WIP

sugar/lib/collection.dart

@@ -21,3 +21,4 @@ export 'src/collection/lists.dart';
 export 'src/collection/maps.dart';
 export 'src/collection/move.dart';
 export 'src/collection/sets.dart';
+export 'src/collection/splay_tree_maps.dart';

sugar/lib/crdt.dart

@@ -0,0 +1,2 @@
+export 'src/crdt/string_index.dart';
+export 'src/crdt/string_index_set.dart';

sugar/lib/src/collection/splay_tree_maps.dart

@@ -0,0 +1,15 @@
+import 'dart:collection';
+
+extension SplayTreeMaps<K, V> on SplayTreeMap<K, V> {
+
+  V? firstValueAfter(K key) => switch (firstKeyAfter(key)) {
+    null => null,
+    final key => this[key],
+  };
+
+  V? lastValueBefore(K key) => switch (lastKeyBefore(key)) {
+    null => null,
+    final key => this[key],
+  };
+
+}

sugar/lib/src/crdt/string_index_set.dart

@@ -0,0 +1,64 @@
+import 'dart:collection';
+
+import 'package:sugar/sugar.dart';
+
+class StringIndexSet<E> extends Iterable<E> {
+
+  /// Whether to strictly validate a string index's format. True by default.
+  final bool strict;
+
+  final SplayTreeMap<String, E> _map;
+  final HashMap<E, String> _inverse;
+  final List<E> _list;
+  final bool Function(E, E) _equals;
+  final int Function(E) _hash;
+
+  StringIndexSet._(
+    this._map,
+    this._inverse,
+    this._list,
+    this._equals,
+    this._hash, {
+    required this.strict,
+  });
+
+
+  void operator []= (Object index, E element) {
+    if (_inverse.containsKey(element)) {
+      return;
+    }
+
+    switch (index) {
+      case final String index:
+        if (_map.firstValueAfter(index) case final after?) {
+          _list.insert(_list.indexWhere((e) => _equals(e, after)), element);
+
+        } else {
+          _list.add(element);
+        }
+
+        _map[index] = element;
+        _inverse[element] = index;
+
+        // Different semantics between map and list
+      case final int index when index < _list.length:
+        final min = _inverse[_list.elementAtOrNull(index - 1)] ?? StringIndex.min;
+        final max = _inverse[_list.elementAtOrNull(index + 1)] ?? StringIndex.max;
+        final string = StringIndex.between(min: min, max: max, strict: strict);
+        _list.insert(index, element);
+
+    }
+  }
+
+
+  @override
+  Iterator<E> get iterator => _list.iterator;
+
+  @override
+  E get first => _list.first;
+
+  set first(E first) {
+
+  }
+
+}

sugar/lib/sugar.dart

@@ -16,6 +16,7 @@ export 'collection_aggregate.dart';
 export 'core.dart';
 export 'core_range.dart';
 export 'core_system.dart';
+export 'crdt.dart';
 export 'math.dart';
 export 'time.dart';
 export 'time_interop.dart';