message integrity

lib/chat/cubits/reconciliation/author_input_source.dart

@@ -1,10 +1,76 @@
+import 'dart:math';
+
+import 'package:async_tools/async_tools.dart';
+import 'package:fast_immutable_collections/fast_immutable_collections.dart';
+import 'package:meta/meta.dart';
 import 'package:veilid_support/veilid_support.dart';
 
 import '../../../proto/proto.dart' as proto;
 
-class AuthorInputSource {
-  AuthorInputSource({required this.messages, required this.cubit});
-
-  final DHTLogStateData<proto.Message> messages;
-  final DHTLogCubit<proto.Message> cubit;
+@immutable
+class InputWindow {
+  const InputWindow(
+      {required this.elements, required this.first, required this.last});
+  final IList<OnlineElementState<proto.Message>> elements;
+  final int first;
+  final int last;
+}
+
+class AuthorInputSource {
+  AuthorInputSource.fromCubit(
+      {required DHTLogStateData<proto.Message> cubitState,
+      required this.cubit}) {
+    _currentWindow = InputWindow(
+        elements: cubitState.elements,
+        first: cubitState.tail - cubitState.elements.length,
+        last: cubitState.tail - 1);
+  }
+
+  ////////////////////////////////////////////////////////////////////////////
+
+  InputWindow get currentWindow => _currentWindow;
+
+  Future<AsyncValue<bool>> updateWindow(
+          int currentPosition, int windowLength) async =>
+      cubit.operate((reader) async {
+        // See if we're beyond the input source
+        if (currentPosition < 0 || currentPosition >= reader.length) {
+          return const AsyncValue.data(false);
+        }
+
+        // Slide the window if we need to
+        var first = _currentWindow.first;
+        var last = _currentWindow.last;
+        if (currentPosition < first) {
+          // Slide it backward, current position is now last
+          first = max((currentPosition - windowLength) + 1, 0);
+          last = currentPosition;
+        } else if (currentPosition > last) {
+          // Slide it forward, current position is now first
+          first = currentPosition;
+          last = min((currentPosition + windowLength) - 1, reader.length - 1);
+        } else {
+          return const AsyncValue.data(true);
+        }
+
+        // Get another input batch futher back
+        final nextWindow = await cubit.loadElementsFromReader(
+            reader, last + 1, (last + 1) - first);
+        final asErr = nextWindow.asError;
+        if (asErr != null) {
+          return AsyncValue.error(asErr.error, asErr.stackTrace);
+        }
+        final asLoading = nextWindow.asLoading;
+        if (asLoading != null) {
+          return const AsyncValue.loading();
+        }
+        _currentWindow = InputWindow(
+            elements: nextWindow.asData!.value, first: first, last: last);
+        return const AsyncValue.data(true);
+      });
+
+  ////////////////////////////////////////////////////////////////////////////
+  final DHTLogCubit<proto.Message> cubit;
+
+  late InputWindow _currentWindow;
 }