path: root/src/inlines.c

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <stdbool.h>
#include <ctype.h>

#include "cmark.h"
#include "html/houdini.h"
#include "utf8.h"
#include "scanners.h"
#include "ast.h"
#include "inlines.h"


typedef struct DelimiterStack {
    struct DelimiterStack *previous;
    struct DelimiterStack *next;
    node_inl *first_inline;
    int delim_count;
    unsigned char delim_char;
    int position;
    bool can_open;
    bool can_close;
} delimiter_stack;

typedef struct Subject {
    chunk input;
    int pos;
    reference_map *refmap;
    delimiter_stack *delimiters;
} subject;

static node_inl *parse_inlines_from_subject(subject* subj);
static int parse_inline(subject* subj, node_inl ** last);

static void subject_from_buf(subject *e, strbuf *buffer, reference_map *refmap);
static int subject_find_special_char(subject *subj);

static unsigned char *bufdup(const unsigned char *buf)
{
    unsigned char *new = NULL;

    if (buf) {
        int len = strlen((char *)buf);
        new = calloc(len + 1, sizeof(*new));
        if(new != NULL) {
            memcpy(new, buf, len + 1);
        }
    }

    return new;
}

static void subject_from_buf(subject *e, strbuf *buffer, reference_map *refmap)
{
    e->input.data = buffer->ptr;
    e->input.len = buffer->size;
    e->input.alloc = 0;
    e->pos = 0;
    e->refmap = refmap;
    e->delimiters = NULL;

    chunk_rtrim(&e->input);
}

static inline int isbacktick(int c)
{
    return (c == '`');
}

static inline unsigned char peek_char(subject *subj)
{
    return (subj->pos < subj->input.len) ? subj->input.data[subj->pos] : 0;
}

static inline unsigned char peek_at(subject *subj, int pos)
{
    return subj->input.data[pos];
}

// Return true if there are more characters in the subject.
static inline int is_eof(subject* subj)
{
    return (subj->pos >= subj->input.len);
}

// Advance the subject.  Doesn't check for eof.
#define advance(subj) (subj)->pos += 1

// Take characters while a predicate holds, and return a string.
static inline chunk take_while(subject* subj, int (*f)(int))
{
    unsigned char c;
    int startpos = subj->pos;
    int len = 0;

    while ((c = peek_char(subj)) && (*f)(c)) {
        advance(subj);
        len++;
    }

    return chunk_dup(&subj->input, startpos, len);
}

// Try to process a backtick code span that began with a
// span of ticks of length openticklength length (already
// parsed).  Return 0 if you don't find matching closing
// backticks, otherwise return the position in the subject
// after the closing backticks.
static int scan_to_closing_backticks(subject* subj, int openticklength)
{
    // read non backticks
    unsigned char c;
    while ((c = peek_char(subj)) && c != '`') {
        advance(subj);
    }
    if (is_eof(subj)) {
        return 0;  // did not find closing ticks, return 0
    }
    int numticks = 0;
    while (peek_char(subj) == '`') {
        advance(subj);
        numticks++;
    }
    if (numticks != openticklength){
        return(scan_to_closing_backticks(subj, openticklength));
    }
    return (subj->pos);
}

// Parse backtick code section or raw backticks, return an inline.
// Assumes that the subject has a backtick at the current position.
static node_inl* handle_backticks(subject *subj)
{
    chunk openticks = take_while(subj, isbacktick);
    int startpos = subj->pos;
    int endpos = scan_to_closing_backticks(subj, openticks.len);

    if (endpos == 0) { // not found
        subj->pos = startpos; // rewind
        return make_str(openticks);
    } else {
        strbuf buf = GH_BUF_INIT;

        strbuf_set(&buf, subj->input.data + startpos, endpos - startpos - openticks.len);
        strbuf_trim(&buf);
        strbuf_normalize_whitespace(&buf);

        return make_code(chunk_buf_detach(&buf));
    }
}

// Scan ***, **, or * and return number scanned, or 0.
// Advances position.
static int scan_delims(subject* subj, unsigned char c, bool * can_open, bool * can_close)
{
    int numdelims = 0;
    unsigned char char_before, char_after;

    char_before = subj->pos == 0 ? '\n' : peek_at(subj, subj->pos - 1);
    while (peek_char(subj) == c) {
        numdelims++;
        advance(subj);
    }
    char_after = peek_char(subj);
    *can_open = numdelims > 0 && !isspace(char_after);
    *can_close = numdelims > 0 && !isspace(char_before);
    if (c == '_') {
        *can_open = *can_open && !isalnum(char_before);
        *can_close = *can_close && !isalnum(char_after);
    }
    return numdelims;
}

/*
static void print_delimiters(subject *subj)
{
    delimiter_stack *tempstack;
    tempstack = subj->delimiters;
    while (tempstack != NULL) {
        printf("Item at %p: %d %d %d %d next(%p) prev(%p)\n",
               tempstack, tempstack->delim_count, tempstack->delim_char,
               tempstack->can_open, tempstack->can_close,
               tempstack->next, tempstack->previous);
        tempstack = tempstack->previous;
    }
}
*/

static void remove_delimiter(subject *subj, delimiter_stack *stack)
{
    if (stack->previous != NULL) {
        stack->previous->next = stack->next;
    }
    if (stack->next == NULL) {
        // top of stack
        subj->delimiters = stack->previous;
    } else {
        stack->next->previous = stack->previous;
    }
    free(stack);
}

static delimiter_stack * push_delimiter(subject *subj,
                    int numdelims,
                    unsigned char c,
                    bool can_open,
                    bool can_close,
                    node_inl *inl_text)
{
    delimiter_stack *istack =
        (delimiter_stack*)malloc(sizeof(delimiter_stack));
    if (istack == NULL) {
        return NULL;
    }
    istack->delim_count = numdelims;
    istack->delim_char = c;
    istack->can_open = can_open;
    istack->can_close = can_close;
    istack->first_inline = inl_text;
    istack->previous = subj->delimiters;
    istack->next = NULL;
    if (istack->previous != NULL) {
        istack->previous->next = istack;
    }
    istack->position = subj->pos;
    return istack;
}

// Parse strong/emph or a fallback.
// Assumes the subject has '_' or '*' at the current position.
static node_inl* handle_strong_emph(subject* subj, unsigned char c, node_inl **last)
{
    int numdelims;
    node_inl * inl_text;
    bool can_open, can_close;

    numdelims = scan_delims(subj, c, &can_open, &can_close);

    inl_text = make_str(chunk_dup(&subj->input, subj->pos - numdelims, numdelims));

    if (can_open || can_close) {
        subj->delimiters = push_delimiter(subj, numdelims, c, can_open, can_close,
                          inl_text);
    }

    return inl_text;
}

static void process_emphasis(subject *subj, delimiter_stack *stack_bottom)
{
    delimiter_stack *closer = subj->delimiters;
    delimiter_stack *opener, *tempstack, *nextstack;
    int use_delims;
    node_inl *inl, *tmp, *emph;

    // move back to first relevant delim.
    while (closer != NULL && closer->previous != stack_bottom) {
        closer = closer->previous;
    }

    // now move forward, looking for closers, and handling each
    while (closer != NULL) {
        if (closer->can_close &&
            (closer->delim_char == '*' || closer->delim_char == '_')) {
            // Now look backwards for first matching opener:
            opener = closer->previous;
            while (opener != NULL && opener != stack_bottom) {
                if (opener->delim_char == closer->delim_char &&
                    opener->can_open) {
                    break;
                }
                opener = opener->previous;
            }
            if (opener != NULL && opener != stack_bottom) {
                // calculate the actual number of delimeters used from this closer
                if (closer->delim_count < 3 || opener->delim_count < 3) {
                    use_delims = closer->delim_count <= opener->delim_count ?
                        closer->delim_count : opener->delim_count;
                } else { // closer and opener both have >= 3 delims
                    use_delims = closer->delim_count % 2 == 0 ? 2 : 1;
                }

                inl = opener->first_inline;

                // remove used delimiters from stack elements and associated inlines.
                opener->delim_count -= use_delims;
                closer->delim_count -= use_delims;
                inl->content.literal.len = opener->delim_count;
                closer->first_inline->content.literal.len = closer->delim_count;

                // free delimiters between opener and closer
                tempstack = closer->previous;
                while (tempstack != NULL && tempstack != opener) {
                    nextstack = tempstack->previous;
                    remove_delimiter(subj, tempstack);
                    tempstack = nextstack;
                }

                // create new emph or strong, and splice it in to our inlines
                // between the opener and closer
                emph = use_delims == 1 ? make_emph(inl->next) : make_strong(inl->next);
                emph->next = closer->first_inline;
                inl->next = emph;
                tmp = emph->content.inlines;
                while (tmp->next != NULL && tmp->next != closer->first_inline) {
                    tmp = tmp->next;
                }
                tmp->next = NULL;

                // if opener has 0 delims, remove it and its associated inline
                if (opener->delim_count == 0) {
                    // replace empty opener inline with emph
                    chunk_free(&(inl->content.literal));
                    inl->tag = emph->tag;
                    inl->next = emph->next;
                    inl->content.inlines = emph->content.inlines;
                    free(emph);
                    emph = inl;
                    // remove opener from stack
                    remove_delimiter(subj, opener);
                }

                // if closer has 0 delims, remove it and its associated inline
                if (closer->delim_count == 0) {
                    // remove empty closer inline
                    tmp = closer->first_inline;
                    emph->next = tmp->next;
                    tmp->next = NULL;
                    cmark_free_inlines(tmp);
                    // remove closer from stack
                    tempstack = closer->next;
                    remove_delimiter(subj, closer);
                    closer = tempstack;
                }
            } else {
                closer = closer->next;
            }
        } else {
            closer = closer->next;
        }
    }
    // free all delimiters in stack down to stack_bottom:
    while (subj->delimiters != stack_bottom) {
        remove_delimiter(subj, subj->delimiters);
    }
}

// Parse backslash-escape or just a backslash, returning an inline.
static node_inl* handle_backslash(subject *subj)
{
    advance(subj);
    unsigned char nextchar = peek_char(subj);
    if (ispunct(nextchar)) {  // only ascii symbols and newline can be escaped
        advance(subj);
        return make_str(chunk_dup(&subj->input, subj->pos - 1, 1));
    } else if (nextchar == '\n') {
        advance(subj);
        return make_linebreak();
    } else {
        return make_str(chunk_literal("\\"));
    }
}

// Parse an entity or a regular "&" string.
// Assumes the subject has an '&' character at the current position.
static node_inl* handle_entity(subject* subj)
{
    strbuf ent = GH_BUF_INIT;
    size_t len;

    advance(subj);

    len = houdini_unescape_ent(&ent,
                   subj->input.data + subj->pos,
                   subj->input.len - subj->pos
                   );

    if (len == 0)
        return make_str(chunk_literal("&"));

    subj->pos += len;
    return make_str(chunk_buf_detach(&ent));
}

// Like make_str, but parses entities.
// Returns an inline sequence consisting of str and entity elements.
static node_inl *make_str_with_entities(chunk *content)
{
    strbuf unescaped = GH_BUF_INIT;

    if (houdini_unescape_html(&unescaped, content->data, (size_t)content->len)) {
        return make_str(chunk_buf_detach(&unescaped));
    } else {
        return make_str(*content);
    }
}

// Clean a URL: remove surrounding whitespace and surrounding <>,
// and remove \ that escape punctuation.
unsigned char *clean_url(chunk *url)
{
    strbuf buf = GH_BUF_INIT;

    chunk_trim(url);

    if (url->len == 0)
        return NULL;

    if (url->data[0] == '<' && url->data[url->len - 1] == '>') {
        houdini_unescape_html_f(&buf, url->data + 1, url->len - 2);
    } else {
        houdini_unescape_html_f(&buf, url->data, url->len);
    }

    strbuf_unescape(&buf);
    return strbuf_detach(&buf);
}

unsigned char *clean_title(chunk *title)
{
       strbuf buf = GH_BUF_INIT;
       unsigned char first, last;

       if (title->len == 0)
               return NULL;

       first = title->data[0];
       last = title->data[title->len - 1];

       // remove surrounding quotes if any:
       if ((first == '\'' && last == '\'') ||
           (first == '(' && last == ')') ||
           (first == '"' && last == '"')) {
               houdini_unescape_html_f(&buf, title->data + 1, title->len - 2);
       } else {
               houdini_unescape_html_f(&buf, title->data, title->len);
       }

       strbuf_unescape(&buf);
       return strbuf_detach(&buf);
}

// Parse an autolink or HTML tag.
// Assumes the subject has a '<' character at the current position.
static node_inl* handle_pointy_brace(subject* subj)
{
    int matchlen = 0;
    chunk contents;

    advance(subj);  // advance past first <

    // first try to match a URL autolink
    matchlen = scan_autolink_uri(&subj->input, subj->pos);
    if (matchlen > 0) {
        contents = chunk_dup(&subj->input, subj->pos, matchlen - 1);
        subj->pos += matchlen;

        return make_autolink(
                     make_str_with_entities(&contents),
                     contents, 0
                     );
    }

    // next try to match an email autolink
    matchlen = scan_autolink_email(&subj->input, subj->pos);
    if (matchlen > 0) {
        contents = chunk_dup(&subj->input, subj->pos, matchlen - 1);
        subj->pos += matchlen;

        return make_autolink(
                     make_str_with_entities(&contents),
                     contents, 1
                     );
    }

    // finally, try to match an html tag
    matchlen = scan_html_tag(&subj->input, subj->pos);
    if (matchlen > 0) {
        contents = chunk_dup(&subj->input, subj->pos - 1, matchlen + 1);
        subj->pos += matchlen;
        return make_raw_html(contents);
    }

    // if nothing matches, just return the opening <:
    return make_str(chunk_literal("<"));
}

// 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, chunk *raw_label)
{
    int startpos = subj->pos;
    int length = 0;

    advance(subj);  // advance past [
    unsigned char c;
    while ((c = peek_char(subj)) && c != '[' && c != ']') {
        if (c == '\\') {
            advance(subj);
            length++;
            if (ispunct(peek_char(subj))) {
                advance(subj);
                length++;
            }
        } else {
            advance(subj);
            length++;
        }
        if (length > MAX_LINK_LABEL_LENGTH) {
            goto noMatch;
        }
    }

    if (c == ']') { // match found
        *raw_label = chunk_dup(&subj->input, startpos + 1, subj->pos - (startpos + 1));
        advance(subj);  // advance past ]
        return 1;
    }

 noMatch:
    subj->pos = startpos; // rewind
    return 0;

}

// Return a link, an image, or a literal close bracket.
static node_inl* handle_close_bracket(subject* subj, node_inl **last)
{
    int initial_pos;
    int starturl, endurl, starttitle, endtitle, endall;
    int n;
    int sps;
    reference *ref;
    bool is_image = false;
    chunk urlchunk, titlechunk;
    unsigned char *url, *title;
    delimiter_stack *opener;
    delimiter_stack *tempstack;
    node_inl *link_text;
    node_inl *inl;
    chunk raw_label;

    advance(subj);  // advance past ]
    initial_pos = subj->pos;

    // look through stack of delimiters for a [ or !
    opener = subj->delimiters;
    while (opener) {
        if (opener->delim_char == '[' || opener->delim_char == '!') {
            break;
        }
        opener = opener->previous;
    }

    if (opener == NULL) {
        return make_str(chunk_literal("]"));
    }

    // If we got here, we matched a potential link/image text.
    is_image = opener->delim_char == '!';
    link_text = opener->first_inline->next;

    // Now we check to see if it's a link/image.

    // First, look for an inline link.
    if (peek_char(subj) == '(' &&
        ((sps = scan_spacechars(&subj->input, subj->pos + 1)) > -1) &&
        ((n = scan_link_url(&subj->input, subj->pos + 1 + sps)) > -1)) {

        // try to parse an explicit link:
        starturl = subj->pos + 1 + sps; // after (
        endurl = starturl + n;
        starttitle = endurl + scan_spacechars(&subj->input, endurl);

        // ensure there are spaces btw url and title
        endtitle = (starttitle == endurl) ? starttitle :
            starttitle + scan_link_title(&subj->input, starttitle);

        endall = endtitle + scan_spacechars(&subj->input, endtitle);

        if (peek_at(subj, endall) == ')') {
            subj->pos = endall + 1;

            urlchunk = chunk_dup(&subj->input, starturl, endurl - starturl);
            titlechunk = chunk_dup(&subj->input, starttitle, endtitle - starttitle);
            url = clean_url(&urlchunk);
            title = clean_title(&titlechunk);
            chunk_free(&urlchunk);
            chunk_free(&titlechunk);
            goto match;

        } else {
            goto noMatch;
        }
    }

    // Next, look for a following [link label] that matches in refmap.
    // skip spaces
    subj->pos = subj->pos + scan_spacechars(&subj->input, subj->pos);
    raw_label = chunk_literal("");
    if (!link_label(subj, &raw_label) || raw_label.len == 0) {
        chunk_free(&raw_label);
        raw_label = chunk_dup(&subj->input, opener->position,
                      initial_pos - opener->position - 1);
    }

    ref = reference_lookup(subj->refmap, &raw_label);
    chunk_free(&raw_label);

    if (ref != NULL) { // found
        url = bufdup(ref->url);
        title = bufdup(ref->title);
        goto match;
    } else {
        goto noMatch;
    }

noMatch:
    // If we fall through to here, it means we didn't match a link:
    remove_delimiter(subj, opener);  // remove this opener from delimiter stack
    subj->pos = initial_pos;
    return make_str(chunk_literal("]"));

match:
    inl = opener->first_inline;
    inl->tag = is_image ? INL_IMAGE : INL_LINK;
    chunk_free(&inl->content.literal);
    inl->content.linkable.label = link_text;
    process_emphasis(subj, opener->previous);
    inl->content.linkable.url   = url;
    inl->content.linkable.title = title;
    inl->next = NULL;
    *last = inl;

    // process_emphasis will remove this delimiter and all later ones.
    // Now, if we have a link, we also want to remove earlier link
        // delimiters. (This code can be removed if we decide to allow links
    // inside links.)
    if (!is_image) {
        opener = subj->delimiters;
        while (opener != NULL) {
            tempstack = opener->previous;
            if (opener->delim_char == '[') {
                remove_delimiter(subj, opener);
            }
            opener = tempstack;
        }
    }

    return NULL;
}

// Parse a hard or soft linebreak, returning an inline.
// Assumes the subject has a newline at the current position.
static node_inl* handle_newline(subject *subj)
{
    int nlpos = subj->pos;
    // skip over newline
    advance(subj);
    // skip spaces at beginning of line
    while (peek_char(subj) == ' ') {
        advance(subj);
    }
    if (nlpos > 1 &&
        peek_at(subj, nlpos - 1) == ' ' &&
        peek_at(subj, nlpos - 2) == ' ') {
        return make_linebreak();
    } else {
        return make_softbreak();
    }
}

// Parse inlines til end of subject, returning inlines.
extern node_inl* parse_inlines_from_subject(subject* subj)
{
    node_inl* result = NULL;
    node_inl** last = &result;
    node_inl* first = NULL;
    while (!is_eof(subj) && parse_inline(subj, last)) {
        if (!first) {
            first = *last;
        }
    }

    process_emphasis(subj, NULL);

    return first;
}

static int subject_find_special_char(subject *subj)
{
    // "\n\\`&_*[]<!"
    static const int8_t SPECIAL_CHARS[256] = {
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1,
        1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

    int n = subj->pos + 1;

    while (n < subj->input.len) {
        if (SPECIAL_CHARS[subj->input.data[n]])
            return n;
        n++;
    }

    return subj->input.len;
}

// Parse an inline, advancing subject, and add it to last element.
// Adjust tail to point to new last element of list.
// Return 0 if no inline can be parsed, 1 otherwise.
static int parse_inline(subject* subj, node_inl ** last)
{
    node_inl* new = NULL;
    chunk contents;
    unsigned char c;
    int endpos;
    c = peek_char(subj);
    if (c == 0) {
        return 0;
    }
    switch(c){
    case '\n':
        new = handle_newline(subj);
        break;
    case '`':
        new = handle_backticks(subj);
        break;
    case '\\':
        new = handle_backslash(subj);
        break;
    case '&':
        new = handle_entity(subj);
        break;
    case '<':
        new = handle_pointy_brace(subj);
        break;
    case '*':
    case '_':
        new = handle_strong_emph(subj, c, last);
        break;
    case '[':
        advance(subj);
        new = make_str(chunk_literal("["));
        subj->delimiters = push_delimiter(subj, 1, '[', true, false, new);
        break;
    case ']':
        new = handle_close_bracket(subj, last);
        break;
    case '!':
        advance(subj);
        if (peek_char(subj) == '[') {
            advance(subj);
            new = make_str(chunk_literal("!["));
            subj->delimiters = push_delimiter(subj, 1, '!', false, true, new);
        } else {
            new = make_str(chunk_literal("!"));
        }
        break;
    default:
        endpos = subject_find_special_char(subj);
        contents = chunk_dup(&subj->input, subj->pos, endpos - subj->pos);
        subj->pos = endpos;

        // if we're at a newline, strip trailing spaces.
        if (peek_char(subj) == '\n') {
            chunk_rtrim(&contents);
        }

        new = make_str(contents);
    }
    if (*last == NULL) {
        *last = new;
    } else if (new) {
        cmark_append_inlines(*last, new);
        *last = new;
    }

    return 1;
}

extern node_inl* parse_inlines(strbuf *input, reference_map *refmap)
{
    subject subj;
    subject_from_buf(&subj, input, refmap);
    return parse_inlines_from_subject(&subj);
}

// Parse zero or more space characters, including at most one newline.
static void spnl(subject* subj)
{
    bool seen_newline = false;
    while (peek_char(subj) == ' ' ||
           (!seen_newline &&
        (seen_newline = peek_char(subj) == '\n'))) {
        advance(subj);
    }
}

// Parse reference.  Assumes string begins with '[' character.
// Modify refmap if a reference is encountered.
// Return 0 if no reference found, otherwise position of subject
// after reference is parsed.
int parse_reference_inline(strbuf *input, reference_map *refmap)
{
    subject subj;

    chunk lab;
    chunk url;
    chunk title;

    int matchlen = 0;
    int beforetitle;

    subject_from_buf(&subj, input, NULL);

    // parse label:
    if (!link_label(&subj, &lab))
        return 0;

    // colon:
    if (peek_char(&subj) == ':') {
        advance(&subj);
    } else {
        return 0;
    }

    // parse link url:
    spnl(&subj);
    matchlen = scan_link_url(&subj.input, subj.pos);
    if (matchlen) {
        url = chunk_dup(&subj.input, subj.pos, matchlen);
        subj.pos += matchlen;
    } else {
        return 0;
    }

    // parse optional link_title
    beforetitle = subj.pos;
    spnl(&subj);
    matchlen = scan_link_title(&subj.input, subj.pos);
    if (matchlen) {
        title = chunk_dup(&subj.input, subj.pos, matchlen);
        subj.pos += matchlen;
    } else {
        subj.pos = beforetitle;
        title = chunk_literal("");
    }
    // parse final spaces and newline:
    while (peek_char(&subj) == ' ') {
        advance(&subj);
    }
    if (peek_char(&subj) == '\n') {
        advance(&subj);
    } else if (peek_char(&subj) != 0) {
        return 0;
    }
    // insert reference into refmap
    reference_create(refmap, &lab, &url, &title);
    return subj.pos;
}