Add plugin for generating tag pages automatically

_plugins/auto_gen_tags.rb

@@ -0,0 +1,45 @@
+# This plugin generates tag pages automatically after saving posts.
+#
+# To enable this plugin, please do the following steps:
+# 1, Create a dir named 'tags' under the root dir.
+# 2. Create a layout, named `_layouts/tag.html,` for tag pages. For example,
+#   ---
+#   layout: default
+#   ---
+#   <h1>
+#     Tag: {{ page.tag }}
+#   </h1>
+#   <ul>
+#   	{% for post in site.posts %}
+#   		{% if post.tags contains page.tag %}
+#   			<li>
+#   			    <a href="{{ post.url }}">{{ post.title }}</a> 
+#   			    ({{ post.date | date_to_string }})<br>
+#   		    </li>
+#   		{% endif %}
+#   	{% endfor %}
+#   </ul>
+#
+# 3. Comment out the `return` statement in the next line.
+return
+# 4. Start jekyll server at local, and the server will parse all posts and
+#    generate tag pages to the `tags/` dir. The tag pages are also generated
+#    on-the-fly after saving posts if jekyll server is running at background.
+
+Jekyll::Hooks.register :posts, :post_write do |post|
+  all_existing_tags = Dir.entries("tags")
+    .map { |t| t.match(/(.*).md/) }
+    .compact.map { |m| m[1] }
+
+  tags = post['tags'].reject { |t| t.empty? }
+  tags.each do |tag|
+    generate_tag_file(tag) if !all_existing_tags.include?(tag)
+  end
+end
+
+def generate_tag_file(tag)
+  File.open("tags/#{tag}.md", "wb") do |file|
+    file << "---\nlayout: tag\ntitle: \"Tag: #{tag}\"\ntag: #{tag}\n---\n"
+  end
+end
+