Markdown to HTML

To render a Markdown tree as HTML.

§1. Main rendering recursion. Do not call this function directly: use the API in Markdown Rendering.

void MDRenderHTML::recurse(OUTPUT_STREAM, markdown_render *rdr, markdown_item *md, int mode) {
    if (rdr == NULL) internal_error("no render details in recursion");
    if (md == NULL) return;
    if (MarkdownVariations::intervene_in_rendering(rdr->variation, rdr, OUT, md, mode)) return;
    int old_mode = mode, tag = (mode & ALT_TEXT_MDRMODE)?FALSE:TRUE;
    switch (md->type) {
        /* Container blocks */
        case BLOCK_QUOTE_MIT:           Render a block quote1.2; break;
        case ALERT_MIT:                 Render an alert1.3; break;
        case UNORDERED_LIST_MIT:        Render an unordered list1.4; break;
        case ORDERED_LIST_MIT:          Render an ordered list1.5; break;
        case UNORDERED_LIST_ITEM_MIT:   Render a list item1.6; break;
        case ORDERED_LIST_ITEM_MIT:     Render a list item1.6; break;
        case FOOTNOTE_BODY_MIT:         Render a footnote body1.7; break;

        /* Leaf blocks */
        case PARAGRAPH_MIT:             Render a paragraph1.8; break;
        case THEMATIC_MIT:              Render a thematic break1.9; break;
        case HEADING_MIT:               Render a heading1.10; break;
        case CODE_BLOCK_MIT:            Render a code block1.11; break;
        case HTML_MIT:                  Render a raw HTML block1.12; break;
        case EMPTY_MIT:                 break;
        case TABLE_MIT:           Render a table1.13; break;
        case TICKBOX_MIT:               Render a tickbox1.14; break;

        /* Inline types */
        case PLAIN_MIT:                 Render plain text1.15; break;
        case LINE_BREAK_MIT:            Render a line break1.16; break;
        case SOFT_BREAK_MIT:            Render a soft line break1.17; break;
        case EMPHASIS_MIT:              Render emphasised text1.18; break;
        case STRONG_MIT:                Render strongly emphasised text1.19; break;
        case CODE_MIT:                  Render inline code sample1.20; break;
        case URI_AUTOLINK_MIT:          Render URI link1.21; break;
        case EMAIL_AUTOLINK_MIT:        Render email link1.22; break;
        case INLINE_HTML_MIT:           Render inline HTML content1.23; break;
        case LINK_MIT:                  Render link1.24; break;
        case IMAGE_MIT:                 Render image1.25; break;
        case LINK_DEST_MIT:             Render link destination1.26; break;
        case STRIKETHROUGH_MIT:         Render struck-through text1.27; break;
        case XMPP_AUTOLINK_MIT:         Render email link1.22; break;
        case TEX_MIT:                   Render maths matter in TeX1.28; break;
        case DISPLAYED_TEX_MIT:         Render maths matter in TeX1.28; break;
        case INWEB_LINK_MIT:            Render Inweb link1.29; break;

        default:                        Recurse1.1; break;
    }
    mode = old_mode;
}

§1.1. Many of these types (and all types not explicitly listed above) perform the following recursion down the Markdown tree. Note that EXISTING_PAR_MDRMODE mode is only used when recursing down the leading edge of the tree: this means that a render at a node can open a paragraph in HTML and then recurse downwards to render the contents even when the first of those contents is known to be a PARAGRAPH_MIT, and the HTML p will not be opened a second time.

Recurse1.1 =

    int m = mode;
    for (markdown_item *c = md->down; c; c = c->next) {
        MDRenderHTML::recurse(OUT, rdr, c, m);
        m = m & (~EXISTING_PAR_MDRMODE);
    }

This code is used in §1, §1.2, §1.3, §1.4.1, §1.8, §1.10, §1.18 and §1.19 and §1.27.

§1.2. Container block items. In these cases we cannot be in ALT_TEXT_MDRMODE mode, since alt-text can only contain inline material, so we need not check this mode before writing HTML tags. See below.

Render a block quote1.2 =

    HTML_OPEN("blockquote");
    WRITE("\n");
    Recurse1.1;
    HTML_CLOSE("blockquote");
    break;

This code is used in §1.

§1.3. Render an alert1.3 =

    int type = Markdown::get_alert_type(md);
    text_stream *name = MarkdownVariations::get_alert_form_name(rdr->variation, type);
    inchar32_t icon = MarkdownVariations::get_alert_form_icon(rdr->variation, type);
    TEMPORARY_TEXT(cl)
    WRITE_TO(cl, "alert");
    for (int i=0; i<Str::len(name); i++) PUT_TO(cl, Characters::tolower(Str::get_at(name, i)));
    HTML_OPEN_WITH("blockquote", "class=\"%S\"", cl);
    DISCARD_TEXT(cl)
    WRITE("\n");
    HTML_OPEN_WITH("p", "class=\"alertheading\"");
    if (icon) { PUT(icon); PUT(' '); }
    if (Str::len(name) > 0) {
        PUT(Str::get_at(name, 0));
        for (int i=1; i<Str::len(name); i++) PUT(Characters::tolower(Str::get_at(name, i)));
    } else {
        WRITE("Alert");
    }
    HTML_CLOSE("p");
    Recurse1.1;
    HTML_CLOSE("blockquote");
    break;

This code is used in §1.

§1.4. Render an unordered list1.4 =

    int skip_ul_rendering = FALSE;
    if (MarkdownVariations::supports(rdr->variation, GADGETS_MARKDOWNFEATURE))
        skip_ul_rendering = MDRender::render_ul_as_carousel(OUT, mode, rdr, md);

    if (skip_ul_rendering == FALSE) {
        HTML_OPEN("ul");
        WRITE("\n");
        Recurse through list1.4.1;
        HTML_CLOSE("ul");
        WRITE("\n");
    }

This code is used in §1.

§1.5. Render an ordered list1.5 =

    int start = Markdown::get_item_number(md->down);
    if (start != 1) {
        HTML_OPEN_WITH("ol", "start=\"%d\"", start);
    } else {
        HTML_OPEN("ol");
    }
    WRITE("\n");
    Recurse through list1.4.1;
    HTML_CLOSE("ol");
    WRITE("\n");

This code is used in §1.

§1.4.1. Recurse through list1.4.1 =

    mode = mode & (~LOOSE_MDRMODE);
    for (markdown_item *ch = md->down; ch; ch = ch->next) {
        if ((ch->next) && (ch->whitespace_follows))
            mode = mode | LOOSE_MDRMODE;
        for (markdown_item *gch = ch->down; gch; gch = gch->next)
            if ((gch->next) && (gch->whitespace_follows))
                mode = mode | LOOSE_MDRMODE;
    }
    Recurse1.1;

This code is used in §1.4 and §1.5.

§1.6. Render a list item1.6 =

    HTML_OPEN("li");
    int nl_issued = FALSE;
    for (markdown_item *ch = md->down; ch; ch = ch->next)
        if (((mode & LOOSE_MDRMODE) == 0) && (ch->type == PARAGRAPH_MIT)) {
            for (markdown_item *gch = ch->down; gch; gch = gch->next)
                MDRenderHTML::recurse(OUT, rdr, gch, mode);
        } else {
            if (nl_issued == FALSE) { nl_issued = TRUE; WRITE("\n"); }
            MDRenderHTML::recurse(OUT, rdr, ch, mode);
        }
    HTML_CLOSE("li");
    WRITE("\n");

This code is used in §1 (twice).

§1.7. Render a footnote body1.7 =

    int displayed_number = md->details;
    if (displayed_number == 1) {
        HTML_OPEN_WITH("ul", "class=\"inwebfootnotetexts\"");
    }
    HTML_OPEN_WITH("li", "class=\"footnote\" id=\"fn:%d\"", md->details);
    HTML_OPEN_WITH("p", "class=\"inwebfootnote\"");
    HTML_OPEN_WITH("sup", "id=\"fnref:%d\"", md->details);
    HTML_OPEN_WITH("a", "href=\"#fn:%d\" rel=\"footnote\"", md->details);
    WRITE("%d", displayed_number);
    HTML_CLOSE("a");
    HTML_CLOSE("sup");
    WRITE("\n");
    markdown_item *r_from = md->down;
    int reopen_for_return_marker = FALSE;
    if ((r_from) && (r_from->type == PARAGRAPH_MIT)) {
        MDRenderHTML::recurse(OUT, rdr, r_from->down, mode);
        r_from = r_from->next;
        if (r_from) {
            HTML_CLOSE("p");
            reopen_for_return_marker = TRUE;
        }
    }
    int m = mode;
    for (markdown_item *c = r_from; c; c = c->next) {
        MDRenderHTML::recurse(OUT, rdr, c, m);
        m = m & (~EXISTING_PAR_MDRMODE);
    }
    if (reopen_for_return_marker) HTML_CLOSE("p");
    HTML_OPEN_WITH("a", "href=\"#fnref:%d\" title=\"return to text\"", md->details);
    WRITE(" &#x21A9;");
    HTML_CLOSE("a");
    HTML_CLOSE("p");
    HTML_CLOSE("li");
    if ((md->next == NULL) || (md->next->type != FOOTNOTE_BODY_MIT)) HTML_CLOSE("ul");
    WRITE("\n");
    break;

This code is used in §1.

§1.8. Leaf blocks. As with container blocks, we cannot be in ALT_TEXT_MDRMODE mode in any of these cases.

Render a paragraph1.8 =

    if ((mode & EXISTING_PAR_MDRMODE) == 0) HTML_OPEN("p");
    mode = mode & (~EXISTING_PAR_MDRMODE);
    Recurse1.1;
    HTML_CLOSE("p");
    break;

This code is used in §1.

§1.9. Render a thematic break1.9 =

    WRITE("<hr />\n");
    break;

This code is used in §1.

§1.10. Render a heading1.10 =

    char *h = "p";
    switch (Markdown::get_heading_level(md)) {
        case 1: h = "h1"; break;
        case 2: h = "h2"; break;
        case 3: h = "h3"; break;
        case 4: h = "h4"; break;
        case 5: h = "h5"; break;
        case 6: h = "h6"; break;
    }
    HTML_OPEN(h);
    TEMPORARY_TEXT(anchor)
    text_stream *url = MarkdownVariations::URL_for_heading(md);
    for (int i=0; i<Str::len(url); i++)
        if (Str::get_at(url, i) == '#')
            for (i++; i<Str::len(url); i++)
                PUT_TO(anchor, Str::get_at(url, i));
    if (Str::len(anchor) > 0) {
        HTML_OPEN_WITH("span", "id=%S", anchor);
    }
    Recurse1.1;
    if (Str::len(anchor) > 0) {
        HTML_CLOSE("span");
    }
    DISCARD_TEXT(anchor)
    HTML_CLOSE(h);
    WRITE("\n");

This code is used in §1.

§1.11. We use the convention that the first word of the info string on a fenced code block is the "language", and give it a CSS class accordingly. A piquant part of CommonMark is that the language does respect entities, but that the body of the code block does not. (It also respects backslash escapes, but we do not render the language in ESCAPES_MDRMODE mode because those have already been taken out at the parsing stage.)

Render a code block1.11 =

    mode = mode & (~ESCAPES_MDRMODE) & (~ENTITIES_MDRMODE);
    TEMPORARY_TEXT(language)
    for (int i=0; i<Str::len(md->info_string); i++) {
        inchar32_t c = Str::get_at(md->info_string, i);
        if ((c == ' ') || (c == '\t')) break;
        PUT_TO(language, c);
    }
    TEMPORARY_TEXT(language_rendered)
    if (Str::len(language) > 0) {
        md->sliced_from = language;
        md->from = 0; md->to = Str::len(language) - 1;
        if (MarkdownVariations::supports(rdr->variation, ENTITIES_MARKDOWNFEATURE))
            MDRender::slice(language_rendered, rdr, md, mode | ENTITIES_MDRMODE);
        else
            MDRender::slice(language_rendered, rdr, md, mode);
    }
    DISCARD_TEXT(language)

    if (MarkdownVariations::supports(rdr->variation, INWEB_SYNTAX_COLOURING_MARKDOWNFEATURE))
        MDRenderHTML::render_code_block(OUT, mode, rdr, md->stashed, language_rendered);
    else Render a code block the default way1.11.1;
    DISCARD_TEXT(language_rendered)

This code is used in §1.

§1.11.1. Render a code block the default way1.11.1 =

    HTML_OPEN("pre");
    if (Str::len(language_rendered) > 0) {
        HTML_OPEN_WITH("code", "class=\"language-%S\"", language_rendered);
    } else {
        HTML_OPEN("code");
    }
    md->sliced_from = md->stashed;
    md->from = 0; md->to = Str::len(md->sliced_from) - 1;
    MDRender::slice(OUT, rdr, md, mode);
    HTML_CLOSE("code");
    HTML_CLOSE("pre");
    WRITE("\n");

This code is used in §1.11.

§1.12. Render a raw HTML block1.12 =

    if (MarkdownVariations::supports(rdr->variation, DISALLOWED_RAW_HTML_MARKDOWNFEATURE)) {
        for (int i=0; i<Str::len(md->stashed); i++)
            if (Str::get_at(md->stashed, i) == '<') {
                TEMPORARY_TEXT(rtag)
                for (int j=i+1; j<Str::len(md->stashed); j++)
                    if (Characters::is_ASCII_letter(Str::get_at(md->stashed, j)))
                        PUT_TO(rtag, Str::get_at(md->stashed, j));
                    else
                        break;
                if (Markdown::tag_should_be_filtered(rtag)) WRITE("&lt;");
                else PUT('<');
                DISCARD_TEXT(rtag)
            } else {
                PUT(Str::get_at(md->stashed, i));
            }
    } else {
        WRITE("%S", md->stashed);
    }

This code is used in §1.

§1.13. Render a table1.13 =

    markdown_item *alignment_markers = md->down;
    if (alignment_markers) {
        int row_count = 0;
        for (markdown_item *md = alignment_markers->next; md; md = md->next) row_count++;
        if (row_count > 0) {
            WRITE("<table>\n");
            WRITE("<thead>\n");
            markdown_item *row = alignment_markers->next;
            WRITE("<tr>\n");
            for (markdown_item *md = row->down, *top = alignment_markers->down;
                md; md = md->next, top = top->next) {
                switch (Markdown::get_alignment(top)) {
                    case 0: WRITE("<th>"); break;
                    case 1: WRITE("<th align=\"left\">"); break;
                    case 2: WRITE("<th align=\"right\">"); break;
                    case 3: WRITE("<th align=\"center\">"); break;
                }
                if (md->down) MDRenderHTML::recurse(OUT, rdr, md->down, mode);
                WRITE("</th>\n");
            }
            WRITE("</tr>\n");
            row = row->next;
            WRITE("</thead>\n");
            if (row_count > 1) {
                WRITE("<tbody>\n");
                while (row) {
                    WRITE("<tr>\n");
                    for (markdown_item *md = row->down, *top = alignment_markers->down;
                        md; md = md->next, top = top->next) {
                        switch (Markdown::get_alignment(top)) {
                            case 0: WRITE("<td>"); break;
                            case 1: WRITE("<td align=\"left\">"); break;
                            case 2: WRITE("<td align=\"right\">"); break;
                            case 3: WRITE("<td align=\"center\">"); break;
                        }
                        if (md->down) MDRenderHTML::recurse(OUT, rdr, md->down, mode);
                        WRITE("</td>\n");
                    }
                    WRITE("</tr>\n");
                    row = row->next;
                }
                WRITE("</tbody>\n");
            }
            WRITE("</table>\n");
        }
    }

This code is used in §1.

§1.14. Render a tickbox1.14 =

    if (Markdown::get_tick_state(md)) {
        WRITE("<input checked=\"\" disabled=\"\" type=\"checkbox\"> ");
    } else {
        WRITE("<input disabled=\"\" type=\"checkbox\"> ");
    }

This code is used in §1.

§1.15. Inline types. Here we might be in ALT_TEXT_MDRMODE mode; that is, we might be rendering Markdown which represents the alt-text for an image. The CommonMark specification calls that an image description, and says:

Though this spec is concerned with parsing, not rendering, it is recommended that in rendering to HTML, only the plain string content of the image description be used.

Exactly what "plain string content" means is unclear but the demonstrated test cases imply that we render inline elements as normal, but without their HTML tags. So from here on, HTML tags can only be opened or closed if the flag tag is TRUE.

Render plain text1.15 =

    MDRender::slice(OUT, rdr, md, mode);
    break;

This code is used in §1.

§1.16. Render a line break1.16 =

    if (tag) WRITE("<br />\n"); else WRITE(" ");
    break;

This code is used in §1.

§1.17. Render a soft line break1.17 =

    MDRenderHTML::char(OUT, '\n', mode);
    break;

This code is used in §1.

§1.18. Render emphasised text1.18 =

    if (tag) HTML_OPEN("em");
    Recurse1.1;
    if (tag) HTML_CLOSE("em");
    break;

This code is used in §1.

§1.19. Render strongly emphasised text1.19 =

    if (tag) HTML_OPEN("strong");
    Recurse1.1;
    if (tag) HTML_CLOSE("strong");
    break;

This code is used in §1.

§1.20. Render inline code sample1.20 =

    if (tag) HTML_OPEN("code");
    mode = mode & (~ESCAPES_MDRMODE);
    mode = mode & (~ENTITIES_MDRMODE);
    MDRender::slice(OUT, rdr, md, mode);
    if (tag) HTML_CLOSE("code");
    break;

This code is used in §1.

§1.21. Render URI link1.21 =

    text_stream *supplied_scheme = NULL;
    if (Markdown::get_add_protocol_state(md)) supplied_scheme = I"http://";
    Render autolink1.21.1;

This code is used in §1.

§1.22. Render email link1.22 =

    text_stream *supplied_scheme = NULL;
    if (Markdown::get_add_protocol_state(md)) {
        supplied_scheme = I"mailto:";
        if (md->type == XMPP_AUTOLINK_MIT) supplied_scheme = I"xmpp:";
    }
    Render autolink1.21.1;

This code is used in §1 (twice).

§1.21.1. Render autolink1.21.1 =

    TEMPORARY_TEXT(address)
    MDRender::slice(address, rdr, md, (mode & (~ESCAPES_MDRMODE)) | URI_MDRMODE);
    if (tag) HTML_OPEN_WITH("a", "href=\"%S%S\"", supplied_scheme, address);
    MDRender::slice(OUT, rdr, md, mode & (~ESCAPES_MDRMODE));
    if (tag) HTML_CLOSE("a");
    DISCARD_TEXT(address)

This code is used in §1.21 and §1.22.

§1.23. Render inline HTML content1.23 =

    mode = mode | RAW_MDRMODE;
    if (Markdown::get_filtered_state(md)) mode = mode | FILTERED_MDRMODE;
    mode = mode & (~ESCAPES_MDRMODE);
    mode = mode & (~ENTITIES_MDRMODE);
    MDRender::slice(OUT, rdr, md, mode);
    break;

This code is used in §1.

§1.24. Render link1.24 =

    TEMPORARY_TEXT(URI)
    TEMPORARY_TEXT(title)
    if (md->details > 0) {
        WRITE_TO(URI, "#fn:%d", md->details);
    } else if (md->down->next) {
        if (md->down->next->type == LINK_DEST_MIT) {
            MDRenderHTML::recurse(URI, rdr, md->down->next, mode);
            if ((md->down->next->next) && (md->down->next->next->type == LINK_TITLE_MIT))
                MDRenderHTML::recurse(title, rdr, md->down->next->next, mode);
        } else if (md->down->next->type == LINK_TITLE_MIT) {
            MDRenderHTML::recurse(title, rdr, md->down->next, mode);
        }
    }
    if (md->details > 0) {
        if (tag) HTML_OPEN_WITH("sup", "id=\"fnred:%d\"", md->details);
        if (tag) HTML_OPEN_WITH("a", "href=\"%S\" rel=\"footnote\"", URI);
    } else if (Str::len(title) > 0) {
        if (tag) HTML_OPEN_WITH("a", "href=\"%S\" title=\"%S\"", URI, title);
    } else {
        if (tag) HTML_OPEN_WITH("a", "href=\"%S\"", URI);
    }
    MDRenderHTML::recurse(OUT, rdr, md->down, mode);
    if (tag) HTML_CLOSE("a");
    if (md->details > 0) {
        if (tag) HTML_CLOSE("sup");
    }
    DISCARD_TEXT(URI)
    DISCARD_TEXT(title)

This code is used in §1.

§1.25. Render image1.25 =

    TEMPORARY_TEXT(URI)
    TEMPORARY_TEXT(title)
    TEMPORARY_TEXT(alt)
    if (md->down->next) {
        if (md->down->next->type == LINK_DEST_MIT) {
            MDRenderHTML::recurse(URI, rdr, md->down->next, mode);
            if ((md->down->next->next) && (md->down->next->next->type == LINK_TITLE_MIT))
                MDRenderHTML::recurse(title, rdr, md->down->next->next, mode);
        } else if (md->down->next->type == LINK_TITLE_MIT) {
            MDRenderHTML::recurse(title, rdr, md->down->next, mode);
        }
    }
    MDRenderHTML::recurse(alt, rdr, md->down, mode | ALT_TEXT_MDRMODE);
    if ((MarkdownVariations::supports(rdr->variation, GADGETS_MARKDOWNFEATURE) == FALSE) ||
        (MDRender::render_gadget(OUT, rdr, mode, alt, URI) == FALSE)) {
        int w = -1, h = -1;
        int j = 0;
        for (int i=1; i<Str::len(URI)-1; i++)
            if (Str::get_at(URI, i) == '@')
                j = i;
        if (j > 0) {
            int x = 0;
            for (int i=j+1; i<Str::len(URI); i++)
                if (Str::get_at(URI, i) == 'x')
                    x = i;
            int bad = FALSE;
            for (int i=j+1; i<Str::len(URI); i++)
                if ((i != x) && (Characters::isdigit(Str::get_at(URI, i)) == FALSE))
                    bad = TRUE;
            if (bad == FALSE) {
                if (x > 0) {
                    if (x > j+1) w = Str::atoi(URI, j+1);
                    if (x < Str::len(URI) - 1) h = Str::atoi(URI, x+1);
                } else {
                    w = Str::atoi(URI, j+1);
                }
                Str::truncate(URI, j);
            }
        }
        MDRender::notify_image(rdr, URI);
        Actually render the image tag1.25.1;
    }
    DISCARD_TEXT(URI)
    DISCARD_TEXT(title)
    DISCARD_TEXT(alt)

This code is used in §1.

§1.25.1. If alt-text for one image contains another image, what should be done? CommonMark implies this is not allowed, but just in case, the decision below is that the second image is replaced by its own alt-text.

Actually render the image tag1.25.1 =

    if (Str::len(title) > 0) {
        if (tag) {
            if ((w > 0) && (h > 0)) {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" title=\"%S\" width=\"%d\" height=\"%d\" /", URI, alt, title, w, h);
            } else if (w > 0) {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" title=\"%S\" width=\"%d\" /", URI, alt, title, w);
            } else if (h > 0) {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" title=\"%S\" height=\"%d\" /", URI, alt, title, h);
            } else {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" title=\"%S\" /", URI, alt, title);
            }
        } else {
            WRITE("%S", alt);
        }
    } else {
        if (tag) {
            if ((w > 0) && (h > 0)) {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" width=\"%d\" height=\"%d\" /", URI, alt, w, h);
            } else if (w > 0) {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" width=\"%d\" /", URI, alt, w);
            } else if (h > 0) {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" height=\"%d\" /", URI, alt, h);
            } else {
                HTML_TAG_WITH("img", "src=\"%S\" alt=\"%S\" /", URI, alt);
            }
        } else {
            WRITE("%S", alt);
        }
    }

This code is used in §1.25.

§1.26. Render link destination1.26 =

    MDRender::slice(OUT, rdr, md->down, mode | URI_MDRMODE);
    break;

This code is used in §1.

§1.27. Render struck-through text1.27 =

    if (tag) HTML_OPEN("del");
    Recurse1.1;
    if (tag) HTML_CLOSE("del");
    break;

This code is used in §1.

§1.28. Render maths matter in TeX1.28 =

    TEMPORARY_TEXT(TEX)
    if (MarkdownVariations::supports(rdr->variation, ALT_TEX_MARKDOWNFEATURE)) {
        int m = RAW_MDRMODE;
        for (markdown_item *c = md->down; c; c = c->next) {
            MDRenderHTML::recurse(TEX, rdr, c, m);
            m = m & (~EXISTING_PAR_MDRMODE);
        }
    } else {
        for (int i=md->from; i<=md->to; i++) {
            inchar32_t c = Markdown::get_at(md, i);
            PUT_TO(TEX, c);
        }
    }
    MDRenderHTML::render_maths(OUT, rdr, TEX, (md->type == DISPLAYED_TEX_MIT)?TRUE:FALSE);
    DISCARD_TEXT(TEX)

This code is used in §1 (twice).

§1.29. Render Inweb link1.29 =

    TEMPORARY_TEXT(address)
    MDRenderHTML::recurse(address, rdr, md->down, RAW_MDRMODE);
    query_results qr = MDRender::resolve_link(rdr, address);
    if (qr.found) {
        if (qr.external) {
            if (tag) MDRenderHTML::link(OUT, rdr, I"", qr.url, mode);
        } else {
            if (tag) MDRenderHTML::link_internal(OUT, qr.url, mode);
        }
        if (qr.token) WRITE("<span class=\"linelabel\">");
        WRITE("%S", qr.title);
        if (qr.token) WRITE("</span>");
        if (tag) MDRenderHTML::end_link(OUT, mode);
    } else {
        HTML_OPEN("b");
        WRITE("[Unresolved link: %S]", address);
        HTML_CLOSE("b");
    }
    DISCARD_TEXT(address)

This code is used in §1.

§2. Character output. Down at the individual character level, there are three mutually exclusive ways to render characters: they all agree on ASCII digits and letters.

void MDRenderHTML::char(OUTPUT_STREAM, inchar32_t c, int mode) {
    if (mode & TOLOWER_MDRMODE) c = Characters::tolower(c);
    if (mode & RAW_MDRMODE) {
        PUT(c);
    } else if (mode & URI_MDRMODE) {
        if (c >= 0x10000) {
            MARKDOWN_URI_HEX(0xF0 + (unsigned char) (c >> 18));
            MARKDOWN_URI_HEX(0x80 + (unsigned char) ((c >> 12) & 0x3f));
            MARKDOWN_URI_HEX( 0x80 + (unsigned char) ((c >> 6) & 0x3f));
            MARKDOWN_URI_HEX(0x80 + (unsigned char) (c & 0x3f));
        } else if (c >= 0x800) {
            MARKDOWN_URI_HEX(0xE0 + (unsigned char) (c >> 12));
            MARKDOWN_URI_HEX(0x80 + (unsigned char) ((c >> 6) & 0x3f));
            MARKDOWN_URI_HEX(0x80 + (unsigned char) (c & 0x3f));
        } else if (c >= 0x80) {
            MARKDOWN_URI_HEX(0xC0 + (unsigned char) (c >> 6));
            MARKDOWN_URI_HEX(0x80 + (unsigned char) (c & 0x3f));
        } else {
            switch (c) {
                case '<': WRITE("&lt;"); break;
                case '&': WRITE("&amp;"); break;
                case '>': WRITE("&gt;"); break;
                case '[': MARKDOWN_URI_HEX((unsigned char) c); break;
                case '\\':MARKDOWN_URI_HEX((unsigned char) c); break;
                case '\"':MARKDOWN_URI_HEX((unsigned char) c); break;
                case ']': MARKDOWN_URI_HEX((unsigned char) c); break;
                case '`': MARKDOWN_URI_HEX((unsigned char) c); break;
                case ' ': MARKDOWN_URI_HEX((unsigned char) c); break;
                default: PUT(c); break;
            }
        }
    } else {
        switch (c) {
            case '<': WRITE("&lt;"); break;
            case '&': WRITE("&amp;"); break;
            case '>': WRITE("&gt;"); break;
            case '"': WRITE("&quot;"); break;
            default: PUT(c); break;
        }
    }
}

§3. Where:

define MARKDOWN_URI_HEX(x) {
        unsigned int z = (unsigned int) x;
        PUT('%');
        MDRender::hex_digit(OUT, z >> 4);
        MDRender::hex_digit(OUT, z & 0x0f);
    }

§4. While this is strictly speaking nothing to do with Markdown, it turns out to be convenient to provide a variant which does not encode " as ":

void MDRenderHTML::escape_text(text_stream *OUT, text_stream *id) {
    for (int i=0; i < Str::len(id); i++) {
        if (Str::get_at(id, i) == '&') WRITE("&amp;");
        else if (Str::get_at(id, i) == '<') WRITE("&lt;");
        else if (Str::get_at(id, i) == '>') WRITE("&gt;");
        else PUT(Str::get_at(id, i));
    }
}

§5. Inweb links.

void MDRenderHTML::link(OUTPUT_STREAM, markdown_render *rdr,
    text_stream *supplied_scheme, text_stream *address, int mode) {
    TEMPORARY_TEXT(url)
    WRITE_TO(url, "%S%S", supplied_scheme, address);
    HTML::begin_link_with_class(OUT, I"external", url);
    DISCARD_TEXT(url)
}
void MDRenderHTML::link_internal(OUTPUT_STREAM, text_stream *nameid, int mode) {
    HTML::begin_link_with_class(OUT, I"internal", nameid);
}
void MDRenderHTML::end_link(OUTPUT_STREAM, int mode) {
    HTML::end_link(OUT);
}

§6. Code block rendering.

void MDRenderHTML::render_code_block(OUTPUT_STREAM, int mode, markdown_render *rdr,
    text_stream *code, text_stream *language_rendered) {
    TEMPORARY_TEXT(colouring)
    void *colours = NULL;
    MDRender::colour_code_block(rdr, language_rendered, code, colouring, &colours);
    if (colours) {
        TEMPORARY_TEXT(prefix)
        MDRender::name_colour_scheme(rdr, prefix, colours);
        HTML_OPEN_WITH("pre", "class=\"%S-displayed-code all-displayed-code code-font\"",
            prefix);
    } else {
        HTML_OPEN_WITH("pre", "class=\"displayed-code all-displayed-code code-font\"");
    }
    MDRenderHTML::render_syntax_coloured(OUT, rdr, code, colouring, colours);
    DISCARD_TEXT(colouring)
    HTML_CLOSE("pre");
    WRITE("\n");
}

void MDRenderHTML::render_syntax_coloured(OUTPUT_STREAM, markdown_render *rdr,
    text_stream *code, text_stream *colouring, void *colours) {
    int ind = Streams::get_indentation(OUT); Streams::set_indentation(OUT, 0);
    int current_colour = -1, colour_wanted = PLAIN_COLOUR;
    for (int i=0; i < Str::len(code); i++) {
        colour_wanted = (int) Str::get_at(colouring, i);
        if (colour_wanted == 0) colour_wanted = PLAIN_COLOUR;
        if (colour_wanted != current_colour) {
            if (current_colour >= 0)
                MDRenderHTML::change_colour(OUT, rdr, -1, colours);
            if (colour_wanted >= 0)
                MDRenderHTML::change_colour(OUT, rdr, colour_wanted, colours);
            current_colour = colour_wanted;
        }
        if (Str::get_at(code, i) == '<') WRITE("&lt;");
        else if (Str::get_at(code, i) == '>') WRITE("&gt;");
        else if (Str::get_at(code, i) == '&') WRITE("&amp;");
        else WRITE("%c", Str::get_at(code, i));
    }
    if (current_colour >= 0) MDRenderHTML::change_colour(OUT, rdr, -1, colours);
    Streams::set_indentation(OUT, ind);
}

void MDRenderHTML::change_colour(OUTPUT_STREAM, markdown_render *rdr, int col,
    void *colours) {
    if (col == -1) {
        HTML_CLOSE("span");
    } else {
        TEMPORARY_TEXT(cl)
        MDRender::name_colour(rdr, cl, col);
        if (Str::len(cl) == 0) {
            PRINT("col: %d\n", col); internal_error("bad colour");
        } else {
            TEMPORARY_TEXT(csname)
            MDRender::name_colour_scheme(rdr, csname, colours);
            HTML_OPEN_WITH("span", "class=\"%S%S\"", csname, cl);
            DISCARD_TEXT(csname)
        }
        DISCARD_TEXT(cl)
    }
}

§7. Maths is rendered using MathJax-friendly formats unless the client has vetoed this: only then do we resort to paraphrasing in plain text.

void MDRenderHTML::render_maths(OUTPUT_STREAM, markdown_render *rdr, text_stream *TEX,
    int displayed) {
    if (MDRender::veto_mathematics(rdr, displayed)) Paraphrase maths as text7.1
    else Render maths as MathJax7.2;
}

§7.1. Paraphrase maths as text7.1 =

    TEMPORARY_TEXT(R)
    MDRender::remove_math_mode(R, TEX, TRUE);
    if (displayed) {
        HTML_OPEN("center");
        MDRender::stream(OUT, rdr, R, 0);
        HTML_CLOSE("center");
    } else {
        MDRender::stream(OUT, rdr, R, 0);
    }
    DISCARD_TEXT(R)

This code is used in §7.

§7.2. In order to support Knuth's webs slightly better, we assume that |...| inside TeX math mode is intended to mean an inline code sample.

Render maths as MathJax7.2 =

    if (displayed) WRITE("$$"); else WRITE("\\INWEBMATH(");
    TEMPORARY_TEXT(escaped)
    MDRender::stream(escaped, rdr, TEX, 0);
    for (int i=0; i<Str::len(escaped); i++) {
        if (Str::get_at(escaped, i) == '|') {
            int found = FALSE;
            for (int j=i+1; j<Str::len(escaped); j++) {
                if (Str::get_at(escaped, j) == '|') {
                    WRITE("\\hbox{\\tt ");
                    for (int k=i+1; k<j; k++) PUT(Str::get_at(escaped, k));
                    WRITE("}");
                    i = j;
                    found = TRUE; break;
                }
            }
            if (found == FALSE) PUT(Str::get_at(escaped, i));
        } else {
            PUT(Str::get_at(escaped, i));
        }
    }
    DISCARD_TEXT(escaped)
    if (displayed) WRITE("$$"); else WRITE("\\INWEBMATH)");

This code is used in §7.