To specify modified versions of the Markdown markup syntax.

• §5. The CommonMark variation
• §6. Github extensions to CommonMark
• §7. Methods for features

§1. In practice, nobody quite uses pure Markdown, because there are always minor additions or removals people want to make in particular use cases. We'll call modified versions of the Markdown syntax "variations", but will provide only one of these here: the baseline of CommonMark.

define MAX_MARKDOWNFEATURES 256

markdown_feature *markdown_feature_registry[MAX_MARKDOWNFEATURES];

void MarkdownVariations::start?Usage of MarkdownVariations::start:
Foundation Module - §8(void) {
    Markdown::create_item_types();
    for (int i=0; i<MAX_MARKDOWNFEATURES; i++) markdown_feature_registry[i] = NULL;
    MarkdownVariations::define_CommonMark();
    MarkdownVariations::define_GFM();
}

markdown_variation *CommonMark_variation = NULL, *GitHub_flavored_Markdown_variation = NULL;

markdown_variation *MarkdownVariations::CommonMark?Usage of MarkdownVariations::CommonMark:
Markdown - §5, §6(void) {
    return CommonMark_variation;
}

markdown_variation *MarkdownVariations::GitHub_flavored_Markdown?Usage of MarkdownVariations::GitHub_flavored_Markdown:
Inform-Flavoured Markdown - §1(void) {
    return GitHub_flavored_Markdown_variation;
}

§2. A variation is essentially a named collection of features, which may affect parsing, or rendering, or both.

All newly-created variations begin with the CommonMark baseline set of features active, and no others. The caller should use MarkdownVariations::add_feature to add a home-brew feature, or MarkdownVariations::remove_feature to take away one of the CommonMark ones.

typedef struct markdown_variation {
    struct text_stream *name;
    int active_built_in_features[NO_DEFINED_MARKDOWNFEATURE_VALUES];
    struct method_set *methods;
    CLASS_DEFINITION
} markdown_variation;

markdown_variation *MarkdownVariations::new?Usage of MarkdownVariations::new:
§5, §6
Inform-Flavoured Markdown - §1(text_stream *name) {
    markdown_variation *variation = CREATE(markdown_variation);
    variation->name = Str::duplicate(name);
    variation->methods = Methods::new_set();
    for (int i=0; i<NO_DEFINED_MARKDOWNFEATURE_VALUES; i++)
        variation->active_built_in_features[i] = FALSE;
    MarkdownVariations::make_baseline_features_active(variation);
    return variation;
}

void MarkdownVariations::add_feature?Usage of MarkdownVariations::add_feature:
§5, §6
Inform-Flavoured Markdown - §1.2, §1.4, §1.6, §1.12, §1.19, §1.22, §1.25, §1.27, §1.30, §1.33(markdown_variation *variation, int feature_id) {
    variation->active_built_in_features[feature_id] = TRUE;
}

void MarkdownVariations::remove_feature?Usage of MarkdownVariations::remove_feature:
Inform-Flavoured Markdown - §1(markdown_variation *variation, int feature_id) {
    variation->active_built_in_features[feature_id] = FALSE;
}

void MarkdownVariations::copy_features_of?Usage of MarkdownVariations::copy_features_of:
Inform-Flavoured Markdown - §1(markdown_variation *to, markdown_variation *from) {
    for (int i=0; i<NO_DEFINED_MARKDOWNFEATURE_VALUES; i++)
        to->active_built_in_features[i] = from->active_built_in_features[i];
}

• The structure markdown_variation is accessed in 2/mth, 2/trs, 4/prp, 4/jsn, 5/mrk, 5/im, 9/pl and here.

§3. A "feature" is an aspect of Markdown syntax or behaviour; any given variation can either support it or not.

typedef struct markdown_feature {
    struct text_stream *name;
    int feature_ID;
    struct method_set *methods;
    CLASS_DEFINITION
} markdown_feature;


markdown_feature *MarkdownVariations::new_feature?Usage of MarkdownVariations::new_feature:
§5, §6
Inform-Flavoured Markdown - §1.2, §1.4, §1.6, §1.12, §1.19, §1.22, §1.25, §1.27, §1.30, §1.33(text_stream *name, int id) {
    markdown_feature *feature = CREATE(markdown_feature);
    if (id >= MAX_MARKDOWNFEATURES) internal_error("too many Markdown features");
    feature->name = Str::duplicate(name);
    feature->feature_ID = id;
    feature->methods = Methods::new_set();
    if (markdown_feature_registry[id]) internal_error("Markdown feature ID clash");
    markdown_feature_registry[id] = feature;
    return feature;
}

• The structure markdown_feature is accessed in 2/mth, 2/trs, 4/prp, 4/jsn, 5/mrk, 5/im, 9/pl and here.

§4. The point of the cumbersome business of both defining an ID and creating an object to represent the same feature was to make this function quick.

int MarkdownVariations::supports?Usage of MarkdownVariations::supports:
§7, §8, §9
Markdown - §5.1
Markdown Phase I - §17.2.1, §17.2.2, §17.2.3, §24.2, §25.2, §25.3, §25.4, §25.5, §25.7, §27
Markdown Phase II - §3, §3.2, §3.3, §3.4, §3.9, §3.9.5.1, §4, §5.2
Markdown Rendering - §2, §2.6, §2.7(markdown_variation *variation, int feature_id) {
    return variation->active_built_in_features[feature_id];
}

§5. The CommonMark variation. Vanilla ice cream is under-rated:

enum BLOCK_QUOTES_MARKDOWNFEATURE from 0
enum ORDERED_LISTS_MARKDOWNFEATURE
enum UNORDERED_LISTS_MARKDOWNFEATURE
enum INDENTED_CODE_BLOCKS_MARKDOWNFEATURE
enum FENCED_CODE_BLOCKS_MARKDOWNFEATURE
enum HTML_BLOCKS_MARKDOWNFEATURE
enum THEMATIC_MARKERS_MARKDOWNFEATURE
enum ATX_HEADINGS_MARKDOWNFEATURE
enum SETEXT_HEADINGS_MARKDOWNFEATURE
enum WEB_AUTOLINKS_MARKDOWNFEATURE
enum EMAIL_AUTOLINKS_MARKDOWNFEATURE
enum INLINE_HTML_MARKDOWNFEATURE
enum BACKTICKED_CODE_MARKDOWNFEATURE
enum LINKS_MARKDOWNFEATURE
enum IMAGES_MARKDOWNFEATURE
enum ASTERISK_EMPHASIS_MARKDOWNFEATURE
enum UNDERSCORE_EMPHASIS_MARKDOWNFEATURE
enum ENTITIES_MARKDOWNFEATURE

markdown_feature *block_quotes_Markdown_feature = NULL;
markdown_feature *ordered_lists_Markdown_feature = NULL;
markdown_feature *unordered_lists_Markdown_feature = NULL;
markdown_feature *indented_code_blocks_Markdown_feature = NULL;
markdown_feature *fenced_code_blocks_Markdown_feature = NULL;
markdown_feature *HTML_blocks_Markdown_feature = NULL;
markdown_feature *thematic_markers_Markdown_feature = NULL;
markdown_feature *ATX_headings_Markdown_feature = NULL;
markdown_feature *setext_headings_Markdown_feature = NULL;

markdown_feature *web_autolinks_Markdown_feature = NULL;
markdown_feature *email_autolinks_Markdown_feature = NULL;
markdown_feature *inline_HTML_Markdown_feature = NULL;
markdown_feature *backticked_code_Markdown_feature = NULL;
markdown_feature *links_Markdown_feature = NULL;
markdown_feature *images_Markdown_feature = NULL;
markdown_feature *asterisk_emphasis_Markdown_feature = NULL;
markdown_feature *underscore_emphasis_Markdown_feature = NULL;

markdown_feature *entities_Markdown_feature = NULL;

void MarkdownVariations::define_CommonMark?Usage of MarkdownVariations::define_CommonMark:
§1(void) {
    block_quotes_Markdown_feature =         MarkdownVariations::new_feature(I"block quotes",         BLOCK_QUOTES_MARKDOWNFEATURE);
    ordered_lists_Markdown_feature =        MarkdownVariations::new_feature(I"ordered lists",        ORDERED_LISTS_MARKDOWNFEATURE);
    unordered_lists_Markdown_feature =      MarkdownVariations::new_feature(I"unordered lists",      UNORDERED_LISTS_MARKDOWNFEATURE);
    indented_code_blocks_Markdown_feature = MarkdownVariations::new_feature(I"indented code blocks", INDENTED_CODE_BLOCKS_MARKDOWNFEATURE);
    fenced_code_blocks_Markdown_feature =   MarkdownVariations::new_feature(I"fenced code blocks",   FENCED_CODE_BLOCKS_MARKDOWNFEATURE);
    HTML_blocks_Markdown_feature =          MarkdownVariations::new_feature(I"HTML blocks",          HTML_BLOCKS_MARKDOWNFEATURE);
    thematic_markers_Markdown_feature =     MarkdownVariations::new_feature(I"thematic markers",     THEMATIC_MARKERS_MARKDOWNFEATURE);
    ATX_headings_Markdown_feature =         MarkdownVariations::new_feature(I"ATX headings",         ATX_HEADINGS_MARKDOWNFEATURE);
    setext_headings_Markdown_feature =      MarkdownVariations::new_feature(I"setext headings",      SETEXT_HEADINGS_MARKDOWNFEATURE);

    web_autolinks_Markdown_feature =        MarkdownVariations::new_feature(I"web autolinks",        WEB_AUTOLINKS_MARKDOWNFEATURE);
    email_autolinks_Markdown_feature =      MarkdownVariations::new_feature(I"email autolinks",      EMAIL_AUTOLINKS_MARKDOWNFEATURE);
    inline_HTML_Markdown_feature =          MarkdownVariations::new_feature(I"inline HTML",          INLINE_HTML_MARKDOWNFEATURE);
    backticked_code_Markdown_feature =      MarkdownVariations::new_feature(I"backticked code",      BACKTICKED_CODE_MARKDOWNFEATURE);
    links_Markdown_feature =                MarkdownVariations::new_feature(I"links",                LINKS_MARKDOWNFEATURE);
    images_Markdown_feature =               MarkdownVariations::new_feature(I"images",               IMAGES_MARKDOWNFEATURE);
    asterisk_emphasis_Markdown_feature =    MarkdownVariations::new_feature(I"emphasis",             ASTERISK_EMPHASIS_MARKDOWNFEATURE);
    underscore_emphasis_Markdown_feature =  MarkdownVariations::new_feature(I"emphasis",             UNDERSCORE_EMPHASIS_MARKDOWNFEATURE);

    entities_Markdown_feature =             MarkdownVariations::new_feature(I"entities",             ENTITIES_MARKDOWNFEATURE);

    CommonMark_variation = MarkdownVariations::new(I"CommonMark 0.30");
}

void MarkdownVariations::make_baseline_features_active?Usage of MarkdownVariations::make_baseline_features_active:
§2(markdown_variation *variation) {
    MarkdownVariations::add_feature(variation, BLOCK_QUOTES_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, ORDERED_LISTS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, UNORDERED_LISTS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, INDENTED_CODE_BLOCKS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, FENCED_CODE_BLOCKS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, HTML_BLOCKS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, THEMATIC_MARKERS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, ATX_HEADINGS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, SETEXT_HEADINGS_MARKDOWNFEATURE);

    MarkdownVariations::add_feature(variation, WEB_AUTOLINKS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, EMAIL_AUTOLINKS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, INLINE_HTML_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, BACKTICKED_CODE_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, LINKS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, IMAGES_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, ASTERISK_EMPHASIS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, UNDERSCORE_EMPHASIS_MARKDOWNFEATURE);

    MarkdownVariations::add_feature(variation, ENTITIES_MARKDOWNFEATURE);
}

§6. Github extensions to CommonMark. See GitHub's specification, which is based on CommonMark, though its description of new features is a little more concise, since these have been added with more prudent syntax.

enum STRIKETHROUGH_MARKDOWNFEATURE
enum TABLES_MARKDOWNFEATURE
enum TASK_LIST_ITEMS_MARKDOWNFEATURE
enum EXTENDED_AUTOLINKS_MARKDOWNFEATURE
enum DISALLOWED_RAW_HTML_MARKDOWNFEATURE

markdown_feature *strikethrough_Markdown_feature = NULL;
markdown_feature *tables_Markdown_feature = NULL;
markdown_feature *task_list_items_Markdown_feature = NULL;
markdown_feature *extended_autolinks_Markdown_feature = NULL;
markdown_feature *disallowed_raw_HTML_Markdown_feature = NULL;

void MarkdownVariations::define_GFM?Usage of MarkdownVariations::define_GFM:
§1(void) {
    strikethrough_Markdown_feature =       MarkdownVariations::new_feature(I"strikethrough",       STRIKETHROUGH_MARKDOWNFEATURE);
    tables_Markdown_feature =              MarkdownVariations::new_feature(I"tables",              TABLES_MARKDOWNFEATURE);
    task_list_items_Markdown_feature =     MarkdownVariations::new_feature(I"task list items",     TASK_LIST_ITEMS_MARKDOWNFEATURE);
    extended_autolinks_Markdown_feature =  MarkdownVariations::new_feature(I"extended autolinks",  EXTENDED_AUTOLINKS_MARKDOWNFEATURE);
    disallowed_raw_HTML_Markdown_feature = MarkdownVariations::new_feature(I"disallowed raw HTML", DISALLOWED_RAW_HTML_MARKDOWNFEATURE);

    GitHub_flavored_Markdown_variation = MarkdownVariations::new(I"GitHub-flavored Markdown 0.29");
    MarkdownVariations::make_GitHub_features_active(GitHub_flavored_Markdown_variation);
}

void MarkdownVariations::make_GitHub_features_active(markdown_variation *variation) {
    MarkdownVariations::add_feature(variation, STRIKETHROUGH_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, TABLES_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, TASK_LIST_ITEMS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, EXTENDED_AUTOLINKS_MARKDOWNFEATURE);
    MarkdownVariations::add_feature(variation, DISALLOWED_RAW_HTML_MARKDOWNFEATURE);
}

§7. Methods for features. New features need to be able to intervene in the parsing or rendering algorithms, so for that we provide methods.

RENDER_MARKDOWN_MTID allows a feature to meddle in how a node is rendered.

enum RENDER_MARKDOWN_MTID

INT_METHOD_TYPE(RENDER_MARKDOWN_MTID, markdown_feature *feature, text_stream *OUT,
    markdown_item *md, int mode)
int MarkdownVariations::intervene_in_rendering?Usage of MarkdownVariations::intervene_in_rendering:
Markdown Rendering - §2(markdown_variation *variation,
    text_stream *OUT, markdown_item *md, int mode) {
    markdown_feature *feature;
    LOOP_OVER(feature, markdown_feature) {
        if (MarkdownVariations::supports(variation, feature->feature_ID)) {
            int rv = FALSE;
            INT_METHOD_CALL(rv, feature, RENDER_MARKDOWN_MTID, OUT, md, mode);
            if (rv) return TRUE;
        }
    }
    return FALSE;
}

§8. POST_PHASE_I_MARKDOWN_MTID allows a feature to restructure or annotate the block tree produced by Phase I parsing; similarly POST_PHASE_II_MARKDOWN_MTID.

enum POST_PHASE_I_MARKDOWN_MTID
enum POST_PHASE_II_MARKDOWN_MTID

VOID_METHOD_TYPE(POST_PHASE_I_MARKDOWN_MTID, markdown_feature *feature,
    markdown_item *tree, md_links_dictionary *link_references)
VOID_METHOD_TYPE(POST_PHASE_II_MARKDOWN_MTID, markdown_feature *feature,
    markdown_item *tree, md_links_dictionary *link_references)
void MarkdownVariations::intervene_after_Phase_I?Usage of MarkdownVariations::intervene_after_Phase_I:
Markdown - §5.1(markdown_variation *variation,
    markdown_item *tree, md_links_dictionary *link_references) {
    markdown_feature *feature;
    LOOP_OVER(feature, markdown_feature) {
        if (MarkdownVariations::supports(variation, feature->feature_ID)) {
            VOID_METHOD_CALL(feature, POST_PHASE_I_MARKDOWN_MTID, tree, link_references);
        }
    }
}
void MarkdownVariations::intervene_after_Phase_II?Usage of MarkdownVariations::intervene_after_Phase_II:
Markdown - §5.2(markdown_variation *variation,
    markdown_item *tree, md_links_dictionary *link_references) {
    markdown_feature *feature;
    LOOP_OVER(feature, markdown_feature) {
        if (MarkdownVariations::supports(variation, feature->feature_ID)) {
            VOID_METHOD_CALL(feature, POST_PHASE_II_MARKDOWN_MTID, tree, link_references);
        }
    }
}

§9. MULTIFILE_MARKDOWN_MTID allows a feature to tell the parser that the content will end up being split across multiple HTML files. If a feature wants to do this, it should then set a "file point" at any top-level positions of its choice, and return TRUE.

enum MULTIFILE_MARKDOWN_MTID

INT_METHOD_TYPE(MULTIFILE_MARKDOWN_MTID, markdown_feature *feature,
    markdown_item *tree, md_links_dictionary *link_references)
int MarkdownVariations::multifile_mode(markdown_variation *variation,
    markdown_item *tree, md_links_dictionary *link_references) {
    if (tree->down) {
        markdown_feature *feature;
        LOOP_OVER(feature, markdown_feature) {
            if (MarkdownVariations::supports(variation, feature->feature_ID)) {
                int rv = FALSE;
                INT_METHOD_CALL(rv, feature, MULTIFILE_MARKDOWN_MTID, tree, link_references);
                if (rv) {
                    MarkdownVariations::assign_URLs_to_headings(tree, link_references);
                    return TRUE;
                }
            }
        }
    }
    return FALSE;
}

void MarkdownVariations::assign_URLs_to_headings(markdown_item *tree,
    md_links_dictionary *link_references) {
    if (tree->down->type != VOLUME_MIT) {
        markdown_item *index = Markdown::new_volume_marker(I"all in one");
        index->next = tree->down; tree->down = index;
    }
    for (markdown_item *md = tree->down; md; md = md->next) {
        if (md->type == VOLUME_MIT) {
            md->down = md->next; md->next = NULL;
            markdown_item *ch = md->down, *prev_ch = NULL;
            while ((ch) && (ch->type != VOLUME_MIT)) { prev_ch = ch, ch = ch->next; }
            if (ch) { prev_ch->next = NULL; md->next = ch; }
        }
    }
    for (markdown_item *vol = tree->down; vol; vol = vol->next) {
        if (vol->type == VOLUME_MIT) {
            if ((vol->down) && (vol->down->type != FILE_MIT)) {
                #ifdef SUPERVISOR_MODULE
                text_stream *home_URL = DocumentationCompiler::home_URL_at_volume_item(vol);
                #endif
                #ifndef SUPERVISOR_MODULE
                text_stream *home_URL = I"index.html";
                #endif
                markdown_item *index = Markdown::new_file_marker(Filenames::from_text(home_URL));
                index->next = vol->down; vol->down = index;
            }
            for (markdown_item *md = vol->down; md; md = md->next) {
                if (md->type == FILE_MIT) {
                    md->down = md->next; md->next = NULL;
                    markdown_item *ch = md->down, *prev_ch = NULL;
                    while ((ch) && (ch->type != FILE_MIT) && (ch->type != VOLUME_MIT)) { prev_ch = ch, ch = ch->next; }
                    if (ch) { prev_ch->next = NULL; md->next = ch; }
                }
            }
        }
    }

    markdown_item *headings[7] = { NULL, NULL, NULL, NULL, NULL, NULL, NULL };
    MarkdownVariations::multifile_r(tree->down, link_references, headings, NULL);
}

§10.

void MarkdownVariations::multifile_r?Usage of MarkdownVariations::multifile_r:
§9(markdown_item *md, md_links_dictionary *link_references,
    markdown_item *headings[7], markdown_item *file_item) {
    int non_heading_found = FALSE;
    for (; md; md = md->next) {
        if (md->type == HEADING_MIT) {
            int L = Markdown::get_heading_level(md);
            headings[L] = md;
            for (int i=L+1; i<=6; i++) headings[i] = NULL;
            text_stream *URL = Str::new();
            if (file_item) {
                WRITE_TO(URL, "%f", Markdown::get_filename(file_item));
            }
            TEMPORARY_TEXT(xref)
            TEMPORARY_TEXT(anchor)
            match_results mr = Regexp::create_mr();
            if (Regexp::match(&mr, md->stashed, U"Chapter (%d+): *(%c*)")) {
                WRITE_TO(xref, "%S", mr.exp[1]);
                WRITE_TO(anchor, "chapter%S", mr.exp[0]);
            } else if (Regexp::match(&mr, md->stashed, U"Chapter (%d+)")) {
                WRITE_TO(xref, "%S", md->stashed);
                WRITE_TO(anchor, "chapter%S", mr.exp[0]);
            } else if (Regexp::match(&mr, md->stashed, U"Section (%d+).(%d+): *(%c*)")) {
                WRITE_TO(xref, "%S", mr.exp[2]);
                WRITE_TO(anchor, "c%Ss%S", mr.exp[0], mr.exp[1]);
            } else if (Regexp::match(&mr, md->stashed, U"Section (%d+).(%d+)")) {
                WRITE_TO(xref, "%S", md->stashed);
                WRITE_TO(anchor, "c%Ss%S", mr.exp[0], mr.exp[1]);
            } else if (Regexp::match(&mr, md->stashed, U"Section (%d+): *(%c*)")) {
                WRITE_TO(xref, "%S", mr.exp[1]);
                WRITE_TO(anchor, "s%S", mr.exp[0]);
            } else if (Regexp::match(&mr, md->stashed, U"Section (%d+)")) {
                WRITE_TO(xref, "%S", md->stashed);
                WRITE_TO(anchor, "s%S", mr.exp[0]);
            } else {
                WRITE_TO(xref, "%S", md->stashed);
                WRITE_TO(anchor, "heading%d", md->id);
            }
            Regexp::dispose_of(&mr);
            if (non_heading_found) {
                WRITE_TO(URL, "#%S", anchor);
            }
            md->user_state = STORE_POINTER_text_stream(URL);
            Markdown::create(link_references, xref, URL, md->stashed);
            DISCARD_TEXT(xref)
            DISCARD_TEXT(anchor)
        } else {
            non_heading_found = TRUE;
        }
        MarkdownVariations::multifile_r(md->down, link_references, headings, (md->type == FILE_MIT)?md:NULL);
    }
}

text_stream *MarkdownVariations::URL_for_heading?Usage of MarkdownVariations::URL_for_heading:
Markdown Rendering - §2.5
Inform-Flavoured Markdown - §1.10, §1.17(markdown_item *md) {
    if ((md) && (md->type == HEADING_MIT) && (Markdown::get_heading_level(md) <= 2))
        if (GENERAL_POINTER_IS_NULL(md->user_state) == FALSE)
            return RETRIEVE_POINTER_text_stream(md->user_state);
    return NULL;
}