Fix footnote reference label text, and add multiple backrefs.

When a footnote is referenced multiple times, we now insert multiple
backrefs linking back to each reference. In order to do this, we had to
change how footnote ref link labels work away from an incrementing
index, and instead use footnote reference label text *plus* an index.

Author: phillmv

src/blocks.c

@@ -485,6 +485,17 @@ static void process_footnotes(cmark_parser *parser) {
         if (!footnote->ix)
           footnote->ix = ++ix;
 
+        // keep track of a) how many times this footnote def has been
+        // referenced, and b) which reference count this footnote ref is at
+        // this is used by renderers when generating links and backreferences.
+        cur->footnote.ix = ++footnote->node->footnote.count;
+
+        // store the footnote reference text label in the footnote ref's node's
+        // `user_data`, so that renderers can use the label when generating
+        // links and backreferences.
+        cur->user_data = parser->mem->calloc(1, (sizeof(char) * cur->as.literal.len) + 1);
+        memmove(cur->user_data, cur->as.literal.data, cur->as.literal.len);
+
         char n[32];
         snprintf(n, sizeof(n), "%d", footnote->ix);
         cmark_chunk_free(parser->mem, &cur->as.literal);

src/commonmark.c

@@ -477,7 +477,7 @@ static int S_render_node(cmark_renderer *renderer, cmark_node *node,
   case CMARK_NODE_FOOTNOTE_REFERENCE:
     if (entering) {
       LIT("[^");
-      OUT(cmark_chunk_to_cstr(renderer->mem, &node->as.literal), false, LITERAL);
+      OUT(node->user_data, false, LITERAL);
       LIT("]");
     }
     break;
@@ -486,9 +486,11 @@ static int S_render_node(cmark_renderer *renderer, cmark_node *node,
     if (entering) {
       renderer->footnote_ix += 1;
       LIT("[^");
-      char n[32];
-      snprintf(n, sizeof(n), "%d", renderer->footnote_ix);
-      OUT(n, false, LITERAL);
+
+      char *str = renderer->mem->calloc(1, (sizeof(char) * node->as.literal.len) + 1);
+      memmove(str, node->as.literal.data, node->as.literal.len);
+
+      OUT(str, false, LITERAL);
       LIT("]:\n");
 
       cmark_strbuf_puts(renderer->prefix, "    ");

src/html.c

@@ -59,17 +59,31 @@ static void filter_html_block(cmark_html_renderer *renderer, uint8_t *data, size
     cmark_strbuf_put(html, data, (bufsize_t)len);
 }
 
-static bool S_put_footnote_backref(cmark_html_renderer *renderer, cmark_strbuf *html) {
+static bool S_put_footnote_backref(cmark_html_renderer *renderer, cmark_strbuf *html, cmark_node *node) {
   if (renderer->written_footnote_ix >= renderer->footnote_ix)
     return false;
   renderer->written_footnote_ix = renderer->footnote_ix;
 
-  cmark_strbuf_puts(html, "<a href=\"#fnref");
-  char n[32];
-  snprintf(n, sizeof(n), "%d", renderer->footnote_ix);
-  cmark_strbuf_puts(html, n);
+  cmark_strbuf_puts(html, "<a href=\"#fnref:");
+  cmark_strbuf_put(html, node->as.literal.data, node->as.literal.len);
   cmark_strbuf_puts(html, "\" class=\"footnote-backref\">↩</a>");
 
+  if (node->footnote.count > 1)
+  {
+    for(int i = 2; i <= node->footnote.count; i++) {
+      char n[32];
+      snprintf(n, sizeof(n), "%d", i);
+
+      cmark_strbuf_puts(html, " <a href=\"#fnref:");
+      cmark_strbuf_put(html, node->as.literal.data, node->as.literal.len);
+      cmark_strbuf_puts(html, ":");
+      cmark_strbuf_put(html, (const unsigned char *)n, strlen(n));
+      cmark_strbuf_puts(html, "\" class=\"footnote-backref\">↩<sup class=\"footnote-ref\">");
+      cmark_strbuf_put(html, (const unsigned char *)n, strlen(n));
+      cmark_strbuf_puts(html, "</sup></a>");
+    }
+  }
+
   return true;
 }
 
@@ -273,7 +287,7 @@ static int S_render_node(cmark_html_renderer *renderer, cmark_node *node,
       } else {
         if (parent->type == CMARK_NODE_FOOTNOTE_DEFINITION && node->next == NULL) {
           cmark_strbuf_putc(html, ' ');
-          S_put_footnote_backref(renderer, html);
+          S_put_footnote_backref(renderer, html, parent);
         }
         cmark_strbuf_puts(html, "</p>\n");
       }
@@ -395,13 +409,12 @@ static int S_render_node(cmark_html_renderer *renderer, cmark_node *node,
         cmark_strbuf_puts(html, "<section class=\"footnotes\">\n<ol>\n");
       }
       ++renderer->footnote_ix;
-      cmark_strbuf_puts(html, "<li id=\"fn");
-      char n[32];
-      snprintf(n, sizeof(n), "%d", renderer->footnote_ix);
-      cmark_strbuf_puts(html, n);
+
+      cmark_strbuf_puts(html, "<li id=\"fn:");
+      cmark_strbuf_put(html, node->as.literal.data, node->as.literal.len);
       cmark_strbuf_puts(html, "\">\n");
     } else {
-      if (S_put_footnote_backref(renderer, html)) {
+      if (S_put_footnote_backref(renderer, html, node)) {
         cmark_strbuf_putc(html, '\n');
       }
       cmark_strbuf_puts(html, "</li>\n");
@@ -410,10 +423,18 @@ static int S_render_node(cmark_html_renderer *renderer, cmark_node *node,
 
   case CMARK_NODE_FOOTNOTE_REFERENCE:
     if (entering) {
-      cmark_strbuf_puts(html, "<sup class=\"footnote-ref\"><a href=\"#fn");
-      cmark_strbuf_put(html, node->as.literal.data, node->as.literal.len);
-      cmark_strbuf_puts(html, "\" id=\"fnref");
-      cmark_strbuf_put(html, node->as.literal.data, node->as.literal.len);
+      cmark_strbuf_puts(html, "<sup class=\"footnote-ref\"><a href=\"#fn:");
+      cmark_strbuf_puts(html, node->user_data);
+      cmark_strbuf_puts(html, "\" id=\"fnref:");
+      cmark_strbuf_puts(html, node->user_data);
+
+      if (node->footnote.ix > 1) {
+        char n[32];
+        snprintf(n, sizeof(n), "%d", node->footnote.ix);
+        cmark_strbuf_puts(html, ":");
+        cmark_strbuf_put(html, (const unsigned char *)n, strlen(n));
+      }
+
       cmark_strbuf_puts(html, "\">");
       cmark_strbuf_put(html, node->as.literal.data, node->as.literal.len);
       cmark_strbuf_puts(html, "</a></sup>");

src/node.h

@@ -76,6 +76,11 @@ struct cmark_node {
 
   cmark_syntax_extension *extension;
 
+  union {
+    int ix;
+    int count;
+  } footnote;
+
   union {
     cmark_chunk literal;
     cmark_list list;