plugins: mark external links with a "↗" symbol

Gemfile

@@ -20,3 +20,5 @@ gem 'color_pound_spec_reporter'
 
 # HTTP bindings to libcurl - https://github.com/taf2/curb
 gem 'curb'
+
+gem "nokogiri", "~> 1.13.10"

Gemfile.lock

@@ -55,15 +55,20 @@ GEM
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
+    mini_portile2 (2.8.1)
     minitest (5.14.2)
     minitest-reporters (1.4.2)
       ansi
       builder
       minitest (>= 5.0)
       ruby-progressbar
+    nokogiri (1.13.10)
+      mini_portile2 (~> 2.8.0)
+      racc (~> 1.4)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (5.0.0)
+    racc (1.6.2)
     rake (13.0.1)
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
@@ -92,7 +97,8 @@ DEPENDENCIES
   kramdown-parser-gfm
   minitest
   minitest-reporters
+  nokogiri (~> 1.13.10)
   rake
 
 BUNDLED WITH
-   2.1.4
+   2.3.26

_plugins/mark_external_links.rb

@@ -0,0 +1,56 @@
+# This file is based on code from https://github.com/riboseinc/jekyll-external-links
+
+require 'nokogiri'
+require 'uri'
+
+# Given hostname and content, updates any found <a> elements as follows:
+#
+# - Adds `rel` attribute
+# - Adds css class for external link icon
+#
+# Only processes external links where `href` starts with "http"
+# and target host does not start with given site hostname.
+def process_content(site_hostname, content, marker_css, link_selector)
+  content = Nokogiri::HTML(content)
+  content.css(link_selector).each do |a|
+    next unless a.get_attribute('href') =~ /\Ahttp/i
+    next if a.get_attribute('href') =~ /\Ahttp(s)?:\/\/#{site_hostname}\//i
+    next if a.inner_html.include? "icon-ext"
+    next if a.inner_html.include? "fa" # another icon is part of the link, e.g fa-github
+    next if a.inner_html.include? a.get_attribute('href') # plain links
+    a.set_attribute('rel', 'external')
+    a.set_attribute('class', marker_css)
+  end
+  return content.to_s
+end
+
+def mark_links_in_page_or_document(page_or_document)
+  site_hostname = URI(page_or_document.site.config['url']).host
+
+  # The link is marked as external by:
+  # (1) setting the rel attribute to external and
+  # (2) appending specified marker css class.
+  marker_css = "icon-ext"
+
+  # Determines which links to mark. E.g., usually we don’t want to mark navigational links.
+  link_selector = 'a:not(.internal-link)'
+
+  # Do not process assets or other non-HTML files
+  unless (page_or_document.respond_to?(:asset_file?) and
+      page_or_document.asset_file?) or
+      page_or_document.output_ext != ".html"
+    page_or_document.output = process_content(
+      site_hostname,
+      page_or_document.output,
+      marker_css,
+      link_selector)
+  end
+end
+
+Jekyll::Hooks.register :documents, :post_render do |doc|
+  mark_links_in_page_or_document(doc)
+end
+
+Jekyll::Hooks.register :pages, :post_render do |page|
+  mark_links_in_page_or_document(page)
+end

assets/css/all.sass

@@ -175,6 +175,16 @@ a
   color: #ccc
   cursor: help
 
+.icon-ext
+  &:after
+    position: relative
+    top: -0.5em
+    font-size: 0.7em
+    content: "↗"
+    color: #aaaaaa
+    display: inline-block
+    text-decoration: none
+
 .backlink-box
   position: relative
   font-size: 0.9em