Updated to only add numbering to slugified heading anchors when absolutely neccessary

app/components/Editor/headingToSlug.js

@@ -1,10 +1,25 @@
 // @flow
 import { escape } from 'lodash';
-import { Node } from 'slate';
+import { Document, Block, Node } from 'slate';
 import slug from 'slug';
 
-export default function headingToSlug(node: Node, index: number = 0) {
-  const slugified = escape(slug(node.text));
-  if (index === 0) return slugified;
-  return `${index}-${slugified}`;
+// finds the index of this heading in the document compared to other headings
+// with the same slugified text
+function indexOfType(document, heading) {
+  const slugified = escape(slug(heading.text));
+  const headings = document.nodes.filter((node: Block) => {
+    if (!node.text) return false;
+    return node.type.match(/^heading/) && slugified === escape(slug(node.text));
+  });
+
+  return headings.indexOf(heading);
+}
+
+// calculates a unique slug for this heading based on it's text and position
+// in the document that is as stable as possible
+export default function headingToSlug(document: Document, node: Node) {
+  const slugified = escape(slug(node.text));
+  const index = indexOfType(document, node);
+  if (index === 0) return slugified;
+  return `${slugified}-${index}`;
 }