fix: Fix message quote generation

packages/moxxmpp/lib/src/message.dart

@@ -1,3 +1,4 @@
+import 'package:moxlib/moxlib.dart';
 import 'package:moxxmpp/src/events.dart';
 import 'package:moxxmpp/src/jid.dart';
 import 'package:moxxmpp/src/managers/base.dart';
@@ -20,6 +21,7 @@ import 'package:moxxmpp/src/xeps/xep_0444.dart';
 import 'package:moxxmpp/src/xeps/xep_0446.dart';
 import 'package:moxxmpp/src/xeps/xep_0447.dart';
 import 'package:moxxmpp/src/xeps/xep_0448.dart';
+import 'package:moxxmpp/src/xeps/xep_0461.dart';
 
 /// Data used to build a message stanza.
 ///
@@ -140,6 +142,11 @@ class MessageManager extends XmppManagerBase {
   /// element to this id. If originId is non-null, then it will create an "origin-id"
   /// child in the message stanza and set its id to originId.
   void sendMessage(MessageDetails details) {
+    assert(
+      implies(details.quoteBody != null, details.quoteFrom != null && details.quoteId != null),
+      'When quoting a message, then quoteFrom and quoteId must also be non-null',
+    );
+
     final stanza = Stanza.message(
       to: details.to,
       type: 'chat',
@@ -148,11 +155,11 @@ class MessageManager extends XmppManagerBase {
     );
 
     if (details.quoteBody != null) {
-      final fallback = '> ${details.quoteBody!}';
-
+      final quote = QuoteData.fromBodies(details.quoteBody!, details.body!);
+        
       stanza
         ..addChild(
-          XMLNode(tag: 'body', text: '$fallback\n${details.body}'),
+          XMLNode(tag: 'body', text: quote.body),
         )
         ..addChild(
           XMLNode.xmlns(
@@ -176,7 +183,7 @@ class MessageManager extends XmppManagerBase {
                 tag: 'body',
                 attributes: <String, String>{
                   'start': '0',
-                  'end': '${fallback.length}'
+                  'end': '${quote.fallbackLength}',
                 },
               )
             ],

packages/moxxmpp/lib/src/xeps/xep_0461.dart

@@ -1,3 +1,4 @@
+import 'package:meta/meta.dart';
 import 'package:moxxmpp/src/managers/base.dart';
 import 'package:moxxmpp/src/managers/data.dart';
 import 'package:moxxmpp/src/managers/handlers.dart';
@@ -5,6 +6,7 @@ import 'package:moxxmpp/src/managers/namespaces.dart';
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stanza.dart';
 
+/// Data summarizing the XEP-0461 data.
 class ReplyData {
   const ReplyData({
     required this.to,
@@ -12,12 +14,57 @@ class ReplyData {
     this.start,
     this.end,
   });
+
+  /// The bare JID to whom the reply applies to
   final String to;
+
+  /// The stanza ID of the message that is replied to
   final String id;
+
+  /// The start of the fallback body (inclusive)
   final int? start;
+
+  /// The end of the fallback body (exclusive)
   final int? end;
+
+  /// Applies the metadata to the received body [body] in order to remove the fallback.
+  /// If either [ReplyData.start] or [ReplyData.end] are null, then body is returned as
+  /// is.
+  String removeFallback(String body) {
+    if (start == null || end == null) return body;
+
+    return body.replaceRange(start!, end, '');
+  }
 }
 
+/// Internal class describing how to build a message with a quote fallback body.
+@visibleForTesting
+class QuoteData {
+  const QuoteData(this.body, this.fallbackLength);
+
+  /// Takes the body of the message we want to quote [quoteBody] and the content of
+  /// the reply [body] and computes the fallback body and its length.
+  factory QuoteData.fromBodies(String quoteBody, String body) {
+    final fallback = quoteBody
+      .split('\n')
+      .map((line) => '> $line\n')
+      .join();
+
+    return QuoteData(
+      '$fallback$body',
+      fallback.length,
+    );
+  }
+  
+  /// The new body with fallback data at the beginning
+  final String body;
+
+  /// The length of the fallback data
+  final int fallbackLength;
+}
+
+/// A manager implementing support for parsing XEP-0461 metadata. The
+/// MessageRepliesManager itself does not modify the body of the message.
 class MessageRepliesManager extends XmppManagerBase {
   @override
   String getName() => 'MessageRepliesManager';

packages/moxxmpp/test/xeps/xep_0461_test.dart

@@ -0,0 +1,44 @@
+import 'package:moxxmpp/moxxmpp.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Test building a singleline quote', () {
+    final quote = QuoteData.fromBodies('Hallo Welt', 'Hello Earth!');
+
+    expect(quote.body, '> Hallo Welt\nHello Earth!');
+    expect(quote.fallbackLength, 13);
+  });
+
+  test('Test building a multiline quote', () {
+    final quote = QuoteData.fromBodies('Hallo Welt\nHallo Erde', 'How are you?');
+
+    expect(quote.body, '> Hallo Welt\n> Hallo Erde\nHow are you?');
+    expect(quote.fallbackLength, 26);
+  });
+
+  test('Applying a singleline quote', () {
+    final body = '> Hallo Welt\nHello right back!';
+    final reply = ReplyData(
+      to: '',
+      id: '',
+      start: 0,
+      end: 13,
+    );
+
+    final bodyWithoutFallback = reply.removeFallback(body);
+    expect(bodyWithoutFallback, 'Hello right back!');
+  });
+
+  test('Applying a multiline quote', () {
+    final body = "> Hallo Welt\n> How are you?\nI'm fine.\nThank you!";
+    final reply = ReplyData(
+      to: '',
+      id: '',
+      start: 0,
+      end: 28,
+    );
+
+    final bodyWithoutFallback = reply.removeFallback(body);
+    expect(bodyWithoutFallback, "I'm fine.\nThank you!");
+  });
+}