{%- assign tags = blank -%}
{%- for post in site.posts -%}
{%- assign post_tags = post.tags | join:'|' | append:'|' -%}
{%- if post_tags != '|' -%}
{%- assign tags = tags | append:post_tags -%}
{%- endif -%}
{%- endfor -%}
{%- assign tags = tags | split:'|' | uniq | sort -%}
{% for tag in tags %}
{{ tag }}
{% for post in site.posts %}
{%- if post.tags contains tag -%}
{%- if post.external_url -%}
{%- assign url = post.external_url -%}
{%- else -%}
{%- assign url = post.url | relative_url -%}
{%- endif -%}
- {{ post.title }}
- {{ post.date | date_to_long_string }}
{%- endif -%}
{% endfor %}
{% endfor %}