model: Add wrapWithBacktickFence, to use with quote-and-reply

For the corresponding logic used in the web app and zulip-mobile,
see web/shared/src/fenced_code.ts.

I believe the logic here differs from that in just this way: it
follows the CommonMark spec more closely by disqualifying
backtick-fence lines where the "info string" has a backtick, since
that's not allowed:

> If the info string comes after a backtick fence, it may not
> contain any backtick characters. (The reason for this restriction
> is that otherwise some inline code would be incorrectly
> interpreted as the beginning of a fenced code block.)

Regarding the new file lib/model/compose.dart, we do have existing
code that could reasonably move here, but it's pretty simple. It's
the code that gives the upload-file Markdown; see
registerUploadStart and registerUploadEnd in
[ContentTextEditingController].

Related: #116

lib/model/compose.dart

@@ -0,0 +1,100 @@
+import 'dart:math';
+
+//
+// Put functions for nontrivial message-content generation in this file.
+//
+// If it's complicated enough to need tests, it should go in here.
+//
+
+// https://spec.commonmark.org/0.30/#fenced-code-blocks
+final RegExp _openingBacktickFenceRegex = (() {
+  // Recognize a fence with "up to three spaces of indentation".
+  // Servers don't recognize fences that start with spaces, as of Server 7.0:
+  //   https://chat.zulip.org/#narrow/stream/6-frontend/topic/quote-and-reply.20fence.20length/near/1588273
+  // but that's a bug, since those fences are valid in the spec.
+  // Still, it's harmless to make our own fence longer even if the server
+  // wouldn't notice the internal fence that we're steering clear of,
+  // and if servers *do* start following the spec by noticing indented internal
+  // fences, then this client behavior will be nice.
+  const lineStart = r'^ {0,3}';
+
+  // The backticks, captured so we can see how many.
+  const backticks = r'(`{3,})';
+
+  // The "info string" plus (meaningless) leading or trailing spaces or tabs.
+  // It can't contain backticks.
+  const trailing = r'[^`]*$';
+  return RegExp(lineStart + backticks + trailing, multiLine: true);
+})();
+
+/// The shortest backtick fence that's longer than any in [content].
+///
+/// Expressed as a number of backticks.
+///
+/// Use this for quote-and-reply or anything else that requires wrapping
+/// Markdown in a backtick fence.
+///
+/// See the CommonMark spec, which Zulip servers should but don't always follow:
+///   https://spec.commonmark.org/0.30/#fenced-code-blocks
+int getUnusedBacktickFenceLength(String content) {
+  final matches = _openingBacktickFenceRegex.allMatches(content);
+  int result = 3;
+  for (final match in matches) {
+    result = max(result, match[1]!.length + 1);
+  }
+  return result;
+}
+
+/// Wrap Markdown [content] with opening and closing backtick fences.
+///
+/// For example, for this Markdown:
+///
+///     ```javascript
+///     console.log('Hello world!');
+///     ```
+///
+/// this function, with `infoString: 'quote'`, gives
+///
+///     ````quote
+///     ```javascript
+///     console.log('Hello world!');
+///     ```
+///     ````
+///
+/// See the CommonMark spec, which Zulip servers should but don't always follow:
+///   https://spec.commonmark.org/0.30/#fenced-code-blocks
+// In [content], indented code blocks
+//   ( https://spec.commonmark.org/0.30/#indented-code-blocks )
+// and code blocks fenced with tildes should make no difference to the
+// backtick fences we choose here; this function ignores them.
+String wrapWithBacktickFence({required String content, String? infoString}) {
+  assert(infoString == null || !infoString.contains('`'));
+  assert(infoString == null || infoString.trim() == infoString);
+
+  StringBuffer resultBuffer = StringBuffer();
+
+  // CommonMark doesn't require closing fences to be paired:
+  //   https://github.com/zulip/zulip-flutter/pull/179#discussion_r1228712591
+  //
+  // - We need our opening fence to be long enough that it won't be closed by
+  //   any fence in the content.
+  // - We need our closing fence to be long enough that it will close any
+  //   outstanding opening fences in the content.
+  final fenceLength = getUnusedBacktickFenceLength(content);
+
+  resultBuffer.write('`' * fenceLength);
+  if (infoString != null) {
+    resultBuffer.write(infoString);
+  }
+  resultBuffer.write('\n');
+  resultBuffer.write(content);
+  if (content.isNotEmpty && !content.endsWith('\n')) {
+    resultBuffer.write('\n');
+  }
+  resultBuffer.write('`' * fenceLength);
+  resultBuffer.write('\n');
+  return resultBuffer.toString();
+}
+
+// TODO more, like /near links to messages in conversations
+//   (also to be used in quote-and-reply)

test/model/compose_test.dart

@@ -0,0 +1,218 @@
+import 'package:checks/checks.dart';
+import 'package:test/scaffolding.dart';
+import 'package:zulip/model/compose.dart';
+
+void main() {
+  group('wrapWithBacktickFence', () {
+    /// Check `wrapWithBacktickFence` on example input and expected output.
+    ///
+    /// The intended input (content passed to `wrapWithBacktickFence`)
+    /// is straightforward to infer from `expected`.
+    /// To do that, this helper takes `expected` and removes the opening and
+    /// closing fences.
+    ///
+    /// Then we have the input to the test, as well as the expected output.
+    void checkFenceWrap(String expected, {String? infoString, bool chopNewline = false}) {
+      final re = RegExp(r'^.*?\n(.*\n|).*\n$', dotAll: true);
+      String content = re.firstMatch(expected)![1]!;
+      if (chopNewline) content = content.substring(0, content.length - 1);
+      check(wrapWithBacktickFence(content: content, infoString: infoString)).equals(expected);
+    }
+
+    test('empty content', () {
+      checkFenceWrap('''
+```
+```
+''');
+    });
+
+    test('content consisting of blank lines', () {
+      checkFenceWrap('''
+```
+
+
+
+```
+''');
+    });
+
+    test('single line with no code blocks', () {
+      checkFenceWrap('''
+```
+hello world
+```
+''');
+    });
+
+    test('multiple lines with no code blocks', () {
+      checkFenceWrap('''
+```
+hello
+world
+```
+''');
+    });
+
+    test('no code blocks; incomplete final line', () {
+      checkFenceWrap(chopNewline: true, '''
+```
+hello
+world
+```
+''');
+    });
+
+    test('three-backtick block', () {
+      checkFenceWrap('''
+````
+hello
+```
+code
+```
+world
+````
+''');
+    });
+
+    test('multiple three-backtick blocks; one has info string', () {
+      checkFenceWrap('''
+````
+hello
+```
+code
+```
+world
+```javascript
+// more code
+```
+````
+''');
+    });
+
+    test('whitespace around info string', () {
+      checkFenceWrap('''
+````
+``` javascript 
+// hello world
+```
+````
+''');
+    });
+
+    test('four-backtick block', () {
+      checkFenceWrap('''
+`````
+````
+hello world
+````
+`````
+''');
+    });
+
+    test('five-backtick block', () {
+      checkFenceWrap('''
+``````
+`````
+hello world
+`````
+``````
+''');
+    });
+
+    test('five-backtick block; incomplete final line', () {
+      checkFenceWrap(chopNewline: true, '''
+``````
+`````
+hello world
+`````
+``````
+''');
+    });
+
+    test('three-, four-, and five-backtick blocks', () {
+      checkFenceWrap('''
+``````
+```
+hello world
+```
+
+````
+hello world
+````
+
+`````
+hello world
+`````
+``````
+''');
+    });
+
+    test('dangling opening fence', () {
+      checkFenceWrap('''
+`````
+````javascript
+// hello world
+`````
+''');
+    });
+
+    test('code blocks marked by indentation or tilde fences don\'t affect result', () {
+      checkFenceWrap('''
+```
+    // hello world
+
+~~~~~~
+code
+~~~~~~
+```
+''');
+    });
+
+    test('backtick fences may be indented up to three spaces', () {
+      checkFenceWrap('''
+````
+ ```
+````
+''');
+      checkFenceWrap('''
+````
+  ```
+````
+''');
+      checkFenceWrap('''
+````
+   ```
+````
+''');
+      // but at 4 spaces of indentation it no longer counts:
+      checkFenceWrap('''
+```
+    ```
+```
+''');
+    });
+
+    test('fence ignored if info string has backtick', () {
+      checkFenceWrap('''
+```
+```java`script
+hello
+```
+''');
+    });
+
+    test('with info string', () {
+      checkFenceWrap(infoString: 'info', '''
+`````info
+```
+hello
+```
+info
+````python
+hello
+````
+`````
+''');
+    });
+  });
+}