Markdown spec
=============

Introduction
------------

md-toc aimes to be as conformant as possible to each supported markdown
parser. What follows is a list of parameters and rules used by md-toc to decide
how to parse markdown files and to generate the table of contents.

.. include:: compatibility_table.rst

Supported markdown parsers
--------------------------

- ``cmark``:

  - "CommonMark parsing and rendering library and program in C".

- ``commonmarker``:

  - a "Ruby wrapper for libcmark (CommonMark parser)".

  - as described on their website: "It also includes extensions to
    the CommonMark spec as documented in the GitHub Flavored Markdown spec,
    such as support for tables, strikethroughs, and autolinking.". For this
    reason we assume that ``commonmarker`` is an alias of ``github``.

- ``github``:

  - uses a forked version of ``cmark`` with some added extensions.
    This language specification is called GitHub Flavored Markdown.

  - there are subtle differences such as
    the disallowed raw HTML extension which affects md-toc.

- ``gitlab``:

  - uses ``commonmarker``. Older versions of md-toc, prior to
    version ``3.0.0``, use ``gitlab`` as an alias of ``redcarpet`` while
    newer versions use ``github`` instead. In the past GitLab used
    Redcarpet as markdown parser.

  - some extensions used in GitLab Flavored Markdown, not to be confused
    with GitHub Flavored Markdown, are different from the ones used in GitHub Flavored Markdown.

  - see also

    - https://docs.gitlab.com/ee/development/documentation/styleguide/#documentation-is-the-single-source-of-truth-ssot

- ``goldmark``:

    - this parser claims to be compliant with CommonMark: `goldmark is compliant with CommonMark 0.29.`.
      For this reason ``goldmark`` is an alias of ``cmark``.

- ``redcarpet``:

  - "The safe Markdown parser, reloaded."


Rules
-----

Headers
```````

Only ATX-style headings are supported in md-toc.

- ``cmark``, ``github``, ``gitlab``: the code used in md-toc is a reverse engineering of the
  behavour described in the following:

  - https://spec.commonmark.org/0.29/#atx-heading

  The escape character ``\`` will be left as-is since they are parsed by
  Github's markdown parser already:

  - https://spec.commonmark.org/0.29/#backslash-escapes

  A line ending character is ``U+000A`` or the ``U+000D`` character,
  respectively ``\n`` and ``\r`` or ``\r\n``.
  Everything following those characters is ignored.
  This has also the benefit to automatically remove
  the trailing newline or carriage return at the end of each line. This also
  includes ATX headers with line endings only as main content, such as
  ``#\n`` or ``#\r``. See also:

  - https://spec.commonmark.org/0.29/#line
  - https://spec.commonmark.org/0.29/#line-ending

  Every other rule for ATX headings is applied.

- ``redcarpet``: see license A

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1444
  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1981

  Line endings are generically ``\n`` or ``\r`` characters. See:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L2845
  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L2854

List item rules
```````````````

Problems
^^^^^^^^

We are interested in sublists indentation rules for all types of lists, and
integer overflows in case of ordered lists.

For ordered lists, we are not concerned about using ``0`` or negative numbers
as list markers so these cases will not be considered. Infact ordred lists
generated by md-toc will always start from ``1``.

Talking about indentation rules, I need to mention that the user is responsible
for generating a correct markdown list according to the parser's rules. Let's
see this example:


  ::

     # foo
     ## bar
     ### baz


no problem here because this is rendered by md-toc, using ``github`` as parser,
with:


  ::

     - [foo](#foo)
       - [bar](#bar)
         - [baz](#baz)


Now, let's take the previous example and reverse the order of the lines:


  ::

     ### baz
     ## bar
     # foo


and this is what md-toc renders using ``github``:


  ::


    - [baz](#baz)
    - [foo](#foo)
    - [bar](#bar)


while the user might expect this:


  ::


        - [baz](#baz)
      - [foo](#foo)
    - [bar](#bar)

Indentation
^^^^^^^^^^^

- ``cmark``, ``github``, ``gitlab``: list indentation for sublists with this parser is based on the
  previous state, as stated in the GitHub Flavored Markdown document, at
  section 5.2:

    "The most important thing to notice is that the position of the text after the
    list marker determines how much indentation is needed in subsequent blocks in
    the list item. If the list marker takes up two spaces, and there are three
    spaces between the list marker and the next non-whitespace character, then
    blocks must be indented five spaces in order to fall under the list item."

  - https://github.github.com/gfm/#list-items

  This is also true with the specular case: if our new list element needs less
  indentation than the one processed currently, we have to use the same number
  of indentation spaces used somewhere earlier in the list.

- ``redcarpet``:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1553
  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1528

  The following C function returns the first non-whitespace character
  after the list marker. The value of ``0`` is returned if the input
  line is not a list element. List item rules are explained in the
  single line comments.

  .. highlight:: c

  ::


      /* prefix_uli • returns unordered list item prefix */
      static size_t
      prefix_uli(uint8_t *data, size_t size)
      {
          size_t i = 0;

          // There can be up to 3 whitespaces before the list marker.
          if (i < size && data[i] == ' ') i++;
          if (i < size && data[i] == ' ') i++;
          if (i < size && data[i] == ' ') i++;

          // The next non-whitespace character must be a list marker and
          // the character after the list marker must be a whitespace.
          if (i + 1 >= size ||
             (data[i] != '*' && data[i] != '+' && data[i] != '-') ||
              data[i + 1] != ' ')
              return 0;

          // Check that the next line is not a header
          // that uses the `-` or `=` characters as markers.
          if (is_next_headerline(data + i, size - i))
              return 0;

          // Return the first non whitespace character after the list marker.
          return i + 2;
      }


  As far as I can tell from the previous and other functions, on a new list
  block the 4 spaces indentation rule applies:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1822
  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1873

  This means that anything that has more than 3 whitespaces is considered as
  sublist. The only exception seems to be for the first sublist in a list
  block, in which that case even a single whitespace counts as a sublist.
  The 4 spaces indentation rule appllies nontheless, so to keep things simple
  md-toc will always use 4 whitespaces for sublists. Apparently, ordered and
  unordered lists share the same proprieties.

  Let's see this example:


  ::


      - I
       - am
           - foo

      stop

      - I
          - am
              - foo


  This is how redcarpet renders it once you run ``$ redcarpet``:


   ::


      <ul>
      <li>I

      <ul>
      <li>am

      <ul>
      <li>foo</li>
      </ul></li>
      </ul></li>
      </ul>

      <p>stop</p>

      <ul>
      <li>I

      <ul>
      <li>am

      <ul>
      <li>foo</li>
      </ul></li>
      </ul></li>
      </ul>


  What follows is an extract of a C function in redcarpet that parses list
  items. I have added all the single line comments.


  .. highlight:: c


  ::


        /* parse_listitem • parsing of a single list item */
        /*  assuming initial prefix is already removed */
        static size_t
        parse_listitem(struct buf *ob, struct sd_markdown *rndr, uint8_t *data,
        size_t size, int *flags)
        {
            struct buf *work = 0, *inter = 0;
            size_t beg = 0, end, pre, sublist = 0, orgpre = 0, i;
            int in_empty = 0, has_inside_empty = 0, in_fence = 0;

            // This is the base case, usually of indentation 0 but it can be
            // from 0 to 3 spaces. If it was 4 spaces it would be a code
            // block.
            /* keeping track of the first indentation prefix */
            while (orgpre < 3 && orgpre < size && data[orgpre] == ' ')
                orgpre++;

            // Get the first index of string after the list marker. Try both
            // ordered and unordered lists
            beg = prefix_uli(data, size);
            if (!beg)
                beg = prefix_oli(data, size);

            if (!beg)
                return 0;

            /* skipping to the beginning of the following line */
            end = beg;
            while (end < size && data[end - 1] != '\n')
                end++;

            // Iterate line by line using the '\n' character as delimiter.
            /* process the following lines */
            while (beg < size) {
                size_t has_next_uli = 0, has_next_oli = 0;

                // Go to the next line.
                end++;

                // Find the end of the line.
                while (end < size && data[end - 1] != '\n')
                    end++;

                // Skip the next line if it is empty.
                /* process an empty line */
                if (is_empty(data + beg, end - beg)) {
                    in_empty = 1;
                    beg = end;
                    continue;
                }

                // Count up to 4 characters of indentation.
                // If we have 4 characters then it might be a sublist.
                // Note that this is an offset and does not point to an
                // index in the actual line string.
                /* calculating the indentation */
                i = 0;
                while (i < 4 && beg + i < end && data[beg + i] == ' ')
                    i++;

                pre = i;

                /* Only check for new list items if we are **not** inside
                 * a fenced code block */
                 if (!in_fence) {
                   has_next_uli = prefix_uli(data + beg + i, end - beg - i);
                   has_next_oli = prefix_oli(data + beg + i, end - beg - i);
                }

                /* checking for ul/ol switch */
                if (in_empty && (
                    ((*flags & MKD_LIST_ORDERED) && has_next_uli) ||
                    (!(*flags & MKD_LIST_ORDERED) && has_next_oli))){
                    *flags |= MKD_LI_END;
                    break; /* the following item must have same list type */
                }

                // Determine if we are dealing with:
                // - an empty line
                // - a new list item
                // - a sublist
                /* checking for a new item */
                if ((has_next_uli && !is_hrule(data + beg + i, end - beg - i)) || has_next_oli) {
                    if (in_empty)
                        has_inside_empty = 1;

                    // The next list item's indentation (pre) must be the same as
                    // the previous one (orgpre), otherwise it might be a
                    // sublist.
                    if (pre == orgpre) /* the following item must have */
                        break;             /* the same indentation */

                    // If the indentation does not match the previous one then
                    // assume that it is a sublist. Check later whether it is
                    // or not.
                    if (!sublist)
                        sublist = work->size;
                }
                /* joining only indented stuff after empty lines */
                else if (in_empty && i < 4 && data[beg] != '\t') {
                    *flags |= MKD_LI_END;
                    break;
                }
                else if (in_empty) {
                    // Add a line delimiter to the next line if it is missing.
                    bufputc(work, '\n');
                    has_inside_empty = 1;
                }

                in_empty = 0;
                beg = end;
            }

            if (*flags & MKD_LI_BLOCK) {
                /* intermediate render of block li */
                if (sublist && sublist < work->size) {
                    parse_block(inter, rndr, work->data, sublist);
                    parse_block(inter, rndr, work->data + sublist, work->size - sublist);
            }
            else
                parse_block(inter, rndr, work->data, work->size);
        }


  According to the code, ``parse_listitem`` is called indirectly by
  ``parse_block`` (via ``parse_list``), but ``parse_block`` is called directly
  by ``parse_listitem`` so the code analysis
  is not trivial. For this reason I might be mistaken about the 4 spaces
  indentation rule.

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L2418
  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1958

  Here is an extract of the ``parse_block`` function with the calls to
  ``parse_list``:

  .. highlight:: c

  ::


      /* parse_block • parsing of one block, returning next uint8_t to parse */
      static void
      parse_block(struct buf *ob, struct sd_markdown *rndr, uint8_t *data, size_t
      size)
      {
          while (beg < size) {

              else if (prefix_uli(txt_data, end))
                beg += parse_list(ob, rndr, txt_data, end, 0);

              else if (prefix_oli(txt_data, end))
                beg += parse_list(ob, rndr, txt_data, end, MKD_LIST_ORDERED);
          }
      }


Overflows
^^^^^^^^^

- ``cmark``, ``github``, ``gitlab``: ordered list markers cannot exceed ``99999999`` according to
  the following. If that is the case then a  ``GithubOverflowOrderedListMarker``
  exception is raised:

  - https://spec.commonmark.org/0.29/#ordered-list-marker

- ``redcarpet``: apparently there are no cases of ordered list marker
  overflows:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1529

Notes on ordered lists
^^^^^^^^^^^^^^^^^^^^^^

- ``cmark``, ``github``, ``gitlab``: ordered list markers may start with any integer (except special cases).
  any following number is ignored and subsequent numeration is progressive:

  - https://spec.commonmark.org/0.29/#start-number

  However, when you try this in practice this is not always true: nested lists
  do not follow the specifications. See:

  - https://github.com/frnmst/md-toc/issues/23

  Markers cannot be negative:

  - https://spec.commonmark.org/0.29/#example-239

- ``redcarpet``: ordered lists do not use the ``start`` HTML attribute:
  any number is ignored and lists starts from 1. See:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/test/MarkdownTest_1.0/Tests/Markdown%20Documentation%20-%20Syntax.html#L323

Link label
``````````

If the user decides to generate the table of contents with the anchor links,
then link label rules will be applied.

- ``cmark``, ``github``, ``gitlab``:

  - https://spec.commonmark.org/0.29/#link-label

  If a line ends in 1 or more '\' characters, this disrupts the anchor
  title. For example ``- [xdmdmsdm\](#xdmdmsdm)`` becomes
  ``<ul><li>[xdmdmsdm](#xdmdmsdm)</li></ul>`` instead of
  ``<ul><li><a href="xdmdmsdm">xdmdmsdm\</a></li></ul>``.
  The workaround used in md-toc is to add a space character at the end of the
  string, so it becomes: ``<ul><li><a href="xdmdmsdm">xdmdmsdm\ </a></li></ul>``

  If the link label contains only whitespace characters a ``GithubEmptyLinkLabel``
  exception is raised.

  If the number of characters inside the link label is over 999 a
  ``GithubOverflowCharsLinkLabel`` is raised.

  If the headers contains ``[`` or ``]``, these characters
  are treated with the following rules.

  - https://spec.commonmark.org/0.29/#link-text
  - https://spec.commonmark.org/0.29/#link-destination

  According to a function in the source code, balanced square brackets do not
  work, however they do when interpeted by the web interface. It is however
  possible that they are supported within the ``handle_close_bracket``
  function.

  - https://github.com/github/cmark/blob/6b101e33ba1637e294076c46c69cd6a262c7539f/src/inlines.c#L881
  - https://github.com/github/cmark/blob/6b101e33ba1637e294076c46c69cd6a262c7539f/src/inlines.c#L994


  Here is the original C function with some more comments added:

  .. highlight:: c

  ::

        // Parse a link label.  Returns 1 if successful.
        // Note:  unescaped brackets are not allowed in labels.
        // The label begins with `[` and ends with the first `]` character
        // encountered.  Backticks in labels do not start code spans.
        static int link_label(subject *subj, cmark_chunk *raw_label) {
          bufsize_t startpos = subj->pos;
          int length = 0;
          unsigned char c;

          // advance past [
          //
          // Ignore the open link label identifier
          // peek_char simply returns the current char if we are
          // in range of the string, 0 otherwise.
          if (peek_char(subj) == '[') {
            advance(subj);
          } else {
            return 0;
          }

          while ((c = peek_char(subj)) && c != '[' && c != ']') {
            // If there is an escape and the next character is (for example)
            // '[' or ']' then,
            // ignore the loop conditions.
            // If there are nested balanced square brakets this loop ends.
            if (c == '\\') {
              advance(subj);
              length++;

              // Puntuation characters are the ones defined at:
              // https://github.github.com/gfm/#ascii-punctuation-character
              if (cmark_ispunct(peek_char(subj))) {
                advance(subj);
                length++;
              }
            } else {
              advance(subj);
              length++;
            }
            // MAX_LINK_LABEL_LENGTH is a constant defined at
            // https://github.com/github/cmark/blob/master/src/parser.h#L13
            if (length > MAX_LINK_LABEL_LENGTH) {
              goto noMatch;
            }
          }

          // If the loop terminates when the current character is ']' then
          // everything between '[' and ']' is the link label...
          if (c == ']') { // match found
            *raw_label =
                cmark_chunk_dup(&subj->input, startpos + 1, subj->pos - (startpos + 1));
            cmark_chunk_trim(raw_label);
            advance(subj); // advance past ]
            return 1;
          }

        // ...otherwise return error.
        // This label always get executed according to C rules.
        noMatch:
          subj->pos = startpos; // rewind
          return 0;
        }


  For simpleness the escape ``[`` and ``]`` rule is used.


- ``redcarpet``:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L998

  Let's inspect this loop:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1017

  .. highlight:: c


  ::


        /* looking for the matching closing bracket */
        for (level = 1; i < size; i++) {
            if (data[i] == '\n')
                text_has_nl = 1;

            else if (data[i - 1] == '\\')
                continue;

            else if (data[i] == '[')
                level++;

            else if (data[i] == ']') {
                level--;
                if (level <= 0)
                    break;
            }
        }

        if (i >= size)
            goto cleanup;


  The cleanup label looks like this:

  .. highlight:: c


  ::


            /* cleanup */
            cleanup:
                rndr->work_bufs[BUFFER_SPAN].size = (int)org_work_size;
                return ret ? i : 0;


  .. highlight:: python

  An example: ``[test \](test \)`` becomes ``[test ](test )`` instead of
  ``<a href="test \">test \</a>``

  Infact, you can see that if the current character is ``\\`` then the the
  current iteration is skipped. If for any chance the next character is ``]``
  then the inline link closing parenthesis detection is ignored. ``i`` becomes
  equal to ``size`` eventually and so we jump to the ``cleanup`` label.
  That lable contains a return statement so that string is not treated as
  inline link anymore. A similar code is implemented also for
  detecting ``(`` and ``)``. See:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1088
  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/markdown.c#L1099

  To solve this we use the same workaround used for ``cmark``, ``github``, ``gitlab``.


Anchor link types and behaviours
````````````````````````````````

- ``cmark``, ``github``: a translated version of the Ruby algorithm is used in md-toc.
  The original one is repored here:

  - https://github.com/jch/html-pipeline/blob/master/lib/html/pipeline/toc_filter.rb

  I could not find the code directly responsable for the anchor link generation.
  See also:

  - https://github.github.com/gfm/
  - https://githubengineering.com/a-formal-spec-for-github-markdown/
  - https://github.com/github/cmark/issues/65#issuecomment-343433978

  See license B.

  Apparently GitHub (and possibly others) filter HTML tags in the anchor links.
  This is an undocumented feature (?) so the ``remove_html_tags`` function was
  added to address this problem. Instead of designing an algorithm to detect HTML tags,
  regular expressions came in handy. All the rules
  present in https://spec.commonmark.org/0.28/#raw-html have been followed by the
  letter. Regular expressions are divided by type and are composed at the end
  by concatenating all the strings. For example:


  ::


        # Comment start.
        COS = '<!--'
        # Comment text.
        COT = '((?!>|->)(?:(?!--).))+(?!-).?'
        # Comment end.
        COE = '-->'
        # Comment.
        CO = COS + COT + COE


  HTML tags are stripped using the ``re.sub`` replace function, for example:


  ::


       line = re.sub(CO, str(), line, flags=re.DOTALL)


  GitHub added an extension in GFM to ignore certain HTML tags, valid at least from versions `0.27.1.gfm.3` to `0.29.0.gfm.0`:

  - https://github.github.com/gfm/#disallowed-raw-html-extension-
  - https://github.com/github/cmark-gfm/blob/fca380ca85c046233c39523717073153e2458c1e/extensions/tagfilter.c

- ``gitlab``: new rules have been written:

  - https://docs.gitlab.com/ee/user/markdown.html#header-ids-and-links

- ``redcarpet``: treats consecutive dash characters by tranforming them
  into a single dash character. A translated version of the C algorithm
  is used in md-toc. The original version is here:

  - https://github.com/vmg/redcarpet/blob/6270d6b4ab6b46ee6bb57a6c0e4b2377c01780ae/ext/redcarpet/html.c#L274

  See license A.

  See also:

  - https://github.com/vmg/redcarpet/issues/618#issuecomment-306476184
  - https://github.com/vmg/redcarpet/issues/307#issuecomment-261793668

Emphasis
^^^^^^^^

To be able to have working anchor links, emphasis must also be removed from the
link destination.

- ``cmark``,  ``github``, ``gitlab``: At the moment the implementation of the removal is incomplete
  because of its complexity. See:

  - https://spec.commonmark.org/0.29/#emphasis-and-strong-emphasis

  The core functions for this feature have been translated directly
  from the original cmark source in C to Python, with some differences:

  1. the ``cmark_utf8proc_charlen`` uses ``length = 1``
     instead of:

     - ``length = cmark_utf8proc_char_len(line[0])``
     - ``length = utf8proc_utf8class[ord(line)]`` (causes list overflow).

     This is what the ``cmark_utf8proc_char_len`` function should look like in md-toc,
     which is taken from ``cmark_utf8proc_encode_char`` in the original source:

     ::

        def cmark_utf8proc_char_len(char: str) -> int:
            # Taken from the cmark_utf8proc_encode_char function
            length = 0
            uc = ord(char)

            assert (uc >= 0)

            if uc < 0x80:
                length = 1
            elif uc < 0x800:
                length = 2
            elif uc == 0xFFFF:
                length = 1
            elif uc == 0xFFFE:
                length = 1
            elif uc < 0x10000:
                length = 3
            elif uc < 0x110000:
                length = 4
            else:
                raise ValueError

            return length

     In Python 3, since all characters are UTF-8 by default, they are all
     represented with length 1. See:

     - https://rosettacode.org/wiki/String_length#Python
     - https://docs.python.org/3/howto/unicode.html#comparing-strings

  The licenses used for all functions with name starting with ``_cmark`` are
  licenses C, D and E.

Code fence
``````````

Code fences are sections of a markdown document where some parsers treat the
text within them as verbatim. Usually the purpose of these sections is to
display source code. Some programming languages use the character ``#`` as a
way to comment a line in the code. For this reason md-toc needs to ignore code
fences in order not to treat the ``#`` character as an ATX-style heading and thus
get parsed as an element of the TOC.

- ``cmark``, ``github``, ``gitlab``: the rules followed are the ones reported on the
  documentation:

  - https://spec.commonmark.org/0.29/#code-fence

- ``redcarpet``: needs to be implemented:

  - https://github.com/vmg/redcarpet/blob/26c80f05e774b31cd01255b0fa62e883ac185bf3/ext/redcarpet/markdown.c#L1389

TOC marker
``````````

A TOC marker is a string that marks that the start and the end of the table
of contents in a markdown file.

By default it was decided to use ``[](TOC)`` as the default TOC marker because
it would result invisible in some markdown parsers. In other cases, however, such
as the one used by Gitea, that particular TOC marker was still visible. HTML
comments seem to be a better solution.

- ``cmark``, ``github``, ``gitlab``:

  - https://spec.commonmark.org/0.29/#html-comment

- ``redcarpet``:

  I cannot find the corresponding code, but I found this:

  - https://github.com/vmg/redcarpet/blob/master/test/MarkdownTest_1.0.3/Tests/Inline%20HTML%20comments.html

Other markdown parsers
----------------------

If you have a look at
https://www.w3.org/community/markdown/wiki/MarkdownImplementations
you will see that there are a ton of different markdown parsers out there.
Moreover, that list has not been updated in a while.

Markdown parsers have different behaviours regarding anchor links. Some of them
implement them while others don't; some act on the duplicate entry problem
while others don't; some strip consecutive dash characters while others don't.
And it's not just about anchor links, as you have read earlier. For example:

- Gitea apparently uses ``goldmark`` as markdown parser. This parser claims
  to be compliant with CommonMark: *goldmark is compliant with CommonMark 0.29.*.
  See:

  - https://github.com/go-gitea/gitea
  - https://github.com/yuin/goldmark
  - https://github.com/go-gitea/gitea/blob/71aca93decc10253133dcd77b64dae5d311d7163/modules/markup/markdown/goldmark.go

  Gitea adds an annoying ``user-content`` substring in the TOC's anchor links. This is true for versions (git tags):

  - v1.13.7
  - v1.13.6
  - v1.13.5
  - v1.13.4
  - v1.13.3
  - v1.13.2
  - v1.13.1
  - v1.13.0
  - v1.12.6
  - v1.12.5
  - v1.12.4
  - v1.12.3
  - v1.12.2
  - v1.12.1
  - v1.11.8
  - v1.12.0
  - v1.11.8
  - v1.11.7
  - v1.11.6
  - v1.11.5
  - v1.11.4
  - v1.11.3
  - v1.11.2
  - v1.11.1
  - v1.11.0

  See:

  - https://github.com/go-gitea/gitea/blob/71aca93decc10253133dcd77b64dae5d311d7163/modules/markup/markdown/goldmark.go#L230
  - https://github.com/go-gitea/gitea/issues/12062
  - https://github.com/go-gitea/gitea/pull/11903
  - https://github.com/go-gitea/gitea/pull/12805

  The ``user-content`` substring does not seem to affect the functionality of the TOC.

  Older versions of Gitea used blackfriday. See:

  - https://github.com/go-gitea/gitea/blob/2a03e96bceadfcc5e18bd61e755980ee72dcdb15/modules/markup/markdown/markdown.go

- Gogs uses Marked as the markdown parser:

  - https://gogs.io/docs
  - https://github.com/chjj/marked
  - https://github.com/chjj/marked/issues/981
  - https://github.com/chjj/marked/search?q=anchor&type=Issues&utf8=%E2%9C%93

- Notabug: *Notabug is powered by a liberated version of gogs*:

  - https://notabug.org/hp/gogs/

- Kramdown: It is unclear if this feature is available. See:

  - https://github.com/gettalong/kramdown/search?q=anchor&type=Issues&utf8=%E2%9C%93

- Gitlab Kramdown

  - https://gitlab.com/gitlab-org/gitlab_kramdown/-/blob/master/lib/gitlab_kramdown/parser/header.rb

Steps to add an unsupported markdown parser
```````````````````````````````````````````

1. Find the source code and/or documents.
2. Find the rules for each section, such as anchor link generation, title
   detection, etc... Rely more on the source code than on the documentation (if
   possible)
3. Add the relevant information on this page.
4. Write or adapt an algorithm for that section.
5. Write unit tests for it.
6. Add the new parser to the CLI interface.

Curiosities
-----------

- GitLab added an extension called ``Table of contents`` to
  its `Gitlab Flavored Mardown`. See:
  https://docs.gitlab.com/ee/user/markdown.html#table-of-contents
- in March 2021 GitHub added an interactive TOC button on the readme files of repositories which works
  works for markdown and other systems.
