Preserve /// markers when wrapping long line comments

When wrapping line comments that start with ///, reuse that marker on
continuation lines instead of hardcoding //. Also treat the full marker
length as non-breakable so long unbreakable tokens (e.g. markdown links)
are left intact rather than splitting after the leading ///.

Fixes #1369

Author: arimu1

core/src/main/java/com/google/googlejavaformat/java/JavaCommentsHelper.java

@@ -145,17 +145,28 @@ private List<String> wrapLineComments(Tok tok, List<String> lines, int column0)
         result.add(line);
         continue;
       }
+      // Preserve the original line-comment marker (`//` or `///`, etc.) on wrapped continuations.
+      // Hardcoding `//` used to inject `//` lines into `///` comments and would also break at the
+      // space after `///`, which mangled long unbreakable tokens such as markdown links
+      // (https://github.com/google/google-java-format/issues/1369).
+      int slashCount = 0;
+      while (slashCount < line.length() && line.charAt(slashCount) == '/') {
+        slashCount++;
+      }
+      // Line comments always start with at least "//".
+      String lineCommentPrefix = line.substring(0, Math.max(slashCount, 2));
+      int prefixLength = lineCommentPrefix.length();
       while (line.length() + column0 > Formatter.MAX_LINE_LENGTH) {
         int idx = Formatter.MAX_LINE_LENGTH - column0;
-        // only break on whitespace characters, and ignore the leading `// `
-        while (idx >= 2 && !CharMatcher.whitespace().matches(line.charAt(idx))) {
+        // only break on whitespace characters, and ignore the leading comment marker
+        while (idx >= prefixLength && !CharMatcher.whitespace().matches(line.charAt(idx))) {
           idx--;
         }
-        if (idx <= 2) {
+        if (idx <= prefixLength) {
           break;
         }
         result.add(line.substring(0, idx));
-        line = "//" + line.substring(idx);
+        line = lineCommentPrefix + line.substring(idx);
       }
       result.add(line);
     }

core/src/test/java/com/google/googlejavaformat/java/FormatterTest.java

@@ -648,6 +648,55 @@ class T {
 """);
   }
 
+  // https://github.com/google/google-java-format/issues/1369
+  @Test
+  public void wrapTripleSlashLineCommentPreservesPrefix() throws Exception {
+    assertThat(
+            new Formatter()
+                .formatSource(
+"""
+class T {
+  void m() {
+    /// one long incredibly unbroken sentence moving from topic to topic so that no-one had a chance to interrupt the speaker at all;
+  }
+}
+"""))
+        .isEqualTo(
+"""
+class T {
+  void m() {
+    /// one long incredibly unbroken sentence moving from topic to topic so that no-one had a chance
+    /// to interrupt the speaker at all;
+  }
+}
+""");
+  }
+
+  // https://github.com/google/google-java-format/issues/1369
+  @Test
+  public void doNotBreakLongUnbreakableTripleSlashLink() throws Exception {
+    assertThat(
+            new Formatter()
+                .formatSource(
+"""
+class T {
+  void m() {
+    /// [Design-doc](8901234567890123456789012345678901234567890123456789012345678901234567890123456789)
+    /// [Design-doc](89012345678901234567890123456789012345678901234567890123456789012345678901234567890)
+  }
+}
+"""))
+        .isEqualTo(
+"""
+class T {
+  void m() {
+    /// [Design-doc](8901234567890123456789012345678901234567890123456789012345678901234567890123456789)
+    /// [Design-doc](89012345678901234567890123456789012345678901234567890123456789012345678901234567890)
+  }
+}
+""");
+  }
+
   @Test
   public void removeTrailingTabsInComments() throws Exception {
     assertThat(

core/src/test/java/com/google/googlejavaformat/java/JavadocFormattingTest.java

@@ -1678,14 +1678,14 @@ <T> T method() {
   }
 
   /// This long line of text looks like a javadoc comment, but is not, because it is separated from
-  // the actual javadoc comment by a plain comment.
+  /// the actual javadoc comment by a plain comment.
   // This is the plain comment.
   /// A third very long line of text, this time a javadoc comment on a field, which again exceeds
   /// the maximum line length.
   String field;
 
   /// A fourth very long line of text, which however is not a javadoc comment so will be wrapped
-  // like a regular // comment.
+  /// like a regular // comment.
 }
 """;
     doFormatTest(input, expected);