List items#

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 Commonmark spec, 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 of indentation, and there are three spaces between the list marker and the next character other than a space or tab, then blocks must be indented five spaces in order to fall under the list item.”

https://spec.commonmark.org/0.30/#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`#

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.

/* 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:

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.

/* 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.

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

/* 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: