Make the logic for the two forms of Javadoc comments more similar.

core/src/main/java/com/google/googlejavaformat/java/javadoc/JavadocFormatter.java

@@ -14,6 +14,7 @@
 
 package com.google.googlejavaformat.java.javadoc;
 
+import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkState;
 import static com.google.googlejavaformat.java.javadoc.JavadocLexer.lex;
 import static java.util.regex.Pattern.CASE_INSENSITIVE;
@@ -60,6 +61,7 @@
 import com.google.googlejavaformat.java.javadoc.Token.TableCloseTag;
 import com.google.googlejavaformat.java.javadoc.Token.TableOpenTag;
 import com.google.googlejavaformat.java.javadoc.Token.Whitespace;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -90,7 +92,7 @@ public static String formatJavadoc(String input, int blockIndent) {
           default ->
               throw new IllegalArgumentException("Input does not start with /** or ///: " + input);
         };
-    String inputForLexer = classicJavadoc ? input : ("///" + markdownCommentText(input));
+    String inputForLexer = classicJavadoc ? classicCommentText(input) : markdownCommentText(input);
     ImmutableList<Token> tokens;
     try {
       tokens = lex(inputForLexer, classicJavadoc);
@@ -208,6 +210,45 @@ private static boolean oneLineJavadoc(String line, int blockIndent) {
 
   private static final CharMatcher NOT_SPACE_OR_TAB = CharMatcher.noneOf(" \t");
 
+  private static final Pattern CLASSIC_PREFIX_PATTERN = Pattern.compile("^[ \\t]*[*][ \\t]?");
+
+  private static String stripJavadocBeginAndEnd(String input) {
+    checkArgument(input.startsWith("/**"), "Missing /**: %s", input);
+    checkArgument(input.endsWith("*/") && input.length() > 4, "Missing */: %s", input);
+    return input.substring("/**".length(), input.length() - "*/".length());
+  }
+
+  /**
+   * Returns the given classic Javadoc comment after removing the leading ∕✱✱, trailing ✱∕, and any
+   * leading asterisks and common leading whitespace on each line.
+   */
+  private static String classicCommentText(String input) {
+    String stripped = stripJavadocBeginAndEnd(input);
+    List<String> lines = stripped.lines().toList();
+    if (lines.isEmpty()) {
+      // Can't happen: it would only happen for `/***/`, but we filter out comments starting `/***`.
+      return "";
+    }
+    // The first line is handled specially in case we have something like `/** * foo\n * bar\n */`.
+    // The end result should not strip the `*` from `* foo`.
+    List<String> processedLines = new ArrayList<>();
+    processedLines.add(lines.get(0));
+    for (String line : lines.subList(1, lines.size())) {
+      Matcher m = CLASSIC_PREFIX_PATTERN.matcher(line);
+      if (m.find()) {
+        processedLines.add(m.replaceFirst(""));
+      } else {
+        // Input line did not have leading `*`. In that case, it's hard to know what is supposed to
+        // be indentation of the comment as a whole and what is supposed to be indentation of the
+        // content. We just strip all leading whitespace.
+        processedLines.add(line.stripLeading());
+      }
+    }
+    // Unlike Markdown comments, stripping common leading whitespace is not mandated by any
+    // specification. But it's not forbidden either.
+    return stripCommonLeadingWhitespace(processedLines);
+  }
+
   /**
    * Returns the given string with the leading /// and any common leading whitespace removed from
    * each line. The resultant string can then be fed to a standard Markdown parser.
@@ -219,6 +260,10 @@ private static String markdownCommentText(String input) {
             .peek(line -> checkState(line.contains("///"), "Line does not contain ///: %s", line))
             .map(line -> line.substring(line.indexOf("///") + 3))
             .toList();
+    return stripCommonLeadingWhitespace(lines);
+  }
+
+  private static String stripCommonLeadingWhitespace(List<String> lines) {
     int leadingSpace =
         lines.stream()
             .filter(line -> NOT_SPACE_OR_TAB.matchesAnyOf(line))

core/src/main/java/com/google/googlejavaformat/java/javadoc/JavadocLexer.java

@@ -14,7 +14,6 @@
 
 package com.google.googlejavaformat.java.javadoc;
 
-import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Verify.verify;
 import static com.google.common.collect.Iterators.peekingIterator;
@@ -76,16 +75,8 @@ static ImmutableList<Token> lex(String input, boolean classicJavadoc) throws Lex
     input = normalizeLineEndings(input);
     MarkdownPositions markdownPositions;
     if (classicJavadoc) {
-      /*
-       * TODO(cpovirk): In theory, we should interpret Unicode escapes (yet output them in their
-       * original form). This would mean mean everything from an encoded ∕✱✱ to an encoded <pre>
-       * tag, so we'll probably never bother.
-       */
-      input = stripJavadocBeginAndEnd(input);
       markdownPositions = MarkdownPositions.EMPTY;
     } else {
-      checkArgument(input.startsWith("///"));
-      input = input.substring("///".length());
       try {
         markdownPositions = MarkdownPositions.parse(input);
       } catch (UnsupportedOperationException e) {
@@ -104,16 +95,6 @@ private static String normalizeLineEndings(String input) {
 
   private static final Pattern NON_UNIX_LINE_ENDING = Pattern.compile("\r\n?");
 
-  private static String stripJavadocBeginAndEnd(String input) {
-    /*
-     * We do this ahead of time so that the main part of the lexer need not say things like
-     * "(?![*]/)" to avoid accidentally swallowing ✱∕ when consuming a newline.
-     */
-    checkArgument(input.startsWith("/**"), "Missing /**: %s", input);
-    checkArgument(input.endsWith("*/") && input.length() > 4, "Missing */: %s", input);
-    return input.substring("/**".length(), input.length() - "*/".length());
-  }
-
   /**
    * An element of the nested contexts we might be in. For example, if we are inside {@code
    * <pre>{@code ...}</pre>} then the stack of nested contexts would be {@code PRE} plus {@code
@@ -248,8 +229,7 @@ private Token readToken() throws LexException {
   private Function<String, Token> consumeToken() throws LexException {
     boolean preserveExistingFormatting = preserveExistingFormatting();
 
-    Pattern newlinePattern = classicJavadoc ? CLASSIC_NEWLINE_PATTERN : MARKDOWN_NEWLINE_PATTERN;
-    if (input.tryConsumeRegex(newlinePattern)) {
+    if (input.tryConsumeRegex(NEWLINE_PATTERN)) {
       somethingSinceNewline = false;
       return preserveExistingFormatting ? ForcedNewline::new : Whitespace::new;
     }
@@ -687,13 +667,11 @@ static boolean hasMultipleNewlines(String s) {
    * We'd remove the trailing whitespace later on (in JavaCommentsHelper.rewrite), but I feel safer
    * stripping it now: It otherwise might confuse our line-length count, which we use for wrapping.
    */
-  private static final Pattern CLASSIC_NEWLINE_PATTERN = compile("[ \t]*\n[ \t]*[*]?[ \t]?");
   /*
-   * With Traditional comments, the initial space and leading `*` characters (if any) are still
-   * present in the input, but with Markdown comments, the leading `///` characters and shared
-   * initial whitespace have been removed at the point where this pattern is applied.
+   * The leading `///` or `*` characters and shared initial whitespace have been removed at the
+   * point where this pattern is applied.
    */
-  private static final Pattern MARKDOWN_NEWLINE_PATTERN = compile("[ \t]*\n");
+  private static final Pattern NEWLINE_PATTERN = compile("[ \t]*\n");
   private static final Pattern BLOCKQUOTE_MARKER_PATTERN = compile("> ?");
 
   // We ensure elsewhere that we match this only at the beginning of a line.

core/src/main/java/com/google/googlejavaformat/java/javadoc/JavadocWriter.java

@@ -343,7 +343,13 @@ void writeMoeEndStripComment(MoeEndStripComment token) {
   void writeHtmlComment(HtmlComment token) {
     requestNewline();
 
-    writeToken(token);
+    List<String> lines = token.value().lines().toList();
+    writeToken(new HtmlComment(lines.get(0)));
+    for (String line : lines.subList(1, lines.size())) {
+      writeNewline(AutoIndent.NO_AUTO_INDENT);
+      output.append(line);
+      remainingOnLine -= line.length();
+    }
 
     requestNewline();
   }

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

@@ -109,8 +109,6 @@ class Test {}
 
   @Test
   public void commentMostlyUntouched() {
-    // This test isn't necessarily what we'd want to do, but it's what we do now, and it's OK-ish.
-    @SuppressWarnings("MisleadingEscapedSpace") // TODO(b/496180372): remove
     String input =
         """
         /**
@@ -129,17 +127,31 @@ class Test {}\
         /**
          * Foo.
          * <!--
-         *abc
+         * abc
          *   def
          * </tr>
-         *-->
+         * -->
          * bar
          */
         class Test {}
         """;
     doFormatTest(input, expected);
   }
 
+  @Test
+  public void markdownHtmlComment() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// <!--
+        /// abc
+        /// -->
+        class Test {}
+        """;
+    String expected = input;
+    doFormatTest(input, expected);
+  }
+
   @Test
   public void moeComments() {
     // We replace moe by MOE to avoid triggering actual MOE rewriting.
@@ -2207,7 +2219,6 @@ public void markdownBlockQuoteInBlockTag() {
 /// > To marry two wives at one time.
 class Test {}
 """;
-    // TODO(emcmanus): the blank lines here should not be present.
     String expected =
 """
 /// A test class.