path: root/src/inlines.c

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

#include "stmd.h"
#include "html/houdini.h"
#include "utf8.h"
#include "scanners.h"

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

static node_inl *parse_chunk_inlines(chunk *chunk, reference_map *refmap);
static node_inl *parse_inlines_while(subject* subj, int (*f)(subject*));
static int parse_inline(subject* subj, node_inl ** last);

static void subject_from_chunk(subject *e, chunk *chunk, reference_map *refmap);
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 = malloc(len + 1);
        memcpy(new, buf, len + 1);
    }

    return new;
}

static inline node_inl *make_link_(node_inl *label, unsigned char *url, unsigned char *title)
{
    node_inl* e = (node_inl*) malloc(sizeof(node_inl));
    e->tag = INL_LINK;
    e->content.linkable.label = label;
    e->content.linkable.url   = url;
    e->content.linkable.title = title;
    e->next = NULL;
    return e;
}

inline static node_inl* make_ref_link(node_inl* label, reference *ref)
{
    return make_link_(label, bufdup(ref->url), bufdup(ref->title));
}

inline static node_inl* make_autolink(node_inl* label, chunk url, int is_email)
{
    return make_link_(label, clean_autolink(&url, is_email), NULL);
}

// Create an inline with a linkable string value.
inline static node_inl* make_link(node_inl* label, chunk url, chunk title)
{
    return make_link_(label, clean_url(&url), clean_title(&title));
}

inline static node_inl* make_inlines(int t, node_inl* contents)
{
    node_inl* e = (node_inl*) malloc(sizeof(node_inl));
    e->tag = t;
    e->content.inlines = contents;
    e->next = NULL;
    return e;
}

// Create an inline with a literal string value.
inline static node_inl* make_literal(int t, chunk s)
{
    node_inl* e = (node_inl*) malloc(sizeof(node_inl));
    e->tag = t;
    e->content.literal = s;
    e->next = NULL;
    return e;
}

// Create an inline with no value.
inline static node_inl* make_simple(int t)
{
    node_inl* e = (node_inl*) malloc(sizeof(node_inl));
    e->tag = t;
    e->next = NULL;
    return e;
}

// Macros for creating various kinds of inlines.
#define make_str(s) make_literal(INL_STRING, s)
#define make_code(s) make_literal(INL_CODE, s)
#define make_raw_html(s) make_literal(INL_RAW_HTML, s)
#define make_linebreak() make_simple(INL_LINEBREAK)
#define make_softbreak() make_simple(INL_SOFTBREAK)
#define make_emph(contents) make_inlines(INL_EMPH, contents)
#define make_strong(contents) make_inlines(INL_STRONG, contents)

// Free an inline list.
extern void free_inlines(node_inl* e)
{
    node_inl * next;
    while (e != NULL) {
        switch (e->tag){
            case INL_STRING:
            case INL_RAW_HTML:
            case INL_CODE:
                chunk_free(&e->content.literal);
                break;
            case INL_LINEBREAK:
            case INL_SOFTBREAK:
                break;
            case INL_LINK:
            case INL_IMAGE:
                free(e->content.linkable.url);
                free(e->content.linkable.title);
                free_inlines(e->content.linkable.label);
                break;
            case INL_EMPH:
            case INL_STRONG:
                free_inlines(e->content.inlines);
                break;
            default:
                break;
        }
        next = e->next;
        free(e);
        e = next;
    }
}

// Append inline list b to the end of inline list a.
// Return pointer to head of new list.
inline static node_inl* append_inlines(node_inl* a, node_inl* b)
{
    if (a == NULL) {  // NULL acts like an empty list
        return b;
    }
    node_inl* cur = a;
    while (cur->next) {
        cur = cur->next;
    }
    cur->next = b;
    return a;
}

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->label_nestlevel = 0;
    e->refmap = refmap;

    chunk_rtrim(&e->input);
}

static void subject_from_chunk(subject *e, chunk *chunk, reference_map *refmap)
{
    e->input.data = chunk->data;
    e->input.len = chunk->len;
    e->input.alloc = 0;
    e->pos = 0;
    e->label_nestlevel = 0;
    e->refmap = refmap;

    chunk_rtrim(&e->input);
}

inline static 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.
inline static 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.
inline static 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
    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.
// Don't advance position.
static int scan_delims(subject* subj, char c, bool * can_open, bool * can_close)
{
    int numdelims = 0;
    char char_before, char_after;
    int startpos = subj->pos;

    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 && numdelims <= 3 && !isspace(char_after);
    *can_close = numdelims > 0 && numdelims <= 3 && !isspace(char_before);
    if (c == '_') {
        *can_open = *can_open && !isalnum(char_before);
        *can_close = *can_close && !isalnum(char_after);
    }
    subj->pos = startpos;
    return numdelims;
}

// Parse strong/emph or a fallback.
// Assumes the subject has '_' or '*' at the current position.
static node_inl* handle_strong_emph(subject* subj, char c)
{
    bool can_open, can_close;
    node_inl * result = NULL;
    node_inl ** last = malloc(sizeof(node_inl *));
    node_inl * new;
    node_inl * il;
    node_inl * first_head = NULL;
    node_inl * first_close = NULL;
    int first_close_delims = 0;
    int numdelims;

    *last = NULL;

    numdelims = scan_delims(subj, c, &can_open, &can_close);
    subj->pos += numdelims;

    new = make_str(chunk_dup(&subj->input, subj->pos - numdelims, numdelims));
    *last = new;
    first_head = new;
    result = new;

    if (!can_open || numdelims == 0) {
        goto done;
    }

    switch (numdelims) {
        case 1:
            while (true) {
                numdelims = scan_delims(subj, c, &can_open, &can_close);
                if (numdelims >= 1 && can_close) {
                    subj->pos += 1;
                    first_head->tag = INL_EMPH;
                    chunk_free(&first_head->content.literal);
                    first_head->content.inlines = first_head->next;
                    first_head->next = NULL;
                    goto done;
                } else {
                    if (!parse_inline(subj, last)) {
                        goto done;
                    }
                }
            }
            break;
        case 2:
            while (true) {
                numdelims = scan_delims(subj, c, &can_open, &can_close);
                if (numdelims >= 2 && can_close) {
                    subj->pos += 2;
                    first_head->tag = INL_STRONG;
                    chunk_free(&first_head->content.literal);
                    first_head->content.inlines = first_head->next;
                    first_head->next = NULL;
                    goto done;
                } else {
                    if (!parse_inline(subj, last)) {
                        goto done;
                    }
                }
            }
            break;
        case 3:
            while (true) {
                numdelims = scan_delims(subj, c, &can_open, &can_close);
                if (can_close && numdelims >= 1 && numdelims <= 3 &&
                        numdelims != first_close_delims) {
                    new = make_str(chunk_dup(&subj->input, subj->pos, numdelims));
                    append_inlines(*last, new);
                    *last = new;
                    if (first_close_delims == 1 && numdelims > 2) {
                        numdelims = 2;
                    } else if (first_close_delims == 2) {
                        numdelims = 1;
                    } else if (numdelims == 3) {
                        // If we opened with ***, we interpret it as ** followed by *
                        // giving us <strong><em>
                        numdelims = 1;
                    }
                    subj->pos += numdelims;
                    if (first_close) {
                        first_head->tag = first_close_delims == 1 ? INL_STRONG : INL_EMPH;
                        chunk_free(&first_head->content.literal);
                        first_head->content.inlines =
                            make_inlines(first_close_delims == 1 ? INL_EMPH : INL_STRONG,
                                    first_head->next);

                        il = first_head->next;
                        while (il->next && il->next != first_close) {
                            il = il->next;
                        }
                        il->next = NULL;

                        first_head->content.inlines->next = first_close->next;

                        il = first_head->content.inlines;
                        while (il->next && il->next != *last) {
                            il = il->next;
                        }
                        il->next = NULL;
                        free_inlines(*last);

                        first_close->next = NULL;
                        free_inlines(first_close);
                        first_head->next = NULL;
                        goto done;
                    } else {
                        first_close = *last;
                        first_close_delims = numdelims;
                    }
                } else {
                    if (!parse_inline(subj, last)) {
                        goto done;
                    }
                }
            }
            break;
        default:
            goto done;
    }

done:
    free(last);
    return result;
}

// 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_autolink(chunk *url, int is_email)
{
    strbuf buf = GH_BUF_INIT;

    chunk_trim(url);

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

    if (is_email)
        strbuf_puts(&buf, "mailto:");

    houdini_unescape_html_f(&buf, url->data, url->len);
    return strbuf_detach(&buf);
}

// Clean a title: remove surrounding quotes and remove \ that escape punctuation.
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.
// Unless raw_label is null, it is set to point to the raw contents of the [].
// Assumes the subject has a '[' character at the current position.
// Returns 0 and does not advance if no matching ] is found.
// Note the precedence:  code backticks have precedence over label bracket
// markers, which have precedence over *, _, and other inline formatting
// markers. So, 2 below contains a link while 1 does not:
// 1. [a link `with a ](/url)` character
// 2. [a link *with emphasized ](/url) text*
static int link_label(subject* subj, chunk *raw_label)
{
    int nestlevel = 0;
    node_inl* tmp = NULL;
    int startpos = subj->pos;

    if (subj->label_nestlevel) {
        // if we've already checked to the end of the subject
        // for a label, even with a different starting [, we
        // know we won't find one here and we can just return.
        // Note:  nestlevel 1 would be: [foo [bar]
        // nestlevel 2 would be: [foo [bar [baz]
        subj->label_nestlevel--;
        return 0;
    }

    advance(subj);  // advance past [
    char c;
    while ((c = peek_char(subj)) && (c != ']' || nestlevel > 0)) {
        switch (c) {
            case '`':
                tmp = handle_backticks(subj);
                free_inlines(tmp);
                break;
            case '<':
                tmp = handle_pointy_brace(subj);
                free_inlines(tmp);
                break;
            case '[':  // nested []
                nestlevel++;
                advance(subj);
                break;
            case ']':  // nested []
                nestlevel--;
                advance(subj);
                break;
            case '\\':
                advance(subj);
                if (ispunct(peek_char(subj))) {
                    advance(subj);
                }
                break;
            default:
                advance(subj);
        }
    }
    if (c == ']') {
        *raw_label = chunk_dup(&subj->input, startpos + 1, subj->pos - (startpos + 1));
        subj->label_nestlevel = 0;
        advance(subj);  // advance past ]
        return 1;
    } else {
        if (c == 0) {
            subj->label_nestlevel = nestlevel;
        }
        subj->pos = startpos; // rewind
        return 0;
    }
}

// Parse a link or the link portion of an image, or return a fallback.
static node_inl* handle_left_bracket(subject* subj)
{
    node_inl *lab = NULL;
    node_inl *result = NULL;
    reference *ref;
    int n;
    int sps;
    int found_label;
    int endlabel, starturl, endurl, starttitle, endtitle, endall;

    chunk rawlabel;
    chunk url, title;

    found_label = link_label(subj, &rawlabel);
    endlabel = subj->pos;

    if (found_label) {
        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;

                url = chunk_dup(&subj->input, starturl, endurl - starturl);
                title = chunk_dup(&subj->input, starttitle, endtitle - starttitle);
                lab = parse_chunk_inlines(&rawlabel, NULL);

                return make_link(lab, url, title);
            } else {
                // if we get here, we matched a label but didn't get further:
                subj->pos = endlabel;
                lab = parse_chunk_inlines(&rawlabel, subj->refmap);
                result = append_inlines(make_str(chunk_literal("[")),
                        append_inlines(lab,
                            make_str(chunk_literal("]"))));
                return result;
            }
        } else {
            chunk rawlabel_tmp;
            chunk reflabel;

            // Check for reference link.
            // First, see if there's another label:
            subj->pos = subj->pos + scan_spacechars(&subj->input, endlabel);
            reflabel = rawlabel;

            // if followed by a nonempty link label, we change reflabel to it:
            if (peek_char(subj) == '[' && link_label(subj, &rawlabel_tmp)) {
                if (rawlabel_tmp.len > 0)
                    reflabel = rawlabel_tmp;
            } else {
                subj->pos = endlabel;
            }

            // lookup rawlabel in subject->reference_map:
            ref = reference_lookup(subj->refmap, &reflabel);
            if (ref != NULL) { // found
                lab = parse_chunk_inlines(&rawlabel, NULL);
                result = make_ref_link(lab, ref);
            } else {
                subj->pos = endlabel;
                lab = parse_chunk_inlines(&rawlabel, subj->refmap);
                result = append_inlines(make_str(chunk_literal("[")),
                        append_inlines(lab, make_str(chunk_literal("]"))));
            }
            return result;
        }
    }
    // If we fall through to here, it means we didn't match a link:
    advance(subj);  // advance past [
    return make_str(chunk_literal("["));
}

// 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();
    }
}

inline static int not_eof(subject* subj)
{
    return !is_eof(subj);
}

// Parse inlines while a predicate is satisfied.  Return inlines.
extern node_inl* parse_inlines_while(subject* subj, int (*f)(subject*))
{
    node_inl* result = NULL;
    node_inl** last = &result;
    while ((*f)(subj) && parse_inline(subj, last)) {
    }
    return result;
}

node_inl *parse_chunk_inlines(chunk *chunk, reference_map *refmap)
{
    subject subj;
    subject_from_chunk(&subj, chunk, refmap);
    return parse_inlines_while(&subj, not_eof);
}

static int subject_find_special_char(subject *subj)
{
    int n = subj->pos + 1;

    while (n < subj->input.len) {
        if (strchr("\n\\`&_*[]<!", 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 '_':
            if (subj->pos > 0) {
                unsigned char prev = peek_at(subj, subj->pos - 1);
                if (isalnum(prev) || prev == '_') {
                    new = make_str(chunk_literal("_"));
                    advance(subj);
                    break;
                }
            }

            new = handle_strong_emph(subj, '_');
            break;
        case '*':
            new = handle_strong_emph(subj, '*');
            break;
        case '[':
            new = handle_left_bracket(subj);
            break;
        case '!':
            advance(subj);
            if (peek_char(subj) == '[') {
                new = handle_left_bracket(subj);
                if (new != NULL && new->tag == INL_LINK) {
                    new->tag = INL_IMAGE;
                } else {
                    new = append_inlines(make_str(chunk_literal("!")), 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 {
        append_inlines(*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_while(&subj, not_eof);
}

// Parse zero or more space characters, including at most one newline.
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;
}