdx] = value.deepCopy(freeze: freeze);
+      } else {
+        values[valueIdx] = value;
+      }
+    }
+
+    return _FieldSet._(
+      message,
+      null, // TODO: event plugin
+      values,
+      _extensions?.deepCopy(),
+      _unknownFields?.deepCopy(),
+      _frozenState, // TODO: Not sure about this part
+      _oneofCases == null ? null : Map.from(_oneofCases!),
+    );
+  }
 }

protobuf/lib/src/protobuf/generated_message.dart

@@ -595,5 +595,12 @@ extension GeneratedMessageGenericExtensions<T extends GeneratedMessage> on T {
   }
 
   /// Returns a writable deep copy of this message.
-  T deepCopy() => info_.createEmptyInstance!() as T..mergeFromMessage(this);
+  T deepCopy({bool freeze = false}) {
+    final T message = info_.createEmptyInstance!() as dynamic;
+    final fieldSet = __fieldSet;
+    if (fieldSet != null) {
+      message.__fieldSet = fieldSet.deepCopy(message, freeze: freeze);
+    }
+    return message;
+  }
 }

protobuf/lib/src/protobuf/pb_list.dart

@@ -31,6 +31,8 @@ class PbList<E> extends ListBase<E> {
       : _wrappedList = List<E>.from(from),
         _check = _checkNotNull;
 
+  PbList._(this._wrappedList, this._check, this._isReadOnly);
+
   @override
   void add(E element) {
     _checkModifiable('add');
@@ -200,4 +202,25 @@ class PbList<E> extends ListBase<E> {
   static Never _readOnlyError(String methodName) {
     throw UnsupportedError("'$methodName' on a read-only list");
   }
+
+  PbList<E> deepCopy({bool freeze = false}) {
+    // TODO: We know the capacity, is it possible to allocate a list with the right size?
+    final wrappedList = <E>[];
+
+    for (final value in _wrappedList) {
+      // TODO: I'm not sure why I need to cast to dynamic below. If `value : E`
+      // and `value : PbMap`, then `value.deepCopy : E`
+      if (value is PbMap) {
+        wrappedList.add(value.deepCopy(freeze: freeze) as dynamic);
+      } else if (value is PbList) {
+        wrappedList.add(value.deepCopy(freeze: freeze) as dynamic);
+      } else if (value is GeneratedMessage) {
+        wrappedList.add(value.deepCopy(freeze: freeze) as dynamic);
+      } else {
+        wrappedList.add(value);
+      }
+    }
+
+    return PbList._(wrappedList, _check, freeze);
+  }
 }

protobuf/lib/src/protobuf/pb_map.dart

@@ -33,6 +33,9 @@ class PbMap<K, V> extends MapBase<K, V> {
         _wrappedMap = Map.unmodifiable(other._wrappedMap),
         _isReadonly = other._isReadonly;
 
+  PbMap._(this.keyFieldType, this.valueFieldType, this._wrappedMap,
+      this._isReadonly);
+
   @override
   V? operator [](Object? key) => _wrappedMap[key];
 
@@ -119,4 +122,25 @@ class PbMap<K, V> extends MapBase<K, V> {
     }
     return this;
   }
+
+  PbMap<K, V> deepCopy({bool freeze = false}) {
+    final wrappedMap = <K, V>{};
+
+    for (final entry in _wrappedMap.entries) {
+      final key = entry.key;
+      final value = entry.value;
+
+      if (value is PbMap) {
+        wrappedMap[key] = value.deepCopy(freeze: freeze) as dynamic;
+      } else if (value is PbList) {
+        wrappedMap[key] = value.deepCopy(freeze: freeze) as dynamic;
+      } else if (value is GeneratedMessage) {
+        wrappedMap[key] = value.deepCopy(freeze: freeze) as dynamic;
+      } else {
+        wrappedMap[key] = value;
+      }
+    }
+
+    return PbMap._(keyFieldType, valueFieldType, wrappedMap, freeze);
+  }
 }

protobuf/lib/src/protobuf/unknown_field_set.dart

@@ -195,6 +195,10 @@ class UnknownFieldSet {
       _throwFrozenMessageModificationError('UnknownFieldSet', methodName);
     }
   }
+
+  UnknownFieldSet deepCopy() {
+    throw 'TODO';
+  }
 }
 
 /// An unknown field in a [UnknownFieldSet].