feat: footnotes (#2384)

* feat: footnotes

* feat: proper DOCX support

packages/editor-ext/src/lib/markdown/utils/turndown.utils.ts

@@ -34,6 +34,8 @@ export function htmlToMarkdown(html: string): string {
     iframeEmbed,
     image,
     video,
+    footnoteRef,
+    footnotesList,
   ]);
   return turndownService.turndown(html).replaceAll('<br>', ' ');
 }
@@ -203,6 +205,56 @@ function image(turndownService: _TurndownService) {
   });
 }
 
+function getFootnoteAnchor(node: HTMLElement): HTMLElement | null {
+  const child = node.firstElementChild as HTMLElement | null;
+  return child?.nodeName === 'A' && child.classList.contains('footnote-ref')
+    ? child
+    : null;
+}
+
+function footnoteRef(turndownService: _TurndownService) {
+  turndownService.addRule('footnoteRef', {
+    filter: function (node: HTMLInputElement) {
+      return node.nodeName === 'SUP' && !!getFootnoteAnchor(node);
+    },
+    replacement: function (_content: string, node: HTMLInputElement) {
+      const anchor = getFootnoteAnchor(node);
+      const number =
+        anchor.getAttribute('data-reference-number') || anchor.textContent;
+      return `[^${number}]`;
+    },
+  });
+}
+
+function footnotesList(turndownService: _TurndownService) {
+  turndownService.addRule('footnotesList', {
+    filter: function (node: HTMLInputElement) {
+      return node.nodeName === 'OL' && node.classList.contains('footnotes');
+    },
+    replacement: function (_content: string, node: HTMLInputElement) {
+      const items = Array.from(node.children).filter(
+        (child) => child.nodeName === 'LI',
+      );
+      const definitions = items.map((li, index) => {
+        const number =
+          (li.getAttribute('id') || '').replace('fn:', '') ||
+          String(index + 1);
+        const markdown = turndownService
+          .turndown((li as HTMLElement).innerHTML)
+          .trim();
+        // continuation lines need a 4-space indent to stay in the footnote
+        const [first, ...rest] = markdown.split('\n');
+        const body = [
+          first,
+          ...rest.map((line: string) => (line.trim() ? `    ${line}` : line)),
+        ].join('\n');
+        return `[^${number}]: ${body}`;
+      });
+      return `\n\n${definitions.join('\n')}\n\n`;
+    },
+  });
+}
+
 function video(turndownService: _TurndownService) {
   turndownService.addRule('video', {
     filter: function (node: HTMLInputElement) {