pandoc-filter

#!/bin/bash

# For external links, appends rel=nofollow
# For headings, converts their inner text to a link to the same header

# Data model reference (for the node.js library because it's easier to read):
# https://github.com/mvhenderson/pandoc-filter-node/blob/master/index.ts

jq '
	walk(
		(try .t catch "") as $t |
		if $t == "Link" and (.c[2][0] | startswith("http")) then
			{
				"t": "Link",
				"c": [
					["", [], [["rel", "nofollow"]]],
					.c[1],
					.c[2]
				]
			}
		elif $t == "Header" then
			{
				"t": "Header",
				"c": [
					.c[0],
					.c[1],
					[
						{
							"t": "Link",
							"c": [
								["", [], []],
								.c[2],
								["#\(.c[1][0])", ""]
							]
						}
					]
				]
			}
		else
			.
		end
	)
'