diff --git a/commitlint.yaml b/commitlint.yaml
index ee82dc00..06238476 100644
--- a/commitlint.yaml
+++ b/commitlint.yaml
@@ -26,3 +26,4 @@ rules:
       - neon_lints
       - nextcloud
       - sort_box
+      - synchronize
diff --git a/packages/synchronize/LICENSE b/packages/synchronize/LICENSE
new file mode 120000
index 00000000..30cff740
--- /dev/null
+++ b/packages/synchronize/LICENSE
@@ -0,0 +1 @@
+../../LICENSE
\ No newline at end of file
diff --git a/packages/synchronize/README.md b/packages/synchronize/README.md
new file mode 100644
index 00000000..97032ba0
--- /dev/null
+++ b/packages/synchronize/README.md
@@ -0,0 +1,3 @@
+# synchronize
+
+A simple generic implementation of https://unterwaditzer.net/2016/sync-algorithm.html
diff --git a/packages/synchronize/analysis_options.yaml b/packages/synchronize/analysis_options.yaml
new file mode 100644
index 00000000..4db3c296
--- /dev/null
+++ b/packages/synchronize/analysis_options.yaml
@@ -0,0 +1 @@
+include: package:neon_lints/dart.yaml
diff --git a/packages/synchronize/lib/src/action.dart b/packages/synchronize/lib/src/action.dart
new file mode 100644
index 00000000..31f5ca03
--- /dev/null
+++ b/packages/synchronize/lib/src/action.dart
@@ -0,0 +1,60 @@
+import 'package:meta/meta.dart';
+import 'package:synchronize/src/object.dart';
+
+/// Action to be executed in the sync process.
+@internal
+@immutable
+sealed class SyncAction<T> {
+  /// Creates a new action.
+  const SyncAction(this.object);
+
+  /// The object that is part of the action.
+  final SyncObject<T> object;
+
+  @override
+  String toString() => 'SyncAction<$T>(object: $object)';
+}
+
+/// Action to delete on object from A.
+@internal
+@immutable
+interface class SyncActionDeleteFromA<T1, T2> extends SyncAction<T1> {
+  /// Creates a new action to delete an object from A.
+  const SyncActionDeleteFromA(super.object);
+
+  @override
+  String toString() => 'SyncActionDeleteFromA<$T1, $T2>(object: $object)';
+}
+
+/// Action to delete an object from B.
+@internal
+@immutable
+interface class SyncActionDeleteFromB<T1, T2> extends SyncAction<T2> {
+  /// Creates a new action to delete an object from B.
+  const SyncActionDeleteFromB(super.object);
+
+  @override
+  String toString() => 'SyncActionDeleteFromB<$T1, $T2>(object: $object)';
+}
+
+/// Action to write an object to A.
+@internal
+@immutable
+interface class SyncActionWriteToA<T1, T2> extends SyncAction<T2> {
+  /// Creates a new action to write an object to A.
+  const SyncActionWriteToA(super.object);
+
+  @override
+  String toString() => 'SyncActionWriteToA<$T1, $T2>(object: $object)';
+}
+
+/// Action to write an object to B.
+@internal
+@immutable
+interface class SyncActionWriteToB<T1, T2> extends SyncAction<T1> {
+  /// Creates a new action to write an object to B.
+  const SyncActionWriteToB(super.object);
+
+  @override
+  String toString() => 'SyncActionWriteToB<$T1, $T2>(object: $object)';
+}
diff --git a/packages/synchronize/lib/src/conflict.dart b/packages/synchronize/lib/src/conflict.dart
new file mode 100644
index 00000000..2f024b92
--- /dev/null
+++ b/packages/synchronize/lib/src/conflict.dart
@@ -0,0 +1,61 @@
+import 'package:meta/meta.dart';
+import 'package:synchronize/src/object.dart';
+
+/// Contains information about a conflict that appeared during sync.
+@immutable
+class SyncConflict<T1, T2> {
+  /// Creates a new conflict.
+  const SyncConflict({
+    required this.id,
+    required this.type,
+    required this.objectA,
+    required this.objectB,
+    this.skipped = false,
+  });
+
+  /// Id of the objects involved in the conflict.
+  final String id;
+
+  /// Type of the conflict that appeared. See [SyncConflictType] for more info.
+  final SyncConflictType type;
+
+  /// Object A involved in the conflict.
+  final SyncObject<T1> objectA;
+
+  /// Object B involved in the conflict.
+  final SyncObject<T2> objectB;
+
+  /// Whether the conflict was skipped by the user, useful for ignoring it later on.
+  final bool skipped;
+
+  @override
+  bool operator ==(final dynamic other) => other is SyncConflict && other.id == id;
+
+  @override
+  int get hashCode => id.hashCode;
+
+  @override
+  String toString() =>
+      'SyncConflict<$T1, $T2>(id: $id, type: $type, objectA: $objectA, objectB: $objectB, skipped: $skipped)';
+}
+
+/// Types of conflicts that can appear during sync.
+enum SyncConflictType {
+  /// New objects with the same id exist on both sides.
+  bothNew,
+
+  /// Both objects with the same id have changed.
+  bothChanged,
+}
+
+/// Ways to resolve [SyncConflict]s.
+enum SyncConflictSolution {
+  /// Overwrite the content of object A with the content of object B.
+  overwriteA,
+
+  /// Overwrite the content of object B with the content of object A.
+  overwriteB,
+
+  /// Skip the conflict and just do nothing.
+  skip,
+}
diff --git a/packages/synchronize/lib/src/journal.dart b/packages/synchronize/lib/src/journal.dart
new file mode 100644
index 00000000..c68ed39c
--- /dev/null
+++ b/packages/synchronize/lib/src/journal.dart
@@ -0,0 +1,33 @@
+import 'package:json_annotation/json_annotation.dart';
+import 'package:synchronize/src/journal_entry.dart';
+
+part 'journal.g.dart';
+
+/// Contains the journal.
+///
+/// Used for detecting changes and new or deleted files.
+@JsonSerializable()
+class SyncJournal {
+  /// Creates a new journal.
+  // Note: This must not be const as otherwise the entries are not modifiable when a const set is used!
+  SyncJournal([final Set<SyncJournalEntry>? entries]) : entries = entries ?? {};
+
+  /// Deserializes a journal from [json].
+  factory SyncJournal.fromJson(final Map<String, dynamic> json) => _$SyncJournalFromJson(json);
+
+  /// Serializes a journal to JSON.
+  Map<String, dynamic> toJson() => _$SyncJournalToJson(this);
+
+  /// All entries contained in the journal.
+  final Set<SyncJournalEntry> entries;
+
+  /// Updates an [entry].
+  void updateEntry(final SyncJournalEntry entry) {
+    entries
+      ..remove(entry)
+      ..add(entry);
+  }
+
+  @override
+  String toString() => 'SyncJournal(entries: $entries)';
+}
diff --git a/packages/synchronize/lib/src/journal.g.dart b/packages/synchronize/lib/src/journal.g.dart
new file mode 100644
index 00000000..87a17272
--- /dev/null
+++ b/packages/synchronize/lib/src/journal.g.dart
@@ -0,0 +1,15 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'journal.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+SyncJournal _$SyncJournalFromJson(Map<String, dynamic> json) => SyncJournal(
+      (json['entries'] as List<dynamic>).map((e) => SyncJournalEntry.fromJson(e as Map<String, dynamic>)).toSet(),
+    );
+
+Map<String, dynamic> _$SyncJournalToJson(SyncJournal instance) => <String, dynamic>{
+      'entries': instance.entries.toList(),
+    };
diff --git a/packages/synchronize/lib/src/journal_entry.dart b/packages/synchronize/lib/src/journal_entry.dart
new file mode 100644
index 00000000..e8111420
--- /dev/null
+++ b/packages/synchronize/lib/src/journal_entry.dart
@@ -0,0 +1,52 @@
+import 'package:collection/collection.dart';
+import 'package:json_annotation/json_annotation.dart';
+import 'package:meta/meta.dart';
+import 'package:synchronize/src/journal.dart';
+
+part 'journal_entry.g.dart';
+
+/// Stores a single entry in the [SyncJournal].
+///
+/// It contains an [id] and ETags for each object, [etagA] and [etagB] respectively.
+@immutable
+@JsonSerializable()
+class SyncJournalEntry {
+  /// Creates a new journal entry.
+  const SyncJournalEntry(
+    this.id,
+    this.etagA,
+    this.etagB,
+  );
+
+  /// Deserializes a journal entry from [json].
+  factory SyncJournalEntry.fromJson(final Map<String, dynamic> json) => _$SyncJournalEntryFromJson(json);
+
+  /// Serializes a journal entry to JSON.
+  Map<String, dynamic> toJson() => _$SyncJournalEntryToJson(this);
+
+  /// Unique ID of the journal entry.
+  final String id;
+
+  /// ETag of the object A.
+  final String etagA;
+
+  /// ETag of the object B.
+  final String etagB;
+
+  @override
+  bool operator ==(final Object other) => other is SyncJournalEntry && other.id == id;
+
+  @override
+  int get hashCode => id.hashCode;
+
+  @override
+  String toString() => 'SyncJournalEntry(id: $id, etagA: $etagA, etagB: $etagB)';
+}
+
+/// Extension to find a [SyncJournalEntry].
+extension SyncJournalEntriesFind on Iterable<SyncJournalEntry> {
+  /// Finds the first [SyncJournalEntry] that has the [SyncJournalEntry.id] set to [id].
+  ///
+  /// Returns `null` if no matching [SyncJournalEntry] was found.
+  SyncJournalEntry? tryFind(final String id) => firstWhereOrNull((final entry) => entry.id == id);
+}
diff --git a/packages/synchronize/lib/src/journal_entry.g.dart b/packages/synchronize/lib/src/journal_entry.g.dart
new file mode 100644
index 00000000..67e59dec
--- /dev/null
+++ b/packages/synchronize/lib/src/journal_entry.g.dart
@@ -0,0 +1,19 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'journal_entry.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+SyncJournalEntry _$SyncJournalEntryFromJson(Map<String, dynamic> json) => SyncJournalEntry(
+      json['id'] as String,
+      json['etagA'] as String,
+      json['etagB'] as String,
+    );
+
+Map<String, dynamic> _$SyncJournalEntryToJson(SyncJournalEntry instance) => <String, dynamic>{
+      'id': instance.id,
+      'etagA': instance.etagA,
+      'etagB': instance.etagB,
+    };
diff --git a/packages/synchronize/lib/src/object.dart b/packages/synchronize/lib/src/object.dart
new file mode 100644
index 00000000..bac03aa8
--- /dev/null
+++ b/packages/synchronize/lib/src/object.dart
@@ -0,0 +1,12 @@
+import 'package:collection/collection.dart';
+
+/// Wraps the actual data contained on each side.
+typedef SyncObject<T> = ({String id, T data});
+
+/// Extension to find a [SyncObject].
+extension SyncObjectsFind<T> on Iterable<SyncObject<T>> {
+  /// Finds the first [SyncObject] that has the `id` set to [id].
+  ///
+  /// Returns `null` if no matching [SyncObject] was found.
+  SyncObject<T>? tryFind(final String id) => firstWhereOrNull((final object) => object.id == id);
+}
diff --git a/packages/synchronize/lib/src/sources.dart b/packages/synchronize/lib/src/sources.dart
new file mode 100644
index 00000000..d07f2e7a
--- /dev/null
+++ b/packages/synchronize/lib/src/sources.dart
@@ -0,0 +1,39 @@
+import 'dart:async';
+
+import 'package:meta/meta.dart';
+import 'package:synchronize/src/conflict.dart';
+import 'package:synchronize/src/object.dart';
+
+/// The source the sync uses to sync from and to.
+@immutable
+abstract interface class SyncSource<T1, T2> {
+  /// List all the objects.
+  FutureOr<List<SyncObject<T1>>> listObjects();
+
+  /// Calculates the ETag of a given [object].
+  ///
+  /// Must be something easy to compute like the mtime of a file and preferably not the hash of the whole content in order to be fast.
+  FutureOr<String> getObjectETag(final SyncObject<T1> object);
+
+  /// Writes the given [object].
+  FutureOr<SyncObject<T1>> writeObject(final SyncObject<T2> object);
+
+  /// Deletes the given [object].
+  FutureOr<void> deleteObject(final SyncObject<T1> object);
+}
+
+/// The sources the sync uses to sync from and to.
+@immutable
+abstract interface class SyncSources<T1, T2> {
+  /// Source A.
+  SyncSource<T1, T2> get sourceA;
+
+  /// Source B.
+  SyncSource<T2, T1> get sourceB;
+
+  /// Automatically find a solution for conflicts that don't matter. Useful e.g. for ignoring new directories.
+  SyncConflictSolution? findSolution(final SyncObject<T1> objectA, final SyncObject<T2> objectB);
+
+  @override
+  String toString() => 'SyncSources<$T1, $T2>(sourceA: $sourceA, sourceB: $sourceB)';
+}
diff --git a/packages/synchronize/lib/src/sync.dart b/packages/synchronize/lib/src/sync.dart
new file mode 100644
index 00000000..19f503d3
--- /dev/null
+++ b/packages/synchronize/lib/src/sync.dart
@@ -0,0 +1,246 @@
+import 'package:synchronize/src/action.dart';
+import 'package:synchronize/src/conflict.dart';
+import 'package:synchronize/src/journal.dart';
+import 'package:synchronize/src/journal_entry.dart';
+import 'package:synchronize/src/object.dart';
+import 'package:synchronize/src/sources.dart';
+
+/// Sync between two [SyncSources]s.
+///
+/// This implementation follows https://unterwaditzer.net/2016/sync-algorithm.html in a generic and abstract way
+/// and should work for any two kinds of sources and objects.
+Future<List<SyncConflict<T1, T2>>> sync<T1, T2>(
+  final SyncSources<T1, T2> sources,
+  final SyncJournal journal, {
+  final Map<String, SyncConflictSolution>? conflictSolutions,
+  final bool keepSkipsAsConflicts = false,
+}) async {
+  final diff = await computeSyncDiff<T1, T2>(
+    sources,
+    journal,
+    conflictSolutions: conflictSolutions,
+    keepSkipsAsConflicts: keepSkipsAsConflicts,
+  );
+  await executeSyncDiff<T1, T2>(
+    sources,
+    journal,
+    diff,
+  );
+  return diff.conflicts;
+}
+
+/// Differences between the two sources.
+class SyncDiff<T1, T2> {
+  /// Creates a new diff.
+  SyncDiff(
+    this.actions,
+    this.conflicts,
+  );
+
+  /// Actions required to solve the difference.
+  final List<SyncAction<dynamic>> actions;
+
+  /// Conflicts without solutions that need to be solved.
+  final List<SyncConflict<T1, T2>> conflicts;
+}
+
+/// Executes the actions required to solve the difference.
+Future<void> executeSyncDiff<T1, T2>(
+  final SyncSources<T1, T2> sources,
+  final SyncJournal journal,
+  final SyncDiff<T1, T2> diff,
+) async {
+  for (final action in diff.actions) {
+    switch (action) {
+      case SyncActionDeleteFromA():
+        await sources.sourceA.deleteObject(action.object as SyncObject<T1>);
+        journal.entries.removeWhere((final entry) => entry.id == action.object.id);
+      case SyncActionDeleteFromB():
+        await sources.sourceB.deleteObject(action.object as SyncObject<T2>);
+        journal.entries.removeWhere((final entry) => entry.id == action.object.id);
+      case SyncActionWriteToA():
+        final objectA = await sources.sourceA.writeObject(action.object as SyncObject<T2>);
+        journal.updateEntry(
+          SyncJournalEntry(
+            action.object.id,
+            await sources.sourceA.getObjectETag(objectA),
+            await sources.sourceB.getObjectETag(action.object as SyncObject<T2>),
+          ),
+        );
+      case SyncActionWriteToB():
+        final objectB = await sources.sourceB.writeObject(action.object as SyncObject<T1>);
+        journal.updateEntry(
+          SyncJournalEntry(
+            action.object.id,
+            await sources.sourceA.getObjectETag(action.object as SyncObject<T1>),
+            await sources.sourceB.getObjectETag(objectB),
+          ),
+        );
+    }
+  }
+}
+
+/// Computes the difference, useful for displaying if a sync is up to date.
+Future<SyncDiff<T1, T2>> computeSyncDiff<T1, T2>(
+  final SyncSources<T1, T2> sources,
+  final SyncJournal journal, {
+  final Map<String, SyncConflictSolution>? conflictSolutions,
+  final bool keepSkipsAsConflicts = false,
+}) async {
+  final actions = <SyncAction<dynamic>>[];
+  final conflicts = <SyncConflict<T1, T2>>{};
+  var objectsA = await sources.sourceA.listObjects();
+  var objectsB = await sources.sourceB.listObjects();
+
+  for (final objectA in objectsA) {
+    final objectB = objectsB.tryFind(objectA.id);
+    final journalEntry = journal.entries.tryFind(objectA.id);
+
+    // If the ID exists on side A and the journal, but not on B, it has been deleted on B. Delete it from A and the journal.
+    if (journalEntry != null && objectB == null) {
+      actions.add(SyncActionDeleteFromA<T1, T2>(objectA));
+      continue;
+    }
+
+    // If the ID exists on side A and side B, but not in journal, we can not just create it in journal, since the two items might contain different content each.
+    if (objectB != null && journalEntry == null) {
+      conflicts.add(
+        SyncConflict<T1, T2>(
+          id: objectA.id,
+          type: SyncConflictType.bothNew,
+          objectA: objectA,
+          objectB: objectB,
+        ),
+      );
+      continue;
+    }
+
+    // If the ID exists on side A, but not on B or the journal, it must have been created on A. Copy the item from A to B and also insert it into journal.
+    if (objectB == null || journalEntry == null) {
+      actions.add(SyncActionWriteToB<T1, T2>(objectA));
+      continue;
+    }
+  }
+
+  for (final objectB in objectsB) {
+    final objectA = objectsA.tryFind(objectB.id);
+    final journalEntry = journal.entries.tryFind(objectB.id);
+
+    // If the ID exists on side B and the journal, but not on A, it has been deleted on A. Delete it from B and the journal.
+    if (journalEntry != null && objectA == null) {
+      actions.add(SyncActionDeleteFromB<T1, T2>(objectB));
+      continue;
+    }
+
+    // If the ID exists on side B and side A, but not in journal, we can not just create it in journal, since the two items might contain different content each.
+    if (objectA != null && journalEntry == null) {
+      conflicts.add(
+        SyncConflict<T1, T2>(
+          id: objectA.id,
+          type: SyncConflictType.bothNew,
+          objectA: objectA,
+          objectB: objectB,
+        ),
+      );
+      continue;
+    }
+
+    // If the ID exists on side B, but not on A or the journal, it must have been created on B. Copy the item from B to A and also insert it into journal.
+    if (objectA == null || journalEntry == null) {
+      actions.add(SyncActionWriteToA<T1, T2>(objectB));
+      continue;
+    }
+  }
+
+  objectsA = await sources.sourceA.listObjects();
+  objectsB = await sources.sourceB.listObjects();
+  final entries = journal.entries.toList();
+  for (final entry in entries) {
+    final objectA = objectsA.tryFind(entry.id);
+    final objectB = objectsB.tryFind(entry.id);
+
+    // Remove all entries from journal that don't exist anymore
+    if (objectA == null && objectB == null) {
+      journal.entries.removeWhere((final e) => e.id == entry.id);
+      continue;
+    }
+
+    if (objectA != null && objectB != null) {
+      final changedA = entry.etagA != await sources.sourceA.getObjectETag(objectA);
+      final changedB = entry.etagB != await sources.sourceB.getObjectETag(objectB);
+
+      if (changedA && changedB) {
+        conflicts.add(
+          SyncConflict<T1, T2>(
+            id: objectA.id,
+            type: SyncConflictType.bothChanged,
+            objectA: objectA,
+            objectB: objectB,
+          ),
+        );
+        continue;
+      }
+
+      if (changedA && !changedB) {
+        actions.add(SyncActionWriteToB<T1, T2>(objectA));
+        continue;
+      }
+
+      if (changedB && !changedA) {
+        actions.add(SyncActionWriteToA<T1, T2>(objectB));
+        continue;
+      }
+    }
+  }
+
+  final unsolvedConflicts = <SyncConflict<T1, T2>>[];
+  for (final conflict in conflicts) {
+    final solution = conflictSolutions?[conflict.id] ?? sources.findSolution(conflict.objectA, conflict.objectB);
+    switch (solution) {
+      case SyncConflictSolution.overwriteA:
+        actions.add(SyncActionWriteToA<T1, T2>(conflict.objectB));
+      case SyncConflictSolution.overwriteB:
+        actions.add(SyncActionWriteToB<T1, T2>(conflict.objectA));
+      case SyncConflictSolution.skip:
+        if (keepSkipsAsConflicts) {
+          unsolvedConflicts.add(
+            SyncConflict<T1, T2>(
+              id: conflict.id,
+              type: conflict.type,
+              objectA: conflict.objectA,
+              objectB: conflict.objectB,
+              skipped: true,
+            ),
+          );
+        }
+      case null:
+        unsolvedConflicts.add(conflict);
+    }
+  }
+
+  return SyncDiff<T1, T2>(
+    _sortActions(actions),
+    unsolvedConflicts,
+  );
+}
+
+List<SyncAction<dynamic>> _sortActions(final List<SyncAction<dynamic>> actions) {
+  final addActions = <SyncAction<dynamic>>[];
+  final removeActions = <SyncAction<dynamic>>[];
+  for (final action in actions) {
+    switch (action) {
+      case SyncActionWriteToA():
+        addActions.add(action);
+      case SyncActionWriteToB():
+        addActions.add(action);
+      case SyncActionDeleteFromA():
+        removeActions.add(action);
+      case SyncActionDeleteFromB():
+        removeActions.add(action);
+    }
+  }
+  return _innerSortActions(addActions)..addAll(_innerSortActions(removeActions).reversed);
+}
+
+List<SyncAction<dynamic>> _innerSortActions(final List<SyncAction<dynamic>> actions) =>
+    actions..sort((final a, final b) => a.object.id.compareTo(b.object.id));
diff --git a/packages/synchronize/lib/synchronize.dart b/packages/synchronize/lib/synchronize.dart
new file mode 100644
index 00000000..5b448e17
--- /dev/null
+++ b/packages/synchronize/lib/synchronize.dart
@@ -0,0 +1,6 @@
+export 'package:synchronize/src/conflict.dart';
+export 'package:synchronize/src/journal.dart';
+export 'package:synchronize/src/journal_entry.dart';
+export 'package:synchronize/src/object.dart';
+export 'package:synchronize/src/sources.dart';
+export 'package:synchronize/src/sync.dart';
diff --git a/packages/synchronize/pubspec.yaml b/packages/synchronize/pubspec.yaml
new file mode 100644
index 00000000..0f5a2126
--- /dev/null
+++ b/packages/synchronize/pubspec.yaml
@@ -0,0 +1,20 @@
+name: synchronize
+version: 1.0.0
+
+environment:
+  sdk: '>=3.0.0 <4.0.0'
+
+dependencies:
+  collection: ^1.0.0
+  json_annotation: ^4.8.1
+  meta: ^1.0.0
+
+dev_dependencies:
+  build_runner: ^2.4.6
+  crypto: ^3.0.0
+  json_serializable: ^6.7.1
+  neon_lints:
+    git:
+      url: https://github.com/nextcloud/neon
+      path: packages/neon_lints
+  test: ^1.24.9
diff --git a/packages/synchronize/pubspec_overrides.yaml b/packages/synchronize/pubspec_overrides.yaml
new file mode 100644
index 00000000..4abc9fdc
--- /dev/null
+++ b/packages/synchronize/pubspec_overrides.yaml
@@ -0,0 +1,4 @@
+# melos_managed_dependency_overrides: neon_lints
+dependency_overrides:
+  neon_lints:
+    path: ../neon_lints
diff --git a/packages/synchronize/test/sync_test.dart b/packages/synchronize/test/sync_test.dart
new file mode 100644
index 00000000..b7baf829
--- /dev/null
+++ b/packages/synchronize/test/sync_test.dart
@@ -0,0 +1,542 @@
+import 'dart:convert';
+import 'dart:math';
+
+import 'package:crypto/crypto.dart';
+import 'package:synchronize/synchronize.dart';
+import 'package:test/test.dart';
+
+abstract class Wrap {
+  Wrap(this.content);
+
+  final String content;
+}
+
+class WrapA extends Wrap {
+  WrapA(super.content);
+}
+
+class WrapB extends Wrap {
+  WrapB(super.content);
+}
+
+class TestSyncState {
+  TestSyncState(
+    this.stateA,
+    this.stateB,
+  );
+
+  final Map<String, WrapA> stateA;
+  final Map<String, WrapB> stateB;
+}
+
+class TestSyncSourceA implements SyncSource<WrapA, WrapB> {
+  TestSyncSourceA(this.state);
+
+  final Map<String, WrapA> state;
+
+  @override
+  Future<List<SyncObject<WrapA>>> listObjects() async =>
+      state.keys.map((final key) => (id: key, data: state[key]!)).toList();
+
+  @override
+  Future<String> getObjectETag(final SyncObject<WrapA> object) async => etagA(object.data.content);
+
+  @override
+  Future<SyncObject<WrapA>> writeObject(final SyncObject<WrapB> object) async {
+    final wrap = WrapA(object.data.content);
+    state[object.id] = wrap;
+    return (id: object.id, data: wrap);
+  }
+
+  @override
+  Future<void> deleteObject(final SyncObject<WrapA> object) async => state.remove(object.id);
+}
+
+class TestSyncSourceB implements SyncSource<WrapB, WrapA> {
+  TestSyncSourceB(this.state);
+
+  final Map<String, WrapB> state;
+
+  @override
+  Future<List<SyncObject<WrapB>>> listObjects() async =>
+      state.keys.map((final key) => (id: key, data: state[key]!)).toList();
+
+  @override
+  Future<String> getObjectETag(final SyncObject<WrapB> object) async => etagB(object.data.content);
+
+  @override
+  Future<SyncObject<WrapB>> writeObject(final SyncObject<WrapA> object) async {
+    final wrap = WrapB(object.data.content);
+    state[object.id] = wrap;
+    return (id: object.id, data: wrap);
+  }
+
+  @override
+  Future<void> deleteObject(final SyncObject<WrapB> object) async => state.remove(object.id);
+}
+
+class TestSyncSources implements SyncSources<WrapA, WrapB> {
+  TestSyncSources(
+    this.sourceA,
+    this.sourceB,
+  );
+
+  factory TestSyncSources.fromState(final TestSyncState state) => TestSyncSources(
+        TestSyncSourceA(state.stateA),
+        TestSyncSourceB(state.stateB),
+      );
+
+  @override
+  final SyncSource<WrapA, WrapB> sourceA;
+
+  @override
+  final SyncSource<WrapB, WrapA> sourceB;
+
+  @override
+  SyncConflictSolution? findSolution(final SyncObject<WrapA> objectA, final SyncObject<WrapB> objectB) => null;
+}
+
+String etagA(final String content) => sha1.convert(utf8.encode('A$content')).toString();
+
+String etagB(final String content) => sha1.convert(utf8.encode('B$content')).toString();
+
+String randomEtag() => sha1.convert(utf8.encode(Random().nextDouble().toString())).toString();
+
+Future<void> main() async {
+  group('sync', () {
+    group('stub', () {
+      test('all empty', () async {
+        final state = TestSyncState({}, {});
+        final sources = TestSyncSources.fromState(state);
+        final journal = SyncJournal();
+
+        final conflicts = await sync(sources, journal);
+        expect(conflicts, isEmpty);
+        expect(state.stateA, isEmpty);
+        expect(state.stateB, isEmpty);
+        expect(journal.entries, isEmpty);
+      });
+
+      group('copy', () {
+        group('missing', () {
+          test('to A', () async {
+            const id = '123';
+            const content = '456';
+            final state = TestSyncState(
+              {},
+              {
+                id: WrapB(content),
+              },
+            );
+            final sources = TestSyncSources.fromState(state);
+            final journal = SyncJournal();
+
+            final conflicts = await sync(sources, journal);
+            expect(conflicts, isEmpty);
+            expect(state.stateA, hasLength(1));
+            expect(state.stateA[id]!.content, content);
+            expect(state.stateB, hasLength(1));
+            expect(state.stateB[id]!.content, content);
+            expect(journal.entries, hasLength(1));
+            expect(journal.entries.tryFind(id)!.etagA, etagA(content));
+            expect(journal.entries.tryFind(id)!.etagB, etagB(content));
+          });
+
+          test('to B', () async {
+            const id = '123';
+            const content = '456';
+            final state = TestSyncState(
+              {
+                id: WrapA(content),
+              },
+              {},
+            );
+            final sources = TestSyncSources.fromState(state);
+            final journal = SyncJournal();
+
+            final conflicts = await sync(sources, journal);
+            expect(conflicts, isEmpty);
+            expect(state.stateA, hasLength(1));
+            expect(state.stateA[id]!.content, content);
+            expect(state.stateB, hasLength(1));
+            expect(state.stateB[id]!.content, content);
+            expect(journal.entries, hasLength(1));
+            expect(journal.entries.tryFind(id)!.etagA, etagA(content));
+            expect(journal.entries.tryFind(id)!.etagB, etagB(content));
+          });
+        });
+
+        group('changed', () {
+          test('to A', () async {
+            const id = '123';
+            const contentA = '456';
+            const contentB = '789';
+            final state = TestSyncState(
+              {
+                id: WrapA(contentA),
+              },
+              {
+                id: WrapB(contentB),
+              },
+            );
+            final sources = TestSyncSources.fromState(state);
+            final journal = SyncJournal({
+              SyncJournalEntry(id, etagA(contentA), randomEtag()),
+            });
+
+            final conflicts = await sync(sources, journal);
+            expect(conflicts, isEmpty);
+            expect(state.stateA, hasLength(1));
+            expect(state.stateA[id]!.content, contentB);
+            expect(state.stateB, hasLength(1));
+            expect(state.stateB[id]!.content, contentB);
+            expect(journal.entries, hasLength(1));
+            expect(journal.entries.tryFind(id)!.etagA, etagA(contentB));
+            expect(journal.entries.tryFind(id)!.etagB, etagB(contentB));
+          });
+
+          test('to B', () async {
+            const id = '123';
+            const contentA = '456';
+            const contentB = '789';
+            final state = TestSyncState(
+              {
+                id: WrapA(contentA),
+              },
+              {
+                id: WrapB(contentB),
+              },
+            );
+            final sources = TestSyncSources.fromState(state);
+            final journal = SyncJournal({
+              SyncJournalEntry(id, randomEtag(), etagB(contentB)),
+            });
+
+            final conflicts = await sync(sources, journal);
+            expect(conflicts, isEmpty);
+            expect(state.stateA, hasLength(1));
+            expect(state.stateA[id]!.content, contentA);
+            expect(state.stateB, hasLength(1));
+            expect(state.stateB[id]!.content, contentA);
+            expect(journal.entries, hasLength(1));
+            expect(journal.entries.tryFind(id)!.etagA, etagA(contentA));
+            expect(journal.entries.tryFind(id)!.etagB, etagB(contentA));
+          });
+        });
+      });
+
+      group('delete', () {
+        test('from A', () async {
+          const id = '123';
+          const content = '456';
+          final state = TestSyncState(
+            {
+              id: WrapA(content),
+            },
+            {},
+          );
+          final sources = TestSyncSources.fromState(state);
+          final journal = SyncJournal({
+            SyncJournalEntry(id, etagA(content), etagB(content)),
+          });
+
+          final conflicts = await sync(sources, journal);
+          expect(conflicts, isEmpty);
+          expect(state.stateA, isEmpty);
+          expect(state.stateB, isEmpty);
+          expect(journal.entries, isEmpty);
+        });
+
+        test('from B', () async {
+          const id = '123';
+          const content = '456';
+          final state = TestSyncState(
+            {},
+            {
+              id: WrapB(content),
+            },
+          );
+          final sources = TestSyncSources.fromState(state);
+          final journal = SyncJournal({
+            SyncJournalEntry(id, etagA(content), etagB(content)),
+          });
+
+          final conflicts = await sync(sources, journal);
+          expect(conflicts, isEmpty);
+          expect(state.stateA, isEmpty);
+          expect(state.stateB, isEmpty);
+          expect(journal.entries, isEmpty);
+        });
+
+        test('from journal', () async {
+          const id = '123';
+          const content = '456';
+          final state = TestSyncState({}, {});
+          final sources = TestSyncSources.fromState(state);
+          final journal = SyncJournal({
+            SyncJournalEntry(id, etagA(content), etagB(content)),
+          });
+
+          final conflicts = await sync(sources, journal);
+          expect(conflicts, isEmpty);
+          expect(state.stateA, isEmpty);
+          expect(state.stateB, isEmpty);
+          expect(journal.entries, isEmpty);
+        });
+      });
+
+      group('conflict', () {
+        test('journal missing', () async {
+          const id = '123';
+          const content = '456';
+          final state = TestSyncState(
+            {
+              id: WrapA(content),
+            },
+            {
+              id: WrapB(content),
+            },
+          );
+          final sources = TestSyncSources.fromState(state);
+          final journal = SyncJournal();
+
+          final conflicts = await sync(sources, journal);
+          expect(conflicts, hasLength(1));
+          expect(conflicts[0].type, SyncConflictType.bothNew);
+          expect(state.stateA, hasLength(1));
+          expect(state.stateA[id]!.content, content);
+          expect(state.stateB, hasLength(1));
+          expect(state.stateB[id]!.content, content);
+          expect(journal.entries, isEmpty);
+        });
+
+        test('both changed', () async {
+          const id = '123';
+          const contentA = '456';
+          const contentB = '789';
+          final state = TestSyncState(
+            {
+              id: WrapA(contentA),
+            },
+            {
+              id: WrapB(contentB),
+            },
+          );
+          final sources = TestSyncSources.fromState(state);
+          final journal = SyncJournal({
+            SyncJournalEntry(id, randomEtag(), randomEtag()),
+          });
+
+          final conflicts = await sync(sources, journal);
+          expect(conflicts, hasLength(1));
+          expect(conflicts[0].type, SyncConflictType.bothChanged);
+          expect(state.stateA, hasLength(1));
+          expect(state.stateA[id]!.content, contentA);
+          expect(state.stateB, hasLength(1));
+          expect(state.stateB[id]!.content, contentB);
+          expect(journal.entries, hasLength(1));
+        });
+
+        group('solution', () {
+          group('journal missing', () {
+            test('skip', () async {
+              const id = '123';
+              const contentA = '456';
+              const contentB = '789';
+              final state = TestSyncState(
+                {
+                  id: WrapA(contentA),
+                },
+                {
+                  id: WrapB(contentB),
+                },
+              );
+              final sources = TestSyncSources.fromState(state);
+              final journal = SyncJournal();
+
+              final conflicts = await sync(
+                sources,
+                journal,
+                conflictSolutions: {
+                  id: SyncConflictSolution.skip,
+                },
+              );
+              expect(conflicts, isEmpty);
+              expect(state.stateA, hasLength(1));
+              expect(state.stateA[id]!.content, contentA);
+              expect(state.stateB, hasLength(1));
+              expect(state.stateB[id]!.content, contentB);
+              expect(journal.entries, isEmpty);
+            });
+
+            test('overwrite A', () async {
+              const id = '123';
+              const contentA = '456';
+              const contentB = '789';
+              final state = TestSyncState(
+                {
+                  id: WrapA(contentA),
+                },
+                {
+                  id: WrapB(contentB),
+                },
+              );
+              final sources = TestSyncSources.fromState(state);
+              final journal = SyncJournal();
+
+              final conflicts = await sync(
+                sources,
+                journal,
+                conflictSolutions: {
+                  id: SyncConflictSolution.overwriteA,
+                },
+              );
+              expect(conflicts, isEmpty);
+              expect(state.stateA, hasLength(1));
+              expect(state.stateA[id]!.content, contentB);
+              expect(state.stateB, hasLength(1));
+              expect(state.stateB[id]!.content, contentB);
+              expect(journal.entries, hasLength(1));
+              expect(journal.entries.tryFind(id)!.etagA, etagA(contentB));
+              expect(journal.entries.tryFind(id)!.etagB, etagB(contentB));
+            });
+
+            test('overwrite B', () async {
+              const id = '123';
+              const contentA = '456';
+              const contentB = '789';
+              final state = TestSyncState(
+                {
+                  id: WrapA(contentA),
+                },
+                {
+                  id: WrapB(contentB),
+                },
+              );
+              final sources = TestSyncSources.fromState(state);
+              final journal = SyncJournal();
+
+              final conflicts = await sync(
+                sources,
+                journal,
+                conflictSolutions: {
+                  id: SyncConflictSolution.overwriteB,
+                },
+              );
+              expect(conflicts, isEmpty);
+              expect(state.stateA, hasLength(1));
+              expect(state.stateA[id]!.content, contentA);
+              expect(state.stateB, hasLength(1));
+              expect(state.stateB[id]!.content, contentA);
+              expect(journal.entries, hasLength(1));
+              expect(journal.entries.tryFind(id)!.etagA, etagA(contentA));
+              expect(journal.entries.tryFind(id)!.etagB, etagB(contentA));
+            });
+          });
+
+          group('both changed', () {
+            test('skip', () async {
+              const id = '123';
+              const contentA = '456';
+              const contentB = '789';
+              final state = TestSyncState(
+                {
+                  id: WrapA(contentA),
+                },
+                {
+                  id: WrapB(contentB),
+                },
+              );
+              final sources = TestSyncSources.fromState(state);
+              final journal = SyncJournal({
+                SyncJournalEntry(id, randomEtag(), randomEtag()),
+              });
+
+              final conflicts = await sync(
+                sources,
+                journal,
+                conflictSolutions: {
+                  id: SyncConflictSolution.skip,
+                },
+              );
+              expect(conflicts, isEmpty);
+              expect(state.stateA, hasLength(1));
+              expect(state.stateA[id]!.content, contentA);
+              expect(state.stateB, hasLength(1));
+              expect(state.stateB[id]!.content, contentB);
+              expect(journal.entries, hasLength(1));
+            });
+
+            test('overwrite A', () async {
+              const id = '123';
+              const contentA = '456';
+              const contentB = '789';
+              final state = TestSyncState(
+                {
+                  id: WrapA(contentA),
+                },
+                {
+                  id: WrapB(contentB),
+                },
+              );
+              final sources = TestSyncSources.fromState(state);
+              final journal = SyncJournal({
+                SyncJournalEntry(id, randomEtag(), randomEtag()),
+              });
+
+              final conflicts = await sync(
+                sources,
+                journal,
+                conflictSolutions: {
+                  id: SyncConflictSolution.overwriteA,
+                },
+              );
+              expect(conflicts, isEmpty);
+              expect(state.stateA, hasLength(1));
+              expect(state.stateA[id]!.content, contentB);
+              expect(state.stateB, hasLength(1));
+              expect(state.stateB[id]!.content, contentB);
+              expect(journal.entries, hasLength(1));
+              expect(journal.entries.tryFind(id)!.etagA, etagA(contentB));
+              expect(journal.entries.tryFind(id)!.etagB, etagB(contentB));
+            });
+
+            test('overwrite B', () async {
+              const id = '123';
+              const contentA = '456';
+              const contentB = '789';
+              final state = TestSyncState(
+                {
+                  id: WrapA(contentA),
+                },
+                {
+                  id: WrapB(contentB),
+                },
+              );
+              final sources = TestSyncSources.fromState(state);
+              final journal = SyncJournal({
+                SyncJournalEntry(id, randomEtag(), randomEtag()),
+              });
+
+              final conflicts = await sync(
+                sources,
+                journal,
+                conflictSolutions: {
+                  id: SyncConflictSolution.overwriteB,
+                },
+              );
+              expect(conflicts, isEmpty);
+              expect(state.stateA, hasLength(1));
+              expect(state.stateA[id]!.content, contentA);
+              expect(state.stateB, hasLength(1));
+              expect(state.stateB[id]!.content, contentA);
+              expect(journal.entries, hasLength(1));
+              expect(journal.entries.tryFind(id)!.etagA, etagA(contentA));
+              expect(journal.entries.tryFind(id)!.etagB, etagB(contentA));
+            });
+          });
+        });
+      });
+    });
+  });
+}